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.
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