Plant hormones are signal molecule produced within plant in low concentration and can trigger defense response against pathogen. These signaling substances are auxin, gibberellins (GAs), cytokinins (CKs), abscisic acid (ABA), ethylene (ET), salicylic acid (SA), jasmonic acid (JA), brassinosteroid (BR) and strigolactones. Phytohormones can cooperate and cross talk with each other (Vanstraelen and Benkova 2012) as well activate defense response against both biotic and abiotic stress. Pathogen evolve diverse strategies to interfere with phytohormone pathway. They secrete effectors to colonize plant. Pathogen effectors manipulate phytohormone pathway by directly altering hormone levels, interfering with phytohormone biosynthesis, or altering or blocking components of phytohormone signaling pathways (Han and Kahmann 2019).
Plants activate their immune system to counter pathogens or herbivorous insects. Pathogens may suppress plant hormone signaling circuit or evade host immunity to infect. Beneficial root-inhabiting microbes also hijack the hormone-regulated immune signaling network to establish a prolonged mutualistic association, indicating the central role of plant hormones in the regulation of plant growth and survival (Pieterse et al., 2012). Interaction of different hormonal networks modulate plant immunity.
AUXIN:
Auxin promotes plant cell division and expansion. Few studies on plant-pathogen interactions identify auxin in pathogenesis and plant defense (Fu and Wang 2011). Auxin signaling interacts with the signaling pathway of all the other known plant hormone (Kazan 2013).
Like plants, large number of pathogens also produce auxin [indole-3-acetic acid (IAA)]. Different microorganisms such as plant growth promoting rhizobacteria, nitrogen fixing symbionts and pathogens produce IAA (Spaepen et al., 2007; Spaepen and Vanderleyden 2011; Duca et al., 2014; Yin et al., 2014). Indole-3-acetic acid control cell division, differentiation and vascular bundle formation, cell enlargement, and responses to light and gravity. Auxin and salicylic acid (SA) pathways act in a mutually antagonistic manner during plant defense, whereas, auxin and jasmonic acid (JA) signaling share commonlalities. Studies show that some pathogens either produce auxins themselves or increase plant auxin biosynthesis upon infection to manipulate the plant’s defense and developmental machinery (Kazan and Manners, 2009). Fungi too synthesize IAA (Gruen, 1959). In plants IAA is found in a conjugated form but a small amount of free IAA is also present. Indole-3-acetic acid conjugates are involved in transport, storage and protection of IAA from enzymatic degradation (Cohen and Bandurski 1978; Ludwig-Muller 2011). Conjugates can control IAA levels in the cell.
Auxins can negatively impact plant defense by interfering with other hormone signaling pathway or with pathogen -associated molecular patterns triggered immunity (PTI) (Robert-Seilaniantz et al., 2011). Elevated level of IAA or enhanced auxin signaling may promote disease development in some plant-pathogen interaction (Kunkel and Harper 2018). IAA plays a role in disease caused by tumorigenic plant pathogen such as Agrobacterium tumefaciens. Here IAA involved in disease development is not synthesized directly by the pathogen but rather is produced by plant cells that has been genetically transformed by A. tumefaciens T-DNA element (Thomashow et al., 1986). IAA induces expression of expansins protein that loosen the cell wall. Loosening of the cell wall is key for plant growth but it also makes the plant vulnerable to pathogen (Ding et al., 2008).
Auxin inhibits SA responses and thus indirectly promotes JA signaling in immunity. Changes in auxin homeostasis influence SA signaling resulting in resistance to biotrophic pathogens (Carna et al., 2014). Qi et al. (2012) reported JA and auxin interact positively in regulating plant resistance to necrotrophic pathogens and that activation of auxin signaling by JA may contribute to plant resistance to necrotrophic pathogens.
To understand the defense response against phytophagous pathogen there are direct and indirect defense. Direct defense involve all structures such as the spines, thorns and trichomes which are used by host plant to counter specific attack by predators. Trichomes are specialized epidermal cell located on aerial parts of plant. Formation of these structural barriers is controlled by combined action of JA, GA and CK (Maes and Goossens 2010; Pattanaik et al., 2014). Auxin response factor, a key component of auxin signaling are required for this process. Relationship between auxin and jasmonate is of particular relevance for control of plant defense responses (Perez-Alonso and Pollmann 2018). Plants limit phytophagous attack by increasing leaf rigidity and stem strength through lignification of their cell walls. Aloni et al. (1990) studied IAA and GA3 control lignin formation in primary phloem fibres and xylem in the stem of Coleus blumei. More over IAA and methyl jasmonate (MeJA) induce the production of anthocyanin in Nicotiana attenuate (Perez-Alonso and Pollmann 2018) which acts as a chemical repellent (Lev-Yadun and Gould 2008). Indirect defense is based on the ability of attacked plant to emit volatile organic compound and ethylene. IAA is capable of stimulating ethylene production (Jones and Kende 1979). Thus IAA, JA and ET contribute to plant defense responses.
See Part III for further information
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References:
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BIOLOGICAL PROCESSES- A NATURE'S GIFT 