Defense responses are conditioned by nutritional and signaling status of the plant. Pathogen overcome physical and biochemical defences and neutralize inducible defense responses to obtain plant nutrient. Plants require energy for defense against pathogens and this requirement is fulfilled by primary metabolite activities. The pathogen invasion bring changes in primary metabolism of the affected part of the plant (Berger et al., 2007). Upon exposure to pathogen plant induce many genes associated with primary metabolic pathway regulating synthesis or degradation of carbohydrate, amino acid and lipid (Rojas et al., 2014). Ward et al. (2010) observed a significant changes in different classes of metabolites indicating metabolite reprogramming during infection, this lead to change in the level of sugars, purines and amino acids.
Plant defense response require energy supply primarily derived from primary metabolic pathway (Bolton 2009). The Verticillium dahiliae infected plant cells would exhibit perturbations in some of the metabolites associated with primary metabolism caused by the induction of plant defence mechanisms and movement of nutrients from host to the pathogen (Buhtz et al., 2015). The induction of various pathogenesis related (PR) genes in Arabidopsis in early stage of Verticillium infectiondepends on ethylene and jasmonic acid (JA) associated signals (Johansson et al., 2006). SA-independent and sugar-dependent pathway for PR-protein gene induction may exist in plant cells (Herbers et al., 1996). Carbohydrate play a vital role in regulating defences responses. Buhtz et al. (2015) observed an accumulation of glucose-6-phosphate and higher level of glycerol-3-phosphate (G-3-P) in infected tomato roots. Their result suggest G-3-P could be involved in systemic acquired resistance (SAR) against Verticillium infection. Sugars enhance plant resistance. It may increase oxidative burst at early stage of infection, stimulate lignification of cell wall and induce PR proteins (Morkunas and Ratajczak 2014). Rate of respiration increases during resistance suggesting cellular metabolism increase to provide energy for the response (Smedegaard-Petersen and Tolstrup 1985). In response to invasion by the pathogen a demand for carbon will move amino acid into energy generating pathway such as tri carboxylic acid cycle pathway (Bolton 2009). Plants may actively mobilize nitrogen source away from infection sites to deprive pathogens of nutrients.
Amino acid metabolic pathway constitute integral part of the immune system. The catabolism of lysine produces the immune signal pipecolic acid (Pip) a cyclic, non-protein amino acid. Pip amplifies plant defense responses and act as a regulator of systemic acquired resistance (SAR), defense priming and local resistance to bacterial pathogens (Zeier 2013). On infection, pathogens require wide range of N source including NH4+ and NO3– and amino acid (Sun et al., 2020). Proline metabolism is involved in oxidative burst and hypersensitive response associated with avirulent pathogen recognition (Zeier 2013). The acylation of amino acids can control plant resistance to pathogens and pests by the formation of protective plant metabolites or by modulation of plant hormone activity. Amino acid conjugates of plant hormones such as indole-3-acetic acid (IAA), JA or salicylic acid (SA) are examples of acylated amino acids (Zeier 2013). Threonine is an amino acid that can provide resistance to Hyaloperonospora arabidopsidis (Stuttmann et al., 2011). Amino acid and their byproduct trigger plant resistance response against pathogens (Zaynab et al., 2019). Wang et al. (2019) studied, different forms of nitrogen (ammonium vs nitrate) regulate cucumber response to Fusarium oxysporum cucumerinum (FOC). They observed nitrate-grown plants accumulated more organic acids while ammonium-grown plants accumulated more amino acids. The altered levels of organic acids and amino acids resulted in different tolerance ability to FOC infection.
Malate is involved in various metabolic pathway and enzyme NADP-malic (NADP-ME) can metabolize it. This enzyme is implicated in defense-related deposition of lignin by providing nicotinamide adenine diphosphate hydrogen (NADPH) for the two NADPH-dependent reductive steps in monolignol biosynthesis (Casati et al., 1999). NADP-ME is involved in production of NADPH for synthesis of activated oxygen species that are produced in order to kill or damage pathogens. Enzyme NADP-ME can provide building blocks and energy for biosynthesis of defense compounds indicating role of malate metabolism in plant defense (Casati et al., 1999).
Lipids provide structural component for the cell wall and cell membrane as well as also provides energy for various metabolic processes and function as signal transduction mediators. Lipid –associated plant defense responses are due to activation of lipases (lipid hydrolysing proteins). Lipases are expressed and activated in plant cell upon pathogen attack (Lee and Park 2019). SA a small phenolic compound synthesized via the shikimate pathway and JA is derived from the fatty acid α-linolenic acid (Lim et al., 2017). Fatty acid (FA) contribute to the generation of antimicrobial oxylipins (FA breakdown product) and biosynthesis of defense hormone JA. Some oxylipins exhibit specific signaling role in plant defense (Lim et al., 2017). FA and lipid regulate SAR.
References:
Berger, S., Benediktyova, Z., Matous, K., Bonfig, K., Mueller, M. J., Nedbal, L. and Roitsch, T. 2007 Visualization of Dynamics of Plant-Pathogen Interaction by Novel Combination of Chlorophyll Fluorescence Imaging and Statistical Analysis: Differential Effects of Virulent and Avirulent Strains of P. syringae and of Oxylipins on A. thaliana. J. Exp. Bot. 58(4): 797 – 806
doi: 10.1039/jxb/er1208
Bolton, M. D. 2009 Primary Metabolism and Plant Defense-Fuel for the Fire. Mol. Plant Microbe Interact. 22(5): 487 – 497
doi: 10.1094/MPMI-22-5-0487
Buhtz, A., Witzel, K., Strehmel, N., Ziegler, J., Abel, S. and Grosch, R. 2015 Perturbations in the Primary Metabolism of Tomato and Arabidopsis thaliana Plants Infected with the Soil-Borne Fungus Verticillium dahiliae. PLoS ONE 10(9): e0138242
doi.org/10.1371/journal.pone.0138242
Casati, P., Drincovich, M. F., Edwards, G. E. and Andreo, C. S. 1999 Malate Metabolism by NADP-malic Enzyme in Plant Defense. Photosynthesis Res. 61(2): 99 – 105
doi.org/10.1023/A:1006209003096
Herbers, K., Meuwly, P., Metraux, J. P. and Sonnewald, U. 1996 Salicylic-acid-Independent Induction of Pathogenesis-Related Protein Transcripts by Sugars is Dependent on Leaf Developmental Stage. FEBS Lett. 397 (2-3): 239 – 244
doi: 10.1016/s0014-5793(96)01183-0
Johansson, A., Staal, J. and Dixelius, C. 2006 Early Responses in the Arabidopsis-Verticillium longisporum Pathosystem are Dependent on NDR1, JA- and ET–Associated Signals via Cytosolic NPR1 and RFO1. Mol. Plant-Microbe Interactions 19(19): 958 – 969
doi: 10.1094/MPMI-19-0958
Lee, H-J. and Park, O. K. 2019 Lipases Associated with Plant Defense against Pathogens. Plant Sci. 279: 51 – 58
doi.org/10.1016/j.plantsci.2018.07.003
Lim, G-H, Singhal, R., Kachroo, A. and Kachroo, P. 2017 Fatty Acid-and Lipid Mediated Signaling Plant Defense. Annu. Rev. Phytopathol. 55(1): 505 -536
doi: 10.1146/annurev-phyto-080516-035406
Morkunas, I. and Ratajczak, L. 2014 The Role of Sugar Signaling in Plant Defense Responses against Fungal Pathogens. Acta Physiologiae Plantarum 36: 1607 – 1619
doi.org/10.1007/s11738-014-1559-z
Rojas, C. M., Senthil-Kumar, M., Tzin, V. and Mysore, K. S. 2014 Regulation of Primary Plant Metabolism during Plant-Pathogen Interactions and its Contribution to Plant Defense. Front Plant Sci. 5:17
doi.org/10.3389/fpls.2014.00017
Smedegaard-Petersen, V. and Tolstrup, K. 1985 The Limiting Effect of Disease Resistance on Yield. Annu. Rev. Phytopathol. 23: 475 – 490
doi.org/10.1146/annurev.py.23.090185.002355
Stuttmann, J., Hubberten, H-M., Rietz, S., Kaur, J., Muskett, P., Guerois, R., Bednarek, P., Hoefgen, R. and Parker, J. E. 2011 Perturbation of Arabidopsis Amino Acid Metabolism Causes Incompatibility with the Adapted Biotrophic Pathogen Hyaloperonospora arabidopsidis [C][W][OA]. Plant Cell 23: 2788 – 2803
doi.org/10.1105/tpc.111.087684
Sun, Y., Wang, M., Mur, L. A. J., Shen, Q. and Guo, S. 2020 Unravelling the Roles of Nitrogen Nutrition in Plant Disease Defences. Int. J. Mol. Sci. 21(2): 572
doi: 10.3390/ijms21020572
Wang, M., Gu, Z., Wang, R., Guo, J., Ling, N., Firbank, L. G. and Guo, S. 2019 Plant Primary Metabolism Regulated by Nitrogen Contributes to Plant-Pathogen Interactions. Plant Cell Physiol. 60(2): 329 – 342
doi: 10.1093/pcp/pcy211
Ward, J. L., Forcat, S., Beckmann, M., Bennett, M., Miller, S. J., Baker, J. M., Hawkins, N. D., Vermeer, C. P., Lu, C., Lin, W., Truman, W. M., Beale, M. H., Draper, J., Mansfield, J. W. and Grant, G. 2010 The Metabolic Transition during Disease Following Infection of Arabidopsis thaliana by Pseudomonas syringae pv. tomato. Plant Journ. 63(3): 443 – 457
doi.org/10.1111/j.1365-313X.2010.04254.x
Zaynab, M., Fatima, M., Sharif, Y., Zafar, M. H., Ali, H. and Khan, K. A. 2019 Role of Primary Metabolites in Plant Defense against Microbial Pathogenesis 137: 103728
doi.org/10.1016/j.micpath.2019.103728
Zeier, J. 2013 New Insights into the Regulation of Plant Immunity by Amino Acid Metabolic Pathways. Plant Cell Environ. 36(12): 2085 – 2103
doi.org/10.1111/pce.12122
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