Phytohormones are small molecules produced within plants that govern diverse physiological processes, including plant defense. Hormonal interactions collectively form hormone signaling networks, which mediate immunity as well as growth and abiotic stress responses.

2. • Phytohormones are small molecules produced within plants that
govern diverse physiological processes, including plant defense,
jasmonate ( JA) and salicylic acid (SA) are major defense-related
phytohormones.
• Other phytohormones, such as Ethylene (ET), Abscisic Acid (ABA),
Auxin, Gibberellins (GAs), Cytokinins (CKs), and Brassinosteroids
(BRs), are also involved in defense responses.
• Hormonal interactions collectively form hormone signaling
networks, which mediate immunity as well as growth and abiotic
stress responses.
• The importance of hormone signaling networks in defense is that
many pathogens interfere with hormone signaling or produce
hormones that increase virulence.
• Signaling pathways mediated by these phytohormones intimately
interact antagonistically or synergistically.

3. Salicylic acid (SA) signaling triggers resistance against biotrophic and
hemibiotrophic pathogens, whereas a combination of Jasmonic acid ( JA) and
Ethylene (ET) signaling activates resistance against necrotrophic pathogens.
These two pathways are mostly antagonistic: elevated biotroph resistance is often
correlated with increased necrotroph susceptibility, and elevated necrotroph
resistance is often correlated with enhanced susceptibility to biotrophs.

4. synergistic and antagonistic interactions between plant stress resistance
hormone signaling pathways

5. .
• Salicylic acid (SA, 2-hydroxy benzoic acid) is one of a
diverse group of phenolic compounds, consisting of an
aromatic ring bearing a hydroxyl group or its functional
derivative, which is synthesized by plants.
• The first suggestion that SA functions as an endogenous
signal for plant disease resistance was made by White and
coworkers ( Antonio and White, 1980), who demonstrated
that injecting SA into the leaves of tobacco plants induced
PR protein accumulation and enhanced resistance to
infection by Tobacco Mosaic Virus (TMV).
• SA treatment has since been shown to induce PR gene
expression and/or enhance resistance in many plant species,
and increased levels of endogenous SA correlate with the
activation of local and/or systemic defense responses.
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6. • Plants synthesize SA via two pathways: the phenylalanine
ammonium lyase (PAL) pathway which operates in the cytosol and
the isochorismate (IC) pathway operates in the chloroplast, both of
which utilize chorismate, the end product of the shikimate pathway,
(Shikimic acid) as a precursor.
• Mutation or silencing of isochorismate synthase 1 (ICS1), which
encodes a key enzyme of the IC pathway in A. thaliana, tomato,
tobacco, and soybean, leads to the loss of pathogen-triggered SA
production.
• Silencing of PAL genes also results in the loss of SA induction upon
pathogen infection in soybean.
• Thus, in dicots, the IC pathway seems to be the major route for SA
biosynthesis in immunity along with PAL pathway as a contributing
pathway.

7. In response to hormonal defense by plants, pathogens evolve several
techniques to be able to cause diease. Eg:
1. The effector Cmu1 from the fungal pathogen Ustilago maydis degrades
chorismate via the chorismate mutase activity, thereby reducing host SA
production and promoting pathogen virulence.
2. The fungal pathogen Verticillium dahliae secretes an isochorismatase,
Vdlsc1, that hydrolyzes IC, thereby reducing SA biosynthesis and
promoting virulence.

8. • Jasmonates are lipid-derived molecules originating
from plastid membrane α-linolenic acid and are
involved in a range of processes from development
to light responses and biotic/abiotic stress signaling.
• Although JA is central to modulating defense against
necrotrophic pathogens, it is increasingly implicated
in other aspects of plant-pathogen interactions,
including SAR.
• In addition to necrotrophic pathogens, JA is
associated with herbivore defense in multiple
angiosperms, such as A. thaliana, Maize, Poplar,
Picea sitchensis, Nicotiana attenuata, and Medicago
truncatula.
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9. • The JA signaling pathway comprises two separate branches:
1. the ERF branch, which in Arabidopsis is regulated by ERF-type
transcription factors such as ERF1 and ORA59 and
2. the MYC branch, which in Arabidopsis is regulated by MYC-type
transcription factors such as MYC2, 3, and 4
• Exogenous JA application induces broad transcriptional reprogramming, a
significant proportion of which is attributable to AtMYC2, a basic helix loop
helix transcription factor and a key regulator of JA responses in plant-microbe
interactions.

11. • Auxins are compounds with aromatic ring and a carboxylic acid group is
involved in almost every aspect of plant growth and development.
• In angiosperms, the tryptophan aminotransferase of Arabidopsis (TAA)
and YUCCA flavin monooxygenase (YUC) pathway is the major auxin
biosynthesis pathway in which Trp is the precursor.
• TIR1 (transport inhibitor response 1) is an F-box protein, and TIR1-related
proteins (AFB1–5) is a well characteized auxin receptors. F-box proteins
target other proteins for degradation via the ubiquitin degradation
pathway.
• AUX-IAA proteins are a group of transcription factors and are negative
regulators of auxin responsive genes. AUX-IAA proteins bind and
inactivate auxin response factors (ARFs), which are usually positive
regulators of auxin signaling. In Arabidopsis, there are 23 ARFs and 29
AUX-IAA proteins. These two protein families homo- and heterodimerize,
resulting in a potentially enormous combination of interactions capable of
fine-tuning specific responses within the auxin signaling pathway.

12. • Auxin acts as molecular glue, promoting association between the
negative regulator (AUX-IAA) and the auxin F-box proteins,
activating the E3 ligase, and leading to AUX-IAA degradation
through ubiquitin-mediated proteasomal degradation. This relieves
repression on ARF proteins and promotes transcription from auxin-
responsive promoters.
• The activation of auxin signaling is often associated with disease
susceptibility. For instance, application of an auxin such as indole-3-
acetic acid (IAA) increases susceptibility against hemibiotrophic
pathogens in rice.
• In A. thaliana, auxin signaling interacts with other phytohormone
signaling pathways, including SA, JA, and ET. For example, auxin
suppresses immunity against the bacterial pathogen Pseudomonas
syringae via SA suppression, whereas SA application stabilizes auxin
resistant 2 (AXR2), a repressor of auxin-mediated transcription,
thereby suppressing auxin signaling.

13. • Consistent with the modulator roles of auxin in plant immunity, many
plant-associated bacteria and fungi (include both mutualistic and
pathogenic microbes) produce auxin or manipulate host auxin
accumulation.
• SA and auxin signaling pathways interact, for the most part,
antagonistically; elevated auxin signaling correlates with increased
susceptibility to biotrophic pathogens.
• Auxin signaling appears to affect SA biosynthesis, through modifying the
stability of negative regulators of response pathway.

14. • GAs are a group of diterpenoid growth hormones
strongly associated with promoting growth,
including stem elongation and germination
• GA modulates plant disease resistance by inducing
the degradation of DELLAs, a class of nuclear
growth repressing proteins that act as central
suppressors of GA signaling.
• The bioactive GAs signal by binding to
intracellular receptors from the GID protein family,
which then complex with DELLA transcription
factors and an E3 ubiquitin ligase. The E3 ubiquitin
ligase polyubiquitinates the DELLA proteins,
tagging them for degradation.
• DELLA proteins can act as repressers of
transcription; thus, the loss of the DELLA
transcription factors in the presence of GA
derepresses gene expression inducing GA response

15. • Negative interaction between GA and DELLA has resulted in
resistance response to biotrophic fungus and susceptibility to
necrotrophic ones under high GA levels.
• Eg: During soybean- Meloidogyne javanica incompatible interaction a
crosstalk between gibberellins and auxins, as well as the participation
of DELLA-like proteins, are crucial to modulate the ROS's levels in
controlling plant immune and stress responses.
• JAZ1 protein, a key repressor of JA signaling, inter-acts in vivo with
DELLA proteins, repressors of the GA pathway. DELLAs prevent
inhibitory JAZ1 interaction with a key transcriptional activator of JA
responses, MYC2, and, thus, enhance the ability of MYC2 to regulate
its target genes. Conversely, GA triggers degradation of DELLAs,
which allows JAZ1to bind MYC2 and suppress MYC2-dependent JA-
signaling outputs.

16. • CKs are phytohormones derived from adenine and involved in the regulation of root
and shoot growth and leaf longevity. They are perceived by membrane-bound histidine
kinase proteins.
• The CK binding pocket is formed by the cyclase and histidine kinase– associated
sensing extracellular (CHASE) domain. In Arabidopsis, three CHASE domain CK
receptors have been characterized—AHK2, AHK3 and AHK4/CRE1/WOL1—with
largely overlapping functions despite differing in their number of transmembrane
domain.
• Upon CK perception, the receptors are autophosphorylated and then transfer the
phosphorylation via a phosphor-relay to Arabidopsis histidine phosphotransfer proteins
(AHPs). Phosphorylation of cytosolic AHPs relocalizes them to the nucleus where they
phosphorylate and activate the response regulators (ARR).
• There are two types of ARRs: Type-A and Type-B. Type-As are negative regulators of
the CK signaling pathway and are induced by CK treatment, indicating negative
feedback of CK on its own signaling pathway. Type-B ARRs are positive regulators of
CK signaling. Upon phosphorylation, Type-B ARRs bind to DNA and activate gene
expression.

17. The role of plant-borne cytokinins in plant immunity
R-protein, UNI, induces cytokinin biosynthesis to
accumulate salicylic acid, which enhances SA-
dependent PATHOGENESIS-RELATED 1 (PR1)
expression. Two R proteins, RPS2 and RPM1, also
activate the transcription of cytokinin- and SA-
responsive genes, ARABIDOPSIS RESPONSE
REGULATOR 5 (ARR5) and PR1, possibly by
inducing cytokinin biosynthesis. Cytokinin
perception through AHK2 and AHK3 cytokinin
receptors initiates transfer of a phosphoryl group
to type-B ARR transcription factors, including
ARR2, which results in the activation of type-A
ARRs. Upon pathogen perception, SA-activated
TGA3 interacts with and recruits ARR2 to
the PR1 promoter to endure hyper-activation
of PR1 by cytokinins. ARR2 overexpressing plants
show enhanced expression of genes involved in
SA signaling, including positive and negative
feedback regulatory modules, whether dependent
or independent of TGA3. Cytokinins also are able
to accumulate NO, which plays an important role
in the hypersensitive response (HR) and closure of
stomata triggered by the perception of bacterial
flagellin through the pattern recognition receptor,
FLS2.
In A. thaliana, CK enhances SA responses, thereby positively contributing to resistance against biotrophic
pathogens.

18. • Abscisic acid (ABA) is a plant hormone. ABA
functions in many plant developmental processes,
including seed and bud domancy, the control of organ
size and stomatal closure. It is especially important
for plants in the response to environmental stress,
including drought, soil salinity, cold
tolerance, freezing tolerance , heat stress and heavy
metal ion tolerance.
• Plants synthesize their ABA via their plastids (an
organelle that does not exist in fungi or animals) as a
compound derived from large (40 carbons) carotenoid
precursor molecules generated via the plastidial 2-C-
methyl-D-erythritol 4-phosphate (MEP) pathway.
• ABA also plays an important role in MAMP-induced
stomatal closure, which are a major route for invasion
into plant tissues by many foliar microbial pathogens.
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19. Genes for the core ABA signaling
pathway, consisting of pyrabactin
resistance 1/PYR1-like regulatory
component of ABA receptor
(PYR/PYL/RCAR), clade A
phosphatase 2Cs (PP2Cs), and Snf1-
related kinases 2 (SnRK2s) which
positively regulate various stress
adaptive responses.
ABA binding to PYLs causes a conformational change that generates a new protein-
protein interaction interface enabling ABA-bound PYLs to bind to and inhibit the active
site of PP2Cs. This alleviates negative regulation on PP2C target SnRK2s, leading to
activation of ABA signaling.
Many pathosystems demonstrate a negative effect from ABA on plant pathogen resistance,
such as the - Botrytis cinerea—tomato
Ralstonia solanacearum—tobacco
Plectosphaerella cucumerina—Arabidopsis
and Magnaporthe oryzae—barley.

20. • ET plays diverse physiological roles in plant growth and development and in
stress responses.
• ET-induced defense-related effector molecules are the so-called pathogenesis
related (PR) proteins. Currently, 17 PR classes have been identified, of which
the majority have been shown to exert direct antimicrobial activity against
fungal species and occasionally against bacterial species.
• Rather than being the principal regulator, ET often modulates defense signaling
pathways, including those regulated by jasmonic acid and salicylic acid.
• Early ET signaling events during these biotic interactions involve activities of
mitogen-activated protein kinases and ETHYLENE RESPONSE FACTOR
(ERF) transcription factors.
• ET contributes positively and negatively to immunity depending on the
pathogen, environmental conditions, and plant species. For instance, in
soybean, ET insensitivity increases severity of disease caused by the
necrotrophic fungus Rhizoctonia solani, whereas enhanced ACS2 expression
promotes resistance against R. solani in Rice.
• ET also contributes to resistance against biotrophic and hemi-biotrophic
pathogens in A. thaliana, soybean, tobacco, and rice.

21. • BRs are are a class of polyhydroxysteroids that
promote plant growth.
• Brassinolide was the first isolated
brassinosteroid in 1979, when pollen
from Brassica napus was shown to promote
stem elongation and cell divisions.
• BRs have been reported to counteract both
abiotic and biotic stress in plants
• Brassinolides (BL) are perceived by the BRI1
(brassinosteroid insensitive 1) leucine-rich
repeat receptor–like kinase (BRI1-associated
recepor kinase 1 or BAK1) located in the
plasma membrane.
Brassinolide

22. Brassinosteroids (BRs) modulate plant interactions with all three types of trophic
pathogens.
In tobacco, pretreatment of plants with brassinolide (BL), the most active BR, gave
rise to increased resistance to the biotrophic bacterial pathogen Pseudomonas
syringae pv. tabaci (Pst) and the biotrophic fungus Oidium sp. (powdery mildew)
In rice plants, BR is able to enhance resistance to the fungal pathogen Magnaporthe
grisea and the bacterial pathogen Xanthomonas oryzae pv. Oryzae.
BR can induce resistance to the viral pathogen tobacco mosaic virus (TMV) in
tobacco. BL treatment enhanced the N-gene-mediated resistance in response to
necrotic-type infection with TMV, resulting in smaller size of lesions and restricted
spread of the virus in the infection site

23. • BRs negatively interact with JA in the regulation of growth processes in
Arabidopsis.
• BRs can also cross-communicate with auxins. As auxins are well known
modulators of plant immunity, either directly or via crosstalk with the SA/JA
signaling network, bidirectional BR-auxin interplay may also contribute to the
ambivalent effects of BRs in disease and resistance.
• BRs also interact with GA. In the rice–P. graminicola interaction, BRs were
dampen effective immune responses by interfering at multiple levels with GA
metabolism Operating at both the level `of biosynthesis regulation and signal
transduction with BR suppressing GA biosynthesis and transcriptionally activating
GA repressor genes.

24. Simplified schematic representation of plant defense signaling networks involving the
hormones ET, SA, JA, and ABA. Necrotrophic pathogen and beneficial microbes induce or
prime ET-and JA-dependent signaling pathways, whereas chewing insects induce JA-and ABA-
signaling pathways. The ET and ABA-regulated branches of the JA pathway are mutually
antagonistic. ET alone or together with JA plays a role in volatile signaling

30. The existence and conservation of hormone cross talk are assumed to bring
fitness advantages to plants simultaneously exposed to multiple stresses.
Eg: A. thaliana plants attacked by herbivores and pathogens of different
lifestyles, experience hormone cross talk, as measured by changes in hormone-
regulated gene expression. The absence of any observed fitness reduction (i.e.,
growth alterations) under these conditions is a consequence of hormone cross
talk, allowing plants to activate specific rather than general defense responses,
thus conserving resources that can be used for growth.
Auxin and CKs are two classic growth
hormones, acting mostly
antagonistically to each other. During
plant immunity, auxin functions mostly
by increasing susceptibility to
pathogens, whereas high levels of CKs
have the opposite effect and enhance
pathogen resistance.
Growth and defense trade-offs mediated
by hormone cross talk also enable precise
defense activation and regulation in certain
tissues or at certain developmental stages.
For example, barley Mlo mutants show
increased callose deposition and cell death
phenotypes in older plants, which are
absent in young seedlings

31. Hormone cross talk participates in defense and growth trade-offs. In rice and
A. thaliana, upregulation of GA signaling by mutation in phytochrome B
(PHYB) impairs JA, leading to increased plant growth and herbivore
susceptibility. In contrast, constitutively activating JA responses increases
defense against herbivores but reduces plant growth through repression of
GA signaling.
In the case of abiotic stress- ABA lowers plant immunity through cross talk
with immune-related hormonal pathways. Such cross talk likely evolved to
modulate the activation of immune responses during adverse abiotic conditions
Eg: conditions of drought signal a lower probability of pathogen attack, as
increased humidity is necessary for sporulation and spore germination in most
biotrophic and necrotrophic fungi and oomycetes and is essential for bacterial
survival and spread. Thus, the mostly negative cross talk of ABA on SA- and
JA-regulated defense responses is likely a response to lower defense activation
when a pathogen attack is not imminent. In addition, because activated
immunity lowers abiotic stress responses, negative ABA effects on JA and SA
signaling can enhance abiotic stress responses, which may be necessary to
increase survival of certain plant species in severe abiotic stress conditions

32. A potential role for hormone cross talk linking defense activation and
growth suppression is control of plant speciation.
Eg: During hybrid necrosis, the F1 progeny derived from a cross
between certain incompatible species/genotypes display severely
stunted growth accompanied by high levels of SA-mediated immunity.
A similar phenotype is also seen during hybrid breakdown, which is
commonly expressed in the F2 progeny. Such phenotypes are
predicted by the Bateson Dobzhansky-Muller model of
incompatibility and are believed to operate as a mechanism of
postzygotic incompatibility that contributes to the maintenance of
gene barriers among species.

33. Spatial Regulation -Plant defense responses are
often the strongest around the site of infection
but taper off with increasing distance in systemic
tissues.
Temporal Regulation- the time between
invasion by the primary and secondary
aggressors may determine whether a trade-off
occurs.
Pathogen-Type Effects- The specificity of a
plant-pathogen interaction may also affect trade-
offs
CASE STUDY

34. 1. GA, ABA, IAA, BL, and CK have recently emerged as important modulators
of plant defenses against microbes.
2. The effect of each hormone on the defense response depends on the pathogen
lifestyle.
3. The induction of the different hormone signaling pathways is predominantly
mediated through inducible ubiquitination of negative regulators followed by
their destruction in the proteasome.
4. The signaling pathways of the different hormones rely on modules composed of
negative and positive regulatory components.
5. The positive and negative regulatory components of hormone pathways are
potential targets to modify hormonal crosstalk during disease and defense.
6. Plants are known to rapidly respond to pathogen and herbivore attack by
reconfiguring their metabolism to reduce pathogen/herbivore food acquisition.
This involves the production of defensive plant secondary compounds, but also
an alteration of the plant primary metabolism to fuel the energetic requirements
of the direct defence.

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