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 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
Aloni, R., Tollier, M. T. and Monties, B. 1990 The Role of Auxin and Gibberellin in Controlling Lignin Formation in Primary Phloem Fibers and in Xylem of Coleus blumei Stems. Plant Physiol. 94(4): 1743 – 1747
Carna, M., Repka, V., Skupa, P. and Sturdik, E. 2014 Auxins in Defense Strategies. Biologia 69(10): 1255 – 1263
Cohen, J. D. and Bandurski, R. S. 1978 The Bound Auxins: Protection of Indole-3-acetic Acid from Peroxidase-catalyzed Oxidation. Planta 139 (3): 203 – 208
Ding, X., Cao, Y., Huang, L., Zhao, J., Xu, C., Li, X. and Wang, S. 2008 Activation of the Indole-3-acetic Acid-Amido Synthetase GH3-8 Suppresses Expansin Expression and Promotes Salicylate- and Jasmonate-Independent Basal Immunity in Rice. Plant Cell 20(1): 228 – 240
Duca, D., Lorv, J., Patten, C. L., Rose, D. and Glick, B. R. 2014 Indole-3-Acetic Acid in Plant-Microbe Interactions. Antonie Van Leeuwenhoek 106(1): 85- 125
Fu, J. and Wang, S. 2011 Insights into Auxin Signaling in Plant-Pathogen Interactions. Front Plant Sci. 2: 74
Gruen, H. E. 1959 Auxins and Fungi. Annu. Rev. Plant Physiol. 10: 405 – 440
Han, X. and Kahmann, R. 2019 Manipulation of Phytohormone Pathways by Effectors of Filamentous Plant Pathogens. Front Plant Sci. 10: 822
Jones, J. F. and Kende, H. 1979 Auxin-Induced Ethylene Biosynthesis in Subapical Stem Sections of Etiolated Seedlings of Pisum sativum L. Planta 46(5): 649 – 656
Kazan, K. and Manners, J. M. 2009 Linking Development to Defense: Auxin in Plant-Pathogen Interactions. Trends Plant Sci. 14(7): 373-382
Kazan, K. 2013 Auxin and the Integration of Environmental Signals into Plant Root Development. Ann. Bot. 112(9): 1655- 1665
Kunkel, B. N. and Harper, C. P. 2018 The Roles of Auxin during Interactions between Bacterial Plant Pathogens and their Hosts. J. Exp. Bot. 69(2): 245 – 254
Lev-Yadun, S. and Gould, K. S. 2008 Role of Anthocyanins in Plant defence. In: “Anthocyanin” Winefield, C., Davies, K. and Gould, K. (eds). Springer, New York, NY. Pages 22- 28
Ludwig-Muller, J. 2011 Auxin Conjugates: Their Role for Plant Development and in Evolution of Land Plants. J. Exp. Bot. 62(6): 1757 – 1773
Maes, L. and Goossens, A. 2010 Hormone-Mediated Promotion of Trichome Initiation in Plants is Conserved but Utilizes Species- and Trichome-Specific Regulatory Mechanisms. Plant Signal Behav. 5(2): 205 – 207
Pattanaik, S., Patra, B., Singh, S. K. and Yuan, L. 2014 An Overview of the Gene Regulatory Network Controlling Trichome Development in the Model Plant Arabidopsis. Front. Plant Sci. 5: 259
Perez-Alonso, M. M. and Pollmann, S. 2018 How Auxin may Contribute to the Regulation of Plant Defense Responses against Herbivory. Austin J. Plant Biol. 4(1): 02- 05
Pieterse, C. M. J., Van der Does, D., Zamioudis, C., Leon-Reyes, A. and Van Wees, S. C. M. 2012 Hormonal Modulation of Plant Immunity. Annu. Rev. Cell Dev. Biol. 28: 489 – 521
Qi, L., Yan, J., Li, Y., Jiang, H., Sun, J., Chen, Q., Li, H., Chu, J., Yan, C., Sun, X., Yu, Y., Li, C. and Li, C. 2012 Arabidopsis thaliana Plants Differentially Modulate Auxin Biosynthesis and Transport during Defense Responses to the Necrotrophic Pathogen Alternaria brassicola. The New Phytologist 195(4): 872 – 882
Robert-Seilaniantz, A., Grant, M. and Jones, J. D. 2011 Hormone Crosstalk in Plant Disease and Defense: More than just Jasmonate-Salicylate Antagonism. Annu. Rev. Phytopathol. 49: 317 – 343
Spaepen, S., Vanderleyden, J. and Remans, R. 2007 Indole-3-Acetic Acid in Microbial and Microorganism-Plant Signaling. FEMS Microbiol. Reviews 31(4): 425 – 448
Spaepen, S. and Vanderleyden, J. 2011 Auxin and Plant-Microbe Interactions. Cold Spring Harb. Perspect Biol. 3(4): a001438
Thomashow, M. F., Hugly, S., Buchholz, W. G. and Thomashow, L. S. 1986 Molecular Basis for the Auxin-Independent Phenotype of Crown Gall Tumor Tissues. Science 231(4738): 616 – 618
Vanstraelen, M. and Benkova, E. 2012 Hormonal Interaction in the Regulation of Plant Development. Annu. Rev. Cell Dev. Biol. 28: 463 – 487
Yin, C., Park, J. J., Gang, D. R. and Hulbert, S. H. 2014 Characterization of a Tryptophan 2-Monooxygenase Gene from Puccinia graminis f. sp. tritici Involved in Auxin Biosynthesis and Rust Pathogenecity. Mol. Plant Microbe Interact. 27(3): 227 – 235