Comparison Of ?-aminobutyric Acid And L-glutamate On Induced Resistance Of Fruit As Well As Their Possible Mechanisms Involved

Fruits are highly susceptible to fungal diseases during the postharvest supply chains.Induction of fruit resistance by elicitors is considered as a safe,efficient,environmentally friendly and sustainable strategy to control postharvest decay of fruit.As a new resource food,?-aminobutyric acid and its direct precursor substance Lglutamate not only play important roles in mammals or plants,but also had been registered as biological pesticide active ingredients by US Environmental Protection Agency(EPA)as early as 1998.But the progress in elucidating their biological role in disease resistance has been disappointingly slow.Therefore,in this work,we used pear fruit as the test material to investigate the effect and possible mechanisms of GABA and L-glutamate on inducing postharvest resistance,and then through the model plant tomato to further study more related mechanisms.The main findings are as follows:1.GABA or L-glutamate could not directly inhibit the postharvest blue mold rot and the spore germinability of Penicillium expansum in pear fruit,but could control the disease by inducing fruit resistance,and its effect was closely related with concentration and induction Time.In addition,1.00 mM GABA or 1.00 mM L-glutamate had no significant difference in the control effect of blue mold rot on pear fruit,whether at room temperature or 4?.In addition,both GABA and L-glutamate had no negative impact on pear fruit quality.2.GABA or L-glutamate treatment could stimulate activity of chitinase(CHI),?-1,3-glucanase(GLU),phenylalanine ammonialyase(PAL),peroxidase(POD)and polyphenol oxidase(PPO)in the wound of pear fruit to varying degrees.In addition,GABA or L-glutamate also could induce the expression of the PR1,GLU,CHI3 and CHI4 genes,indicating that GABA or L-glutamate treatment increased the disease resistance of pear fruit by activating defense-related enzymes and genes expression,and there might be a priming mechanism existing in GABA inducing pear fruit resistance.3.GABA or L-glutamate treatment could increase the content of glutamate,GABA,arginine,methionine,leucine and lysine in the wound of pear fruit,while glutamate,GABA and arginine could effectively control the blue mold rot in pear fruit.In addition,either of the incidence of the pear fruit treated with GABA synthesis or metabolic inhibitor were not significantly different from the control group.It was shown that GABA or L-glutamate treatment could effectively induce postharvest pear fruit resistance might be closely related to induced amino acid accumulation and GABA shunt.4.Both of 1 mM GABA and L-glutamate treatment could significantly induce the resistance against gray mold rot on cherry tomato fruit,and L-glutamate was more effective than GABA to some extent.5.Both of GABA and L-glutamate treatment could induce the expression of PR genes(PR1a,PR2 a,PR3a,PR3 b,PR5a and PR5b)in cherry tomato fruits to different degrees,indicating that GABA or L-glutamate treatment might improve the disease resistance of tomato fruit by activating PR gene expressions.6.GABA and L-glutamate could significantly induce the key genes(ISC1,PAL5,NPR1,WRKY1,TGA1 and TGA2)in the salicylic acid synthesis and signal pathway,indicating that GABA or L-glutamate treatment induced tomato fruit disease resistance might be associated with salicylic acid pathway.7.After induction of GABA or L-glutamate for 24 h,cherry tomato fruit could be significantly induced the accumulation of seven kinds of amino acids(glutamate,aspartic acid,methionine,phenylalanine,lysine,histidine and arginine).Among these seven amino acid,L-glutamate,lysine,methionine,and arginine could significantly inhibit the blue mold rot on pear fruit.In addition,GABA or L-glutamate treatment could significantly induce the expression of GAD1,GAD3 and GABA-TP1,indicating that GABA or L-glutamate treatment might also enhance tomato fruit resistance by inducing amino acid accumulation and activating GABA shunt.8.GABA or L-glutamate treatment could activate SlGLR genes in cherry tomato fruit,especially GLR2.6.When tomato fruit was treated with DNQX(glutamate receptor inhibitor),or DNQX combined with GABA and L-glutamic acid treatment,the inhibition effect of tomato fruit against gray mold rot disappeared.Therefore,we speculate that SlGLRs might be involved in the process of GABA or L-glutamate treatment inducing tomato fruit disease resistance.9.The transcriptome data showed that 121 differentially expressed genes were up-regulated and 81 genes down-regulated in cherry tomato fruits after GABA treatment,while 120 differentially expressed genes were up-regulated and 93 genes were down-regulated after L-glutamate treatment.KEGG analysis indicated that amino acid metabolism,secondary metabolism,plant-pathogen interaction,lipid metabolism and energy metabolism pathway might be involved in GABA or L-glutamate induction to improve tomato fruit disease resistance.