Effect Of L-Glutamate On Inhibiting Postharvest Diseases By Inducing Resistance In Fruit And The Possible Defense Mechanisms Involved

Diseases caused by fungal pathogens resulted in significant economic losses of fruit in the field and the postharvest phase.Currently,the application of hazardous fungicides is the simplest and most effective approach for disease management.In fact,increasing plant natural resistance by physical,biological and chemical elicitors has emerged as a viable alternative strategy for disease control to reduce the use of chemical fungicides.L-glutamate is a multi-functional amino acid which occupies a central position in amino acid metabolism and nitrogen assimilation.In addition to its structural role in proteins,glutamate function as a precursor for many defense compounds.While multitude of studies have investigated the involvement of glutamate in plant abiotic stress,scarce information is available on the role of glutamate in pathogen attack.Possibly,the application of glutamate may hold great potential for environmentally friendly,economically sound and sustainable management of fungal pathogens.This study aimed to gain further knowledge on the effect of exogenous glutamate as a commercial value and promising elicitor on postharvest pathogens in fruit.A multidisciplinary approach was pursued,combining transcriptomics,proteomics and metabolomics.The main findings are as follows:(1)Exogenous application of L-glutamate could provide effective protection for fruit against fungal pathogens,possibly by inducing resistance.Glutamate was not only concentration effective but also time effective.Preharvest application of glutamate broaden the spectrum of use of glutamate.(2)Exogenous application of y-aminobutyric acid(GABA)induced resistance against the necrotrophic pathogen A.alternata at least in part by activating GABA shunt which could restrict the levels of cell death caused by reactive oxygen species and replenish the tricarboxylic acid(TCA)cycle.RT-qPCR analyses showed that GABA shunt was quickly activated by glutamate,which is a precursor for GABA.GABA shunt might play a vital role in the resistance mechanisms underpinning glutamate-induced plant immunity.(3)A large variety of defense-related enzymes and genes involved in the glutamine synthetase/glutamate synthase cycle,energy-generated metabolism,such as the glycolysis and the TCA cycle were activated by glutamate in tomato fruit.It appeared that the activation of these pathways as mentioned above was strongly associated with resistance mechanisms.(4)RT-qPCR analyses revealed that application of exogenous glutamate resulted in the activation of expression of the SA-responsive genes PAL,NPR1,TGA1,TGA2,WRKY70 and PR,while the expression of ethylene signaling pathway genes ACS,ACO,ETR3,ETR4 as well as jasmonic acid signaling pathway genes LOX1,AOS2,AOC,MYC2 and PI-? were down-regulated.The glutamate-mediated resistance mechanisms partly depends on the salicylic acid signaling pathway,while ethylene and jasmonic acid biosynthesis and signaling pathways were suppressed by glutamate.(5)iTRAQ analyses showed that 97 proteins were up-regulated and 42 proteins were down-regulated between the control and glutamate-treated tomato.These proteins were involved in plant-pathogen interaction,phenylpropanoid biosynthesis,oxidative phosphorylation,other glycan degradation,fatty acid metabolism.(6)GC-MS analyses showed the effect of glutamate on the contents of metabolites in tomato fruit.The result further suggested that energy metabolisms,amino acid metabolisms and plant hormone metabolisms might contribute to the glutamate-mediated resistance mechanisms.(7)The transcriptomics analyses showed that 623 genes were up-regulated and 647 genes were down-regulated between the control and glutamate-treated citrus,while 234 genes were up-regulated and 193 genes were down-regulated between the P.digitatum treatment and glutamate + P.digitatum treatment.These genes were related to carbon metabolism,amino acid metabolism,plant hormone metabolism and secondary metabolism.In summary,the work presented here indicated that exogenous application of L-glutamate could provide effective protection for fruit against postharvest pathogens,possibly by inducing resistance.It appeared that the GABA shunt,carbon and nitrogen metabolism and plant hormone metabolism were strongly associated with resistance mechanism underpinning glutamate-induced plant immunity.