Article

ISSN 0006-2979, Biochemistry (Moscow), 2026, Vol. 91, No. 4, pp. 543-560 © Pleiades Publishing, Ltd., 2026.

543

REVIEW

Plant Innate Immunity: Crosstalk of Signaling Pathways

Boris I. Skulachev

, Anastasia K. Atabekova

, Alexander A. Lezzhov

and Andrey G. Solovyev

2,a

Faculty of Bioengineering and Bioinformatics, Moscow State University, 119234 Moscow, Russia

Belozersky Institute of Physico-Chemical Biology, Moscow State University, 119992 Moscow, Russia

e-mail: solovyev@belozersky.msu.ru

Received December 13, 2025

Revised February 27, 2026

Accepted March 14, 2026

Abstract— The innate immunity of plants is a dynamic, multilevel system traditionally divided into pat-

tern-triggered immunity (PTI) and effector-triggered immunity (ETI). Despite being activated by different

types of receptors localized in different cell compartments, PTI and ETI are currently considered interde-

pendent components of a single defense system. This view suggests that, due to various positive interac-

tions between these two pathways, the innate immunity of plants is more than the sum of PTI and ETI.

Available data indicate that PTI and ETI enhance each other synergistically, increasing the concentration

of signaling molecules, such as components of kinase cascades, reactive oxygen species, calcium ions, and

phytohormones. This leads to the activation of defense genes, providing a local response to pathogens and

the development of systemic plant resistance.

DOI: 10.1134/S0006297925604289

Keywords: innate plant immunity, receptors of pathogen patterns, receptors of pathogen effectors, resisto-

somes, kinase cascades, reactive oxygen species, calcium signaling, transcriptional reprogramming, hypersen-

sitive response, local resistance, systemic resistance

* To whom correspondence should be addressed.

INTRODUCTION

Plants are exposed to attacks by pathogenic bac-

teria, fungi, and viruses, for which a plant organism

serves as a natural habitat and a source of nutrients

required for growth. As a result of the long-term co-

evolution of plants and phytopathogens, sophisticated

molecular interaction mechanisms have emerged that

enable plants to mount defense responses against in-

fection while allowing pathogens to overcome such

defenses [1]. In particular, plant evolution has led to

the emergence of innate immunity, which is based

on defense response genes that encode proteins ca-

pable of detecting attacks by pathogens and activat-

ing nonspecific protective responses, both local and

systemic [2]. In addition, plants possess an adaptive

immune system based on RNA silencing that provides

sequence-specific protection against certain patho-

gens, predominantly viruses [3].

Traditionally, plant innate immunity has been

considered as a two-layered system. The first layer,

known as pattern-triggered immunity (PTI), is activat-

ed upon recognition of conserved pathogen-associat-

ed molecular patterns (PAMPs) or damage-associated

molecular patterns (DAMPs) by pattern recognition

receptors (PRRs) on the cell surface. To suppress

PTI, many pathogens produce effector proteins. In

response, plants activate a second, typically stronger

immune response known as effector-triggered immu-

nity (ETI), which involves the recognition of pathogen

effectors by specific cytoplasmic receptors [4].

PTI and ETI had been initially considered as two

distinct branches of plant innate immunity, a concept

portrayed in the well-known “zigzag model” describ-

ing the development of plant immune responses [1].

However, subsequent studies have revealed extensive

interconnections between PTI and ETI. Although these

responses are mediated by different classes of recep-

tors localized in distinct cellular compartments, both

pathways converge on a common set of downstream

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BIOCHEMISTRY (MOSCOW) Vol. 91 No. 4 2026

defense responses, such as activation of mitogen-acti-

vated protein kinase (MAPK) cascades, calcium influx,

production of reactive oxygen species (ROS), synthe-

sis of phytohormones, and transcriptional reprogram-

ming. In recent years, substantial progress has been

made in understanding how PTI and ETI interact to

establish effective immunity. Increasing evidence in-

dicates that these two pathways are tightly intercon-

nected and, in some cases, functionally inseparable

across multiple levels of immune response, ranging

from pathogen recognition to the activation of de-

fense gene expression [1].

In this review, we summarize current insights

into the mechanisms underlying PTI and ETI, as well

as diverse modes of interaction between these com-

ponents of plant innate immunity. We also discuss

the development of local and systemic resistance to

pathogens that arises upon activation of PTI andETI.

PTI AND ETI: RECEPTORS AND CORECEPTORS

The classical distinction between PTI and ETI is

largely based on the types of receptors involved in

pathogen recognition. PTI is activated by PRRs located

at the cell surface, which recognize PAMPs. Depend-

ing on the pathogen type, PAMPs may be further clas-

sified as virus-associated molecular patterns (VAMPs)

in the case of viral infection or microbe-associated

molecular patterns (MAMPs) in the case of bacterial

or fungal infection. There are also damage-associated

molecular patterns (DAMPs), which are plant-derived

molecules, such as fragments of cell wall components

generated during pathogen attack, that are capable

of activating immune responses [1, 5]. PRRs contain

a transmembrane α-helical domain and a variable

extracellular domain (ECD) that mediates recogni-

tion of diverse PAMPs depending on the type of ECD.

Some PRRs also possess a cytoplasmic kinase domain

and are therefore classified as receptor-like kinases

(RLKs), whereas PRRs lacking a structured cytoplas-

mic domain are referred to as receptor-like proteins

(RLPs) [6, 7]. Based on the structure of their ECDs,

PRRs can be divided into several classes. The larg-

est group comprises receptors containing leucine-rich

repeat (LRR) domains, while other classes include re-

ceptors with lysine motifs (LysM) and lectin, wall-as-

sociated kinase (WAK), S-locus, malectin-like, pro-

line-rich, and cysteine-rich repeat domains [8]. Upon

interaction with a pathogen, RLKs and RLPs recognize

PAMPs via their extracellular domains, undergo con-

formational changes, and dynamically associate with

their coreceptors and receptor-like cytoplasmic kinas-

es (RLCKs), forming signaling complexes that initiate

intracellular signaling cascades (Fig.  1). RLCKs lack

extracellular and transmembrane domains, playing a

central role in transducing signals from the cell sur-

face to intracellular pathways that lead to pathogen

resistance [7, 9].

In general, PTI-associated immune responses pro-

ceed as follows: activation of PRRs and their core-

ceptors upon PAMP recognition leads to phosphory-

lation of RLCKs, which subsequently phosphorylate

downstream signaling components, thereby initiating

intracellular signal transduction through activation of

MAPK cascades, ion channels, and ROS production.

These events result in large-scale transcriptional re-

programming and the activation of defense respons-

es, including cell wall reinforcement, synthesis of

antimicrobial compounds such as phytoalexins and

phytoncides, and production of phytohormones that

coordinate local and systemic responses, ultimately

conferring resistance (Fig.  1) [8].

PRR coreceptors are most commonly represented

by LRR-RLK receptors of the SERK family (SERK1-5,

somatic embryogenesis receptor kinases) and by

members of the NIK (NSP-interacting kinase) family

(NIK1-3) [5, 7]. For example, recognition of bacterial

flagellin or its conserved peptide fragment flg22 by

the receptor FLS2 (flagellin sensing  2) in Arabidopsis

thaliana results in rapid phosphorylation of the re-

ceptor kinase domain and its dimerization with the

coreceptor SERK3, also known as BAK1 (BRI1-associ-

ated kinase  1). This complex subsequently interacts

with the RLCK BIK1 (botrytis-induced kinase  1) [10].

Conformational changes lead to the release of phos-

phorylated BIK1 from the complex for activation of

downstream signaling components [11].

Coreceptors of the NIK family have been shown

to play a role in antiviral immunity. A mechanistic

model of antiviral signaling via NIK1 suggests that

viral PAMPs are recognized by yet unidentified PRRs,

leading to phosphorylation and oligomerization of

NIK1. Activated NIK1 mediates phosphorylation of

ribosomal protein L10 (RPL10) at Ser104, thus pro-

moting its translocation into the nucleus, resulting

in global translational repression [12, 13]. Notably,

in addition to its positive role in antiviral respons-

es, NIK1 also participates in antibacterial immunity,

where it functions as a negative regulator due to

its constitutive association with FLS2 and BAK1. In

nik1-knockout plants, flg22-induced activation of PTI

is enhanced, including increased MAPK activity, ele-

vated levels of ROS and salicylic acid (SA), and in-

creased expression of the PR1 gene and other PTI

components such as WRKY30, FRK1, and PP2C. Such

effects can be explained by the constitutive interac-

tion of NIK1 with FLS2 and BAK1. These interactions

likely prevent autoimmune response in the absence

of infection. Upon flg22 treatment, activation of

FLS2 leads to phosphorylation of NIK1 at Thr474 by

BAK1, thereby triggering antiviral signaling through

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Fig.  1. Schematic representation of signaling pathways underlying PTI and ETI in plants. Recognition of PAMPs and DAMPs

by cell-surface PRRs induces conformational changes in the receptors, their association with coreceptors, and phosphory-

lation of RLCKs. Activated RLCKs initiate multiple intracellular signaling cascades, including activation of MAPK cascades,

phosphorylation of RBOHD leading to ROS burst, and activation of classical ion channels that mediate Ca

influx and

downstream signaling via CPKs. Pathogen-derived effectors that suppress PTI are recognized by cytoplasmic NLR (NBS-LRR)

receptors. Sensor NLRs (CNLs and TNLs) are responsible for effector recognition, whereas helper NLRs (RNLs) are activated

downstream of sensor NLR signaling. NLR activation results in their oligomerization, resulting in formation of resistosomes.

CNL- and RNL-resistosomes integrate into the plasma membrane and function as Ca

channels, promoting a robust cal-

cium influx. TNL (TIR-NLR) resistosomes exhibit NADase activity, generating secondary signaling molecules that, through

interaction with EDS1, lead to the activation of RNLs. The increase in cytosolicCa

concentration that results from resis-

tosome functioning activates CPKs, which then phosphorylate TFs and multiple signaling components, including RBOHD,

thereby amplifying ROS production. ROS, in turn, activate Ca

channels, forming a positive feedback loop. Collectively,

activeCa

, MAPK, and ROS signaling pathways lead to large-scale transcriptional reprogramming and activation of defense

gene expression. Some CNLs and TNLs may also function directly in the nucleus; however, whether this occurs in the form

of resistosomes remains to be determined.

the NIK1-mediated RPL10 phosphorylation and re-

pression of translation-associated genes. Therefore,

flg22 treatment can promote antiviral resistance in a

NIK1-dependent manner [13]. Thus, NIK1 represents

a unique example of a receptor kinase involved in

autoimmune regulation, as well as in antiviral and

antibacterial immunity.

Coreceptor kinases, such as SERK and NIK, are

often targets of pathogen effectors [14]. Inhibition of

these kinases suppresses PTI; however, in response

to such effectors, plants activate the second layer of

innate immunity – ETI. ETI receptors, which recog-

nize pathogen effectors, are cytoplasmic proteins that

contain LRR domains, nucleotide-binding sites (NBSs),

and variable N-terminal domains. These receptors are

collectively referred to as NBS-LRR proteins (NLRs).

Based on the structure of their N-terminal domains,

NLRs are classified into CNLs (coiled-coil domain),

TNLs (Toll/interleukin-1 receptor domain), and RNLs

(CC

RPW8

domain) [15]. Functionally, NLRs can be di-

vided into sensor NLRs (CNLs and TNLs), which rec-

ognize pathogen effectors, and helper NLRs (RNLs),

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which do not directly recognize effectors, but rath-

er transmit signals from sensor NLRs. Examples of

helper NLRs include ADR1 (activated disease resis-

tance  1), NRG1 (N requirement gene  1), and members

of the NRC (NB-LRR protein required for HR-associ-

ated cell death) family. ADR1 can function with both

CNL and TNL receptors, whereas NRG1 is associated

with TNL-mediated signaling [16, 17]. NRC proteins

are required for the function of sensor CNLs in the

Solanaceae family [18].

Several models have been proposed to describe

effector recognition by NLRs. In the direct recognition

model, the NLR binds the effector via its LRR domain.

In the guard model, NLRs detect modifications of host

target proteins (“guardees”) induced by pathogen ef-

fectors. In the decoy model, plants have evolved to

express proteins that mimic genuine effector targets

but lack their key immune functions. Modification of

these decoys by effectors is recognized by NLRs, trig-

gering NLR activation. Finally, in the integrated de-

coy model, decoy domains are incorporated directly

into NLR proteins (e.g., HMA, WRKY, BED, and NOI

domains); such domains are present in up to 10% of

all NLRs [19-21].

While the C-terminal LRR domain is directly in-

volved in the effector recognition, the NBS domain

acts as a molecular switch that binds adenosine di-

phosphate (ADP) in the passive state and adenosine

triphosphate (ATP) in the active state. Upon ligand

recognition, the exchange of ADP for ATP in the NBS

domain triggers conformational changes that lead to

the formation of NLR oligomers called resistosomes.

Pentameric and hexameric resistosomes formed by

CNLs and RNLs can integrate into the plasma mem-

brane and function as Ca

-permeable ion channels,

whereas tetrameric TNL resistosomes function as

holoenzymes bearing NADase activity and generate

secondary signaling molecules derived from NAD

that activate RNL proteins (Fig.  1) [22, 23]. Proper

functioning of the TNL/RNL signaling module also

requires intermediate complexes with lipase-like pro-

teins of the EDS1 (enhanced disease susceptibility  1)

family [24]. Binding of TNL resistosome-derived sig-

naling molecules to EDS1-PAD4 or EDS1-SAG101 het-

erodimers induces conformational changes in PAD4

(phytoalexin-deficient 4) and SAG101, promoting in-

teraction with RNL proteins and their activation.

The activated RNLs in turn undergo conformational

rearrangements exposing N-terminal α-helices; this

leads to dissociation of EDS1 and RNL oligomeriza-

tion, resulting in the formation of membrane-associ-

ated resistosomes with ion channel activity [9, 25-28].

In addition, proper NLR function depends on HSP90

chaperones and RAR1/SGT1 co-chaperones, which

form complexes with NLR proteins to ensure their

correct folding and functioning [15, 20].

In general, upon effector recognition, activat-

ed NLRs oligomerize into resistosomes, triggering

a strong Ca

influx into the cytoplasm. This acti-

vates nicotinamide adenine dinucleotide phosphate

(NADPH) oxidases and ROS production, also trigger-

ing downstream hormonal signaling, transcriptional

reprogramming, and programmed cell death associat-

ed with the hypersensitive response (HR), a hallmark

of ETI. In some cases, activated NLRs can translocate

to the nucleus and interact with transcription factors

(TFs), directly modulating gene expression (Fig.  1).

These processes ultimately lead to local and systemic

immune responses, providing resistance at the site of

infection and throughout the plant.

Despite the differences in initiation mechanisms,

PTI and ETI activate overlapping and complementa-

ry reaction cascades, involve the same components of

signaling pathways, and mutually regulate each other

at various stages, including the receptor level. In fact,

in recent years, increasing evidence suggest both the

regulation of ETI by PTI receptors and the regulation

of PTI by ETI receptors.

Many studies highlight the importance of PRR

signaling for ETI. Since pathogens that induce ETI

also contain PAMPs, it is challenging to dissect indi-

vidual contributions of PTI and ETI. Notably, ETI-asso-

ciated HR induced by Pseudomonas syringae effectors

(AvrRpt2, AvrPphB, AvrRps4) is significantly reduced

in PRR or coreceptor mutants, including fls2/efr, fls2/

efr/cerk1, bak1-5/bkk1-1, and bak1-5/bkk1-1/cerk1.

Furthermore, in Arabidopsis, PTI coreceptors BAK1

and BKK1 are required for restricting the infection by

Hyaloperonospora arabidopsidis, whose effectors are

recognized by TNL RPP2 and RPP4 [29]. Estradiol-de-

pendent expression of bacterial effectors AvrRps4 and

AvrRpt2 recognized by TNL RRS1/RPS4 and CNL RPS2,

respectively, allowed to analyze ETI in the absence

of PTI. Under these conditions, ETI responses were

relatively weak and were not associated with ROS ac-

cumulation or HR induction [30, 31]. These findings

indicate that PTI components are required for full

ETI functionality and act to potentiate ETI-mediated

pathogen restriction (Fig.  2) [31, 32]. Recent studies

have revealed even tighter interdependence between

PTI and ETI. RLCKs, key components of PTI signaling,

have been shown to constitutively inhibit NLR oligo-

merization through phosphorylation. Upon PTI activa-

tion, RLCKs dissociate from NLRs and are recruited

to activated PRRs, relieving ETI inhibition. Thus, PTI

induction can enhance ETI signaling [33].

It has recently been discovered that PTI can also

suppress ETI. This phenomenon, termed pre-PTI-me-

diated ETI suppression (PES), involves attenuation of

AvrRpt2-induced ETI that occurs after the prior in-

duction of a weak PTI-response by flg22 in Arabidop-

sis [22]. In the dde2/ein2/pad4/sid2 mutant defective

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Fig.  2. Schematic representation of interconnections be-

tween PTI and ETI underlying plant innate defense re-

sponses. Upon pathogen attack, PAMPs are recognized by

cell-surface PRRs, leading to the PTI activation. At the same

time, pathogen-derived effectors produced to suppress PTI

are recognized by intracellular NLRs, resulting in the ETI

activation. PTI enhances ETI responses, whose components

may remain weak in the absence of PTI. Conversely, ETI

reinforces PTI by increasing the accumulation of key signal-

ing components and amplifying downstream signaling. This

positive regulatory feedback loop, in which ETI acts as an

amplifier of PTI responses, leads to a large-scale transcrip-

tional reprogramming and establishment of both local and

systemic resistance.

in multiple hormone signaling pathways [including

jasmonic acid (JA), ethylene (ET), PAD4 and SA branch-

es], AvrRpm1-induced HR is suppressed if plants are

pretreated with PAMPs [34]. These findings suggest

that PTI can regulate ETI either positively or nega-

tively, with negative regulation potentially serving to

limit the resource-consuming ETI when PTI alone is

sufficient.

A large amount of data also suggests that PTI can

be regulated by ETI. Indeed, activation of NLRs rapid-

ly increases both transcript and protein levels of key

PTI components, including components of receptor

complexes and downstream signaling factors. Theac-

tivation of NLRs such as RPM1, RPS2, RPS5, RPS4, and

RPP4 promotes accumulation of BAK1, SOBIR1, BIK1/

PBL, RBOHD, MPK3, and MPK6 independently of PTI

induction [29, 31, 32]. Moreover, activation of the

PRR RLP23 requires ETI signaling components such

as RNL ADR1, as well as EDS1 and PAD4 [35, 36].

As a result, ETI activation strengthens PTI re-

sponses (Fig.  2). For instance, ETI induced by recog-

nition of the effector AvrRps4 by the receptors RRS1

and RPS4 enhances flg22-triggered ROS production

and cell death [32]. Thus, ETI triggered by different

effectors amplifies and reinforces PTI responses in-

duced by PAMPs.

THE ROLE OF SIGNALING MOLECULES

IN PTI–ETI INTERACTIONS

Signaling pathways underlying PTI and ETI share

common hubs, including MAPK cascades, ROS, Ca

signaling, and phytohormone pathways, which are

involved in the interaction between PTI and ETI [4,

9, 37].

MAPK. PTI is characterized by rapid activation

of MAPK cascades following ligand recognition by re-

ceptors, whereas ETI induces a slower but stronger

and more sustained MAPK activation [38]. While the

components of MAPK cascades directly phosphory-

lated by RLCK kinases have been well characterized

in PTI signaling, the mechanisms by which NLR sig-

naling activates MAPK cascades remain unclear. Two

major MAPK cascades are involved in the positive

regulation of PTI. The first is initiated by MAP ki-

nase kinase kinases (MAPKKKs), such as MAP3K1

(also known as MEKK1), leading to the activation of

MAPK4 and MAPK11. The second cascade involves

MAP3K3, MAP3K5, and YODA kinase, which activate

MAPK3 and MAPK6 [39]. In addition to positive regu-

lation, there is also negative regulation of PTI mediat-

ed by the MEKK1–MKK1/2–MPK4 cascade [40]. MAPK

cascades can be activated either by transmembrane

PRRs or by RLCKs, particularly those belonging to

subfamilies VII and XII [9, 41]. Identifying specific

factors responsible for MAPK activation during ETI

remains challenging, as ETI amplifies PTI-derived

signals and the two pathways act synergistically [9].

It has been shown that TNL receptors such as

RRS1/RPS4 and RPP4 are unable to trigger MAPK ac-

tivation in transgenic Arabidopsis expressing AvrRps4

or AvrRpp4 in the absence of PRR signaling. This sug-

gests that TNL-associated MAPK phosphorylation is

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mediated through the PTI pathway. In contrast, MAPK

activation via CNL receptors, such as RPS2, RPS5, and

RPM1 in Arabidopsis, appears to occur independently

of PRR signaling [31, 32].

ROS. Both PTI- and ETI-mediated immune re-

sponses lead to increased intracellular ROS levels.

ROS are typically generated as byproducts of aerobic

metabolism, but they also have essential signaling

functions in plant immunity. ROS production during

immune responses is primarily mediated by NADPH

oxidases (respiratory burst oxygen homologs, RBOHs)

[9]. For example, during flg22-induced PTI, activation

of the PRR-BAK1 complex leads to activation of RLCKs

BIK1 and PBL1 (PBS1-like  1), which then phosphory-

late RBOHD, resulting in a rapid ROS burst. Additional

regulators, such as the MAP4K protein SIK1, further

enhance this process by transphosphorylating BIK1,

thereby stabilizing it and synergistically promoting

RBOHD activation and ROS production  [42]. Calci-

um signaling also plays a critical role in regulating

RBOHD activity: calcium-dependent protein kinases

(CPKs) phosphorylate RBOHD in response to chang-

es in cytosolic Ca

levels [9]. Thus, ROS integrate of

multiple signaling pathways, including calcium and

MAPK signaling, and represent one of the key links

between PRR- and NLR-mediated immune responses

(Fig.  1).

In the context of ETI, ROS production is more

intense and prolonged than in PTI and is associat-

ed with the induction of programmed cell death

(PCD) [43]. In addition to post-translational modifica-

tions of RBOH proteins, ETI involves transcriptional

activation of RBOH genes. ETI is further character-

ized by sustained MAPK activation and enhanced Ca

influx, which together establish a positive feedback

loop that regulates ROS production. Therefore, during

theETI, ROS burst is sustained and amplified by var-

ious positive feedback loops. Recent studies have

shown that the ETI-associated ROS burst requires

PRR-mediated signaling and that this signaling is es-

sential for reaching peak phosphorylation levels of

RBOHD during ETI [31, 32]. These findings suggest

that ROS accumulation during ETI is largely driven

by PTI signaling through PRR/coreceptor complexes.

Calcium. Ca

ions serve as universal second

messengers in eukaryotic cells. Upon pathogen recog-

nition, the cytosolic Ca

concentration in plant rap-

idly increases, triggering multiple defense respons-

es. PTI components, such as RLCKs (e.g., BIK1 and

PBL1), regulate activation of plasma membrane Ca

channels, including cyclic nucleotide-gated channels

(CNGCs), hyperosmolality-gated calcium-permeable

channel (OSCAs), and glutamate receptor-like chan-

nels (GLRs) [9, 44]. Upon attack and subsequent in-

fection by a pathogen, two peaks in the Ca

signal

are typically observed that differ in duration and

amplitude: a transient peak associated with PTI and

a more sustained peak associated with ETI [9]. The

sustained Ca

peak during ETI is mediated by NLR

resistosomes, which integrate into the plasma mem-

brane and function as Ca

-permeable channels, fa-

cilitating a robust influx of calcium into the cytosol.

Interestingly, integration of resistosomes into the plas-

ma membrane can compromise membrane integrity,

potentially generating DAMPs that further stimulate

PTI responses [45]. Apart from resistosomes, classical

channels also appear to contribute to ETI re-

sponses. For example, Arabidopsis CNGC2 and CNGC4

play an important role in AvrRpt2/RPS2-mediated HR,

whereas CNGC11 and CNGC12 contribute to ETI re-

sponses against Hyaloperonospora parasitica [41].

As described above, Ca

signaling is also closely

linked to MAPK cascades and ROS production via ac-

tivation of NADPH oxidases, ensuring synergetic reg-

ulation of these signaling networks [9].

Phytohormones. Phytohormonal signaling is

an important component of plant innate immunity.

Phytohormones are endogenous signaling molecules

that regulate plant growth, development, and stress

responses at very low concentrations [46]. Phytohor-

mones are traditionally divided into growth-related

hormones (auxins, gibberellins, cytokinins, brassino-

steroids, and strigolactones) and stress-related hor-

mones, including SA, JA, ET, and abscisic acid (ABA).

However, recent studies indicate that the boundar-

ies between these categories are being increasingly

blurred, as growth hormones influence certain as-

pects of stress-dependent metabolism, whereas stress

hormones participate in the regulation of some physi-

ological processes[47]. A substantial body of evidence

has been accumulated on the role of phytohormonal

signaling in plant defense responses [48, 49]. Due to

the broad scope of this topic, which cannot be fully

covered in this review, only one aspect of phytohor-

monal signaling is discussed below, namely the role

of phytohormones in the interaction between PTI

and ETI.

SA plays a key role in phytohormonal signaling

that occurs upon activation of innate immunity mech-

anisms [50], contributing to both local and systemic

defense responses (see below) and mediating cross-

talk between PTI and ETI. Activation of both PTI and

ETI leads to increased SA levels in plants [51]. This

increase is mediated by TFs such as SARD1 (systemic

acquired resistance deficient  1) and CBP60g (calm-

odulin-binding protein 60g), which activate promot-

ers of genes encoding proteins involved in SA bio-

synthesis, including ICS1 (isochorismate synthase  1),

EDS5 (enhanced disease susceptibility  5), and PBS3

(avrPphB susceptible  3) [52,53]. Members of the NPR

(non-expressor of pathogenesis-related genes) fami-

ly act as intracellular SA receptors; NPR1 functions

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as a positive regulator of SA signaling, whereas

NPR3 and NPR4 act as negative regulators [54, 55].

NPR1, NPR3, and NPR4 interact with TFs of the TGA

(TGACG-binding) family, collectively regulating the

transcription of SA-responsive genes involved in plant

defense [53]. In addition to activating defense gene

expression, SA plays an important role in regulating

the PTI/ETI interactions. Treatment with exogenous

SA increases the levels of receptors and signaling

components involved in both PTI and ETI [56-58],

whereas activation of both pathways is strongly im-

paired in npr1- and npr4-mutant plants [59]. Thus,

SA establishes a positive feedback loop that ampli-

fies immune responses and synchronizes the pro-

duction of PTI- and ETI-associated proteins, thereby

coordinating the activation of these two signaling

pathways.

Along with the positive regulation, phytohor-

mones can also negatively regulate PTI and ETI, for

example, through antagonistic interactions between

the SA and JA signaling pathways. SA signaling is typ-

ically activated in response to biotrophic pathogens,

whereas JA and ET signaling pathways are associ-

ated with responses to necrotrophic pathogens [60].

SA signaling induces TFs that repress transcription

of JA-responsive genes, while coronatine, a func-

tional JA analog, induces NAC TFs that suppress the

ICS1 gene promoter activity, thereby reducing SA lev-

els [61]. Thus, activation of JA-dependent responses

to necrotrophic pathogens can suppress PTI and ETI

pathways, which require SA signaling for full activa-

tion.

REGULATION OF TRANSCRIPTION

Signaling cascades initiated by PTI and ETI lead

to extensive reprogramming of the plant transcrip-

tional landscape, thereby activating defense mech-

anisms. This reprogramming is coordinated in the

nucleus through multiple regulatory mechanisms,

including the activity of diverse TFs, chromatin re-

modeling complexes, and post-translational modifica-

tions of proteins [37, 62]. Notably, due to the close

interconnection between PTI and ETI pathways, most

transcriptional changes described for these responses

largely overlap [63].

Key TF families involved in plant immune re-

sponses include WRKY, AP2/ERF, NAC, bZIP, bHLH,

and CAMTA (calmodulin-binding transcription acti-

vator) [37, 62, 64]. The activity of these TFs is often

regulated via post-translational modifications me-

diated by upstream immune signaling components.

For example, WRKY33, WRKY28, CBP60g, CAMTA3,

MYB51, and ORA59 (octadecanoid-responsive arabi-

dopsis  59) are directly phosphorylated by both CPKs

activated upon Ca

influx and MAPK cascades [37,

62]. Studies of WRKY33 demonstrate that different

signaling pathways modulate its activity through dis-

tinct mechanisms: phosphorylation by CPKs enhances

its DNA-binding activity, whereas phosphorylation via

MAPK pathway increases its transactivation activity.

Furthermore, SUMOylation of WRKY33 stabilizes its

interaction with MAPKs, forming a sustained positive

feedback loop [62, 65]. ORA59, a central regulator of

the JA/ET signaling, alters its DNA-binding preferenc-

es depending on the phosphorylation status, thereby

switching between activation of different groups of

genes. In contrast, SA-dependent ubiquitination tar-

gets ORA59 for proteasomal degradation  [62]. Thus,

post-translational modifications enable flexible recon-

figuration of transcriptional networks in response to

changes in hormonal environment.

Activation of TFs following ligand recognition

can occur not only through canonical signaling cas-

cades but also via direct interaction of TFs with ETI

receptors  [37]. For instance, the barley CNL receptor

MLA10 modulates the activity of WRKY and MYB TFs,

thereby directly controlling defense gene expression

in the nucleus. Similar mechanisms have been de-

scribed for the A.  thaliana nuclear TNL receptor pair

RPS4/RRS1, which forms a complex with WRKY TFs

and functions as a nuclear regulator of gene expres-

sion [37]. Another well-characterized TNL receptor

encoded by the N resistance gene in some Nicotiana

species interacts with SPL6 (squamosa promoter-bind-

ing protein-like  6) in the presence of the tobacco

mosaic virus effector p50  [66]. The TNL receptor

SNC1 (suppressor of NPR1-1, constitutive  1) has been

shown to oligomerize in the nucleus, with its nuclear

localization being essential for activation of its de-

fense function [67]. Subsequent studies demonstrated

that SNC1 interacts with nuclear co-repressors of the

TOPLESS family and acts as an amplifier of immune

signaling. This function depends on both its oligom-

erization and NADase activity, suggesting that SNC1

may function in the nucleus in a resistosome-like

form (Fig.  1) [68].

Upon immune response, activated signaling com-

ponents can induce post-translational modifications

not only in TFs but also in RNA polymerase  II (PolII).

For example, MPK3/6, activated during PTI and ETI,

phosphorylate CDKC1 and CDKC2 (cyclin-dependant

kinase complex), which in turn phosphorylate the

C-terminal domain (CTD) of PolII, enhancing its tran-

scriptional activity. Conversely, the CTD phosphatase

PHOSPHATASE-LIKE  3 (CPL3) can dephosphorylate

the CTD, thereby acting as a negative regulator of

transcription [62, 69].

The structure of chromatin plays a critical role

in the regulation of transcription. Analyses using

ATAC-seq, DNA-seq, and MNase-seq have shown that

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BIOCHEMISTRY (MOSCOW) Vol. 91 No. 4 2026

activation of immune responses leads to remodeling

of nucleosome architecture, thereby increasing acces-

sibility of key regulatory DNA regions to TFs. Certain

chromatin-remodeling ATPases, such as PKR2 and

RAD54, promote immunity, whereas others, such as

EDA16 and SWP73A, suppress it, indicating a com-

plex relationship between chromatin remodeling and

immune responses [62]. Phosphorylated WRKY33, in

cooperation with the SWR1 chromatin remodeling

complex, promotes accumulation of histone modifi-

cations such as H3K4me3 and facilitates replacement

of H2A with H2A.Z in H2A-H2B dimers. These chang-

es support sustained expression of genes involved in

phytoalexin-mediated defense responses [70]. Tran-

scriptional regulation can also occur through non-

canonical functions of ARGONAUTE1 (AGO1). AGO1

associates with chromatin at specific genomic loci

via interactions with specific small RNAs and the

SWI/SNF (SWItch/sucrose non-fermentable) chromatin

remodeling complex, particularly its subunits SWI3B

and SWI3D, thereby promoting recruitment of PolII.

These AGO1 functions may be regulated by phytohor-

mones as well as flg22-induced PTI [71].

DNA methylation patterns are also altered during

immune responses, and it is also known that chang-

es in methylation status of specific genomic regions

are critical for proper immune response. For exam-

ple, DNA demethylases reduce methylation levels at

the regulatory regions of flg22-induced defense genes,

facilitating TF binding [72]. Transcription of PTI-in-

duced genes can also be regulated by long noncoding

RNAs (lncRNAs), such as ELENA1, whose production

is induced by PAMPs including flg22 and elf18 (EF-Tu

N-terminal peptide). ELENA1 enhances the expression

of SA-responsive defense genes, including PR1 (patho-

genesis-related  1) [73, 74].

Nuclear import and export of transcriptional ma-

chinery components represent a selective regulatory

mechanism controlling the plant immune transcrip-

tome. For instance, the nuclear pore complex com-

ponent CPR5 (constitutive expressor of PR-5) plays a

key role in regulating ETI-induced PCD. ETI signaling

induces conformational changes in CPR5 that alter

nuclear pore permeability, enabling nuclear import of

defense-related proteins such as NPR1 and ABI5, there-

by promoting transcriptional reprogramming [62].

Notably, NPR1 is considered a key transducer of ROS

signaling into transcriptional reprogramming. Under

normal conditions, NPR1 forms cytoplasmic oligo-

mers stabilized by disulfide bonds. These bonds are

disrupted upon immune activation and subsequent

decrease in redox potential, leading to the formation

of NPR1 monomers [62,  75]. According to the classi-

cal model, NPR1 monomers translocate to the nucleus

and function as transcriptional coactivators together

with TGA TFs [75]. However, more recent studies

suggest that NPR1 functions as a dimer, interacting

with two TGA3 dimers to form an enhanceosome in

the nucleus [76]. The above-mentioned NPC protein

CPR5 also performs additional regulatory functions

during ETI. ETI signaling can disrupt CPR5 interac-

tion with cyclin-dependent kinase inhibitors SIAMESE

(SIM) and SIAMESE-RELATED1 (SMR1), leading to

the activation of the E2F TF, promoting ETI-associ-

ated PCD [62]. Moreover, CPR5 exhibits RNA-binding

activity and can associate with various transcripts,

including that of AGO1, resulting in alternative

splicing [77].

Immune responses can also lead to phosphory-

lation of splicing factors, which regulate defensive

genes. For example, activation of MPK4 induces al-

ternative splicing of pre-mRNAs encoding WRKY TFs,

CPK kinases, and splicing regulators themselves [62].

During ETI, alternative splicing serves as an import-

ant mechanism for regulating NLR expression, there-

by preventing autoimmune responses. In the absence

of pathogen effectors, alternatively spliced isoforms

of TNL mRNAs are degraded via the nonsense-medi-

ated mRNA decay (NMD) pathway. However, in the

presence of effectors, NMD is suppressed, allow-

ing expression of fully functional versions of TNL

genes [78]. Alternative splicing also regulates phyto-

hormone signaling. For example, the functioning of

JAZ genes, key regulators of JA signaling, is controlled

by splicing factors PRP39a and PRP40, which are re-

cruited by the mediator subunit MED25 [79].

PLANT RESISTANCE

The final stage of immune responses is the es-

tablishment of resistance to pathogens, which can be

either local or systemic.

Local resistance: hypersensitive response and

extreme resistance. HR represents a form of local

resistance characterized by rapid cell death, which

leads to the simultaneous elimination of the invad-

ing pathogen [15, 80, 81]. HR develops as a result of

ETI; simultaneous co-activation of PTI significantly

enhances the strength of HR; in some cases, HR de-

velopment is impossible in the absence of PTI com-

ponents [30, 32]. HR is preceded by a series of early

events, including an increase in ROS and nitric oxide

(NO) levels, as well as changes in cytosolic Ca

con-

centration. HR is also typically accompanied by lipid

peroxidation, cell wall reinforcement, and transcrip-

tional reprogramming [15].

Weak Ca

, MAPK, and ROS signals generated

during PTI are typically insufficient to trigger

HR-induced PCD; rather, their amplification via ETI

is required, leading to more intense and prolonged

signals that induce PCD. One of the key players

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BIOCHEMISTRY (MOSCOW) Vol. 91 No. 4 2026

in this process is resistosome, which induces a ro-

bust Ca

 influx. Enhanced MAPK signaling and the

associated ROS production are also critical for PCD in-

duction. ROS accumulation promotes SA biosynthesis,

which in turn further enhances ROS production via

two mechanisms: inhibition of the mitochondrial elec-

tron transport chain and reduction in the antioxidant

enzyme activity. These processes establish a self-am-

plifying loop of SA and ROS accumulation, ultimately

leading to cell death. However, SA can also act as a

negative regulator of HR. This effect is thought to be

related to its role in SAR, where tight control of lo-

cal PCD is required for proper systemic response [15,

82, 83]. SA is not the only phytohormone involved in

HR regulation. For example, ET can influence ETI-as-

sociated PCD. ET signaling components such as ETR1

and EIN2 have been shown to accelerate PCD pro-

gression, contributing to the expansion of ETI-induced

necrotic lesions [84].

Thus, the HR is a type of immune response in

which localized cell death serves as a key mechanism

for limiting pathogen spread. This form of response

illustrates a plant strategy in which the death of

some cells is a price paid for maintaining the integri-

ty of the organism and activating systemic resistance,

highlighting the importance of the balance between

localized cell death and systemic defense.

Another form of local resistance, described in the

context of viral infection, is extreme resistance (ER),

in which infected cells do not undergo cell death or

phenotypic changes, while pathogen replication and

spread are blocked at very early stages[85]. The mo-

lecular mechanisms underlying ER remain poorly un-

derstood. Like HR, ER is initiated by NLR receptors,

and depending on virus load, the same NLR protein

may trigger either HR or ER. Notably, both HR and

ER can induce SAR, indicating shared features be-

tween these immune responses [86]. At this point, it

remains unclear whether ER and HR are manifesta-

tions of a single type of resistance or fundamentally

different types of resistance that are induced sequen-

tially. In the case of potato virus  X (PVX) infection,

the key gene controlling ER in Solanum stoloniferum

is Rx1 (resistance to potato virus  X  1). Recent studies

indicate that the protein encoded by the Rx1 locus

mediates transcript-specific translational repression

of viral RNA encoding the capsid protein, without

affecting global cellular translation. In addition, nu-

cleocytoplasmic transport of Rx1 is required for ER

establishment [86, 87]. Thus, ER represents a unique

form of local plant immunity characterized by as-

ymptomatic yet highly effective suppression of viral

infection.

Systemic resistance: SAR and ISR. Biotic stress

not only induces the local primary response described

above, but also initiates the development of systemic

resistance, which is typically divided into systemic

acquired resistance (SAR) and induced systemic re-

sistance (ISR) [88, 89].

SAR develops as a consequence of a localized

defense response, initially triggered by both PTI and

ETI, and involves resistance to a broad spectrum of

pathogens, which is observed in other, uninfected

parts of the plant distant from the site of infection

[90, 91]. SAR typically persists for 2-3 weeks but may

last longer and, in some cases, can be transmitted to

subsequent generations, a phenomenon referred to as

transgenerational resistance[89]. SAR is characterized

by elevated expression of defense-related genes, in-

cluding those that encode PR proteins[90]. For SAR to

be established, it is evident that the development of a

local defense response must lead to the generation of

a signal or signals that can be transported through-

out the plant systemically and activate the expression

of specific genes in distant parts of the plant [92].

In recent decades, significant efforts have been di-

rected toward identifying the components necessary

for generating such a signal and determining the na-

ture of the signal itself.

One of the central regulators of SAR is SA. Partial

or complete suppression of SA accumulation leads to

attenuation or loss of SAR[90]. Early studies suggest-

ed that SA itself does not exhibit systemic mobility

but plays a key role in inducing the SAR response,

being synthesized de novo in healthy leaves, to which

a mobile SAR signal is transmitted from pathogen-in-

fected parts of the plant [93]. However, more recent

findings indicate that SA may also be systemically

transported, although its mobility alone is insufficient

to induce SAR. In this model, SA likely acts coopera-

tively with other mobile signals[94, 95]. Nowadays it

is known that one of the key mobile signals is methyl

salicylate (MeSA), a derivative of SA [96]. In plants,

SA and MeSA exist in a state of dynamic equilibrium.

When SA levels rise in response to infection due to

the activation of PTI and ETI, MeSA is synthesized.

Conversely, after MeSA is transported systemically,

it is hydrolyzed, leading to the accumulation of SA

(Fig.  3). These processes are mediated by enzymes

such as SA-methyltransferase (SAMT), SA-binding

protein  2 (SABP2), and methylesterase (MES) [97, 88].

The requirement of SAMT and MES for SAR has been

demonstrated in potato, Arabidopsis, and tobacco

plants [88, 98]. Thus, systemic transport of MeSA, its

conversion back to SA in distal tissues, and possibly

the transport ofSA itself and other SAR signals collec-

tively lead to activation of SA-dependent gene expres-

sion in uninfected tissues, including genes encoding

defense proteins. Notably, due to its volatile nature,

MeSA can also mediate plant-to-plant communica-

tion, inducing pathogen resistance in neighboring

plants [88]. Another class of volatile compounds

SKULACHEV et al.552

BIOCHEMISTRY (MOSCOW) Vol. 91 No. 4 2026

Fig.  3. Two major signaling pathways leading to SAR. Both pathways are activated by PTI and ETI. In the SA/MeSA-de-

pendent pathway (left), pathogen infection of plant tissues triggers PTI/ETI, leading to the local accumulation of SA and

subsequent formation of MeSA, which acts as a mobile signal that is transported to the upper leaves and induces SAR.

The Pip/NHP-dependent pathway (right) involves the production of Pip upon PTI/ETI activation and its subsequent conver-

sion into NHP, which functions as a mobile SAR signal. In upper leaves, MeSA is converted back to SA, whereas NHP activates

expression of genes involved in SA biosynthesis and perception. Thus, signaling through both MeSA and NHP pathways

leads to SA accumulation in distal tissues, ultimately triggering SAR. The NHP signal amplification pathway, involving AzA,

G3P, NO and ROS, which may also act as systemic SAR signals, is not shown.

that have a similar function in plant-to-plant com-

munication are monoterpenes, such as pinenes,

whose biosynthesis is also induced during PTI and

ETI [99, 100].

An important component of SAR activation is

the non-proteinogenic amino acid Pip (pipecolic

acid) [101]. During infection, Pip levels increase sig-

nificantly in pathogen-infected leaves, and elevated

Pip levels are also observed in systemic tissues. Fur-

thermore, exogenous application of Pip results in SAR

induction [102]. Pip functions through its biological-

ly active derivative, N-hydroxypipecolic acid (NHP),

which is synthesized from Pip by the enzyme FMO1

(flavin-dependent monooxygenase  1). In plants carry-

ing mutations in the FMO1 gene, SAR development

is strongly suppressed [101]. Accumulating evidence

indicates that NHP is the key signaling molecule in

the Pip-dependent SAR activation pathway. NHP is ca-

pable of systemic transport throughout the plant, and

its exogenous application induces SAR, complements

mutations in Pip biosynthesis genes, and activates

expression of both these genes and FMO1 gene [91].

Thus, activation of Pip biosynthesis at infection sites

leads to local accumulation of NHP and its sys-

temic transport to distal tissues, where NHP pro-

motes further Pip synthesis and its conversion into

NHP (Fig.  3).

In addition to NHP, several other molecules

have been identified as mobile SAR signals, includ-

ing azelaic acid (AzA), glycerol-3-phosphate (G3P),

NO, and ROS, the synthesis of which is activated by

NHP [83, 101]. These components not only mediate

long-distance signal transmission but also contribute

to signal amplification, which occurs both at infec-

tion sites and in systemic leaves during SAR develop-

ment [91].

Thus, two closely interconnected major signal-

ing pathways mediate SAR: the SA-dependent path-

way and the Pip/NHP-dependent pathway [92]. NHP

activates transcription of a number of genes in the

SA biosynthesis pathway, including ICS1, EDS5, and

CBP60g, as well as genes encoding SA receptors such

as NPR1[101]. Thereby, NHP enhances SA signaling in

two ways, leading to the activation of defense gene

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BIOCHEMISTRY (MOSCOW) Vol. 91 No. 4 2026

expression and establishment of SAR in distant parts

of the plant. While SA-mediated signaling represents

the primary mechanism of Pip action, a minor SA-in-

dependent branch of Pip/NHP signaling that also con-

tributes to SAR has been described [103].

In contrast to SAR, which is induced by patho-

gens, ISR is triggered by non-pathogenic, plant

growth-promoting rhizobacteria and fungi. Neverthe-

less, ISR, like SAR, confers resistance to a broad spec-

trum of pathogens. For example, in cucumber plants,

growth-promoting rhizobacteria have been shown

to induce systemic resistance against bacterial, fun-

gal, and viral pathogens, as well as nematodes [104].

ISR is not characterized by a full activation of de-

fense responses, as occurs in SAR, but rather by a

physiological state known as priming, in which plants

develop a faster and stronger response to subsequent

pathogen infection than unprimed plants [105]. Cur-

rent evidence suggests that ISR initiation resembles

that of SAR and involves recognition of microbial

molecular patterns by plant receptors. However, in

the case of SAR, this recognition leads to a full-scale

immune response, whereas ISR results in a moder-

ate activation. For instance, flagellin from the plant

growth-promoting bacteria Burkholderia phytofirmans

activates ISR in grapevine through the FLS2 receptor

[106], whose activation in other contexts induces PTI

and SAR. Notably, the immune response triggered by

B.  phytofirmans flagellin is weaker and less sustained

than that induced by pathogenic flagellin[106]. Thus,

activation of functionally identical receptors can lead

to SAR during plant interactions with pathogens and

to ISR in the case of interactions with bacteria and

fungi that are beneficial to plant growth. Several

mechanisms may underline this difference. Structur-

al variations in microbial patterns may alter receptor

activation, as demonstrated for B.  phytofirmans flg22,

which contains amino acid substitutions compared to

flg22 of pathogens and induces weaker immune re-

sponses [106]. Additionally, it is known that low-mo-

lecular-weight compounds produced by pathogenic

bacteria can inhibit PTI [107], and a similar mecha-

nism is suggested to operate in beneficial fungi and

bacteria as well [104]. Furthermore, ISR activation

and the onset of priming state involve signaling via

the JA/ET pathways [108, 109], which may antago-

nize SA signaling required for full activation of PTI

(see above). Together, these observations suggest that

beneficial microbes employ multiple strategies to lim-

it the strength of plant immune responses, thereby

facilitating mutually beneficial interactions with host

plants.

In summary, systemic resistance in plants results

from both local and systemic defense responses in-

volving complex interactions between innate immu-

nity pathways and long-distance signaling networks.

CONCLUSIONS

Recent studies demonstrate that ETI operates

through components of PTI and amplifies PTI signal-

ing, while PTI, in turn, synergistically enhances ETI

signaling. As a result, the crosstalk between PTI and

ETI leads to mutual potentiation of these pathways,

ensuring effective plant innate immunity and the es-

tablishment of resistance to a wide range of patho-

gens.

Although the mutual dependence of these two

pathways has long been recognized, its extent re-

mains unclear. It appears that ETI is more dependent

on PTI than vice  versa. However, PTI alone is gen-

erally insufficient to confer robust resistance, which

requires signal amplification by ETI. While ETI sig-

naling is ultimately responsible for establishing re-

sistance, it is PTI that can be considered as the main

mechanism of defense against pathogens. To suppress

PTI, pathogens deploy effector proteins, which in turn

trigger ETI. Activation of ETI leads to increased accu-

mulation of key PTI components and enhanced sig-

naling, thereby counteracting effector-mediated sup-

pression of PTI by pathogens. Within this model, ETI

can be viewed as a PTI signal amplifier necessary for

the development of effective resistance. Further in-

vestigation of the interdependence between PTI and

ETI, as well as the molecular mechanisms underlying

their interactions, will undoubtedly be a main focus

of research in the field of plant innate immunity in

the coming years.

In addition, further research is needed to better

understand the mechanisms underlying the activation

of PTI and ETI receptors, as well as other signaling

components that mediate signal transduction and in-

teractions required for effective functioning of plant

innate immunity. A deeper understanding of the rela-

tionship between PTI and ETI will not only contribute

to our knowledge of plant immunity in general, but

may also facilitate the development of new strategies

in crop genetics and breeding to enhance resistance

to pathogens.

Abbreviations

ADR1 activated disease resistance  1

AGO argonaute

AzA azelaic acid

BAK1 brassinosteroid intensitive  1 (BRI1)-as-

sociated kinase

BIK1 botrytis-induced kinase  1

BKK1 BAK-like  1

CAMTA calmodulin-binding transcription acti-

vator

CBP60g calmodulin-binding protein  60G

CDKC cyclin-dependent kinase complex

CNGC cyclic nucleotide-gated ion channel

SKULACHEV et al.554

BIOCHEMISTRY (MOSCOW) Vol. 91 No. 4 2026

CNL coiled-coil-NLR

CPK calcium-dependent protein kinase

CPR5 constitutive expressor of PR-5

CTD C-terminal domain

DAMP damage-associated molecular pattern

ECD extracellular domain

EDS1 enhanced disease susceptibility  1

ER extreme resistance

ET ethylene

ETI effector-triggered immunity

FLS2 flagellin-sensitive  2

FMO1 flavin-dependent monooxygenase  1

G3P glycerol 3-phosphate

GLR glutamate receptor-like channel

HR hypersensitive response

ICS1 isochorismate synthase  1

ISR induced systemic resistance

JA jasmonic acid

JAZ jasmonate ZIM-domain

lncRNA long non-coding RNA

LRR leucine-rich repeat

LysM lysin motif

MAMP microbe-associated molecular pattern

MAPK (MPK) mitogen-activated protein kinase

MeSA methyl salycilate

NBS nucleotide binding site

NHP N-hydroxypipecolic acid

NIK1 NSP-interacting kinase

NLR NBS-LRR

NMD nonsense-mediated mRNA decay

NPR nonexpressor of PR-genes

NRC NB-LRR protein required for HR-associ-

ated cell death

NRG1 N requirement gene  1

NSP nuclear shuttle protein

ORA59 octadecanoid-responsive arabidopsis  59

OSCA hyperosmolality-gated calcium-perme-

able channel

PAD4 phytoalexin-deficient  4

PAMP pathogen-associated molecular pattern

PBL1 PBS1-like  1

PBS avrPphB susceptible

PCD programmed cell death

PES pre-PTI-mediated ETI suppression

Pip pipecolic acid

PR pathogenesis-related

PRR pattern-recognition receptor

PTI pattern-triggered immunity

PVX potato virus  X

RBOH respiratory burst oxygen homologue

RLCK receptor-like cytoplasmic kinase

RLK receptor-like kinase

RLP receptor-like protein

RNL RPW8-NBS-LRR

ROS reactive oxygen species

RPL10 ribosomal protein  L10

RPS resistance to Pseudomonas syringae

RPW8 resistance to powdery mildew  8

RRS resistance to Ralstonia solanacearum

Rx1 resistance to Potato virus  X  1

SA salicylic acid

SAG101 senescence-associated gene  101

SAR systemic acquired resistance

SARD1 systemic acquired resistance defi-

cient  1

SERK somatic embryogenesis receptor kinase

SNC suppressor of NPR1-1 constitutive  1

SOBIR1 suppressor of  BIR1

SPL6 squamosa promoter binding pro-

tein-like  6

SWI/SNF SWItch/Sucrose non-fermentable

TF transcription factor

TGA TGACG-binding

TIR toll-interleukin

TNL TIR-NLR

VAMP virus-associated molecular pattern

WAK wall-associated kinase

Contributions

All authors participated in writing and editing the

manuscript.

Funding

This study was conducted under state assign-

ment of Lomonosov Moscow State University

(AAAA-A19-119042590057-9).

Ethics approval and consent to participate

This work does not contain any studies involving hu-

man and animal subjects.

Conflict of interest

The authors declare that they have no conflicts of

interest.

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