Effects Of Indole-3-acetic Acid On Induced Resistance Of Fruit And Its Relationship With ?-Aminobutyric Acid

Diseases caused by fungal pathogens are one of the main reasons of losses of fruits during the postharvest storage,transportation and selling.Chemical fungicides are the primary method to minimize postharvest decay,but the types and using range are limited due to their adverse effects on the environment and human health.Induction of natural resistance in fruits using physical,chemical,and biological elicitors that lead a long-lasting inhibition to a broad range of disease-causing organisms has received increasing attention in the recent years.Considered to be plant growth regulator generally,auxin is also related to microbial pathogenesis and plant defense.Although the functions of auxin on plant disease and defense are widely explored,only a few studies focus on the effects of auxin alone as a chemical elicitor towards postharvest diseases in fruit and its mechanisms remain unknown.This paper focused on the efficacy of auxin,especially the indole-3-acetic acid(IAA),on induction of disease resistance of fruits and and the possible action mechanisms involved,by analyzing the effects of IAA on pathogen,fruit decay,fruit quality,resistance-related physiological and biochemical indexes and gene expressions of fruit,typical resistance-related hormonal biosynthetic and signaling pathways,?-aminobutyric acid(GABA)shunt and so on.The main findings are as follows:(1)Both endogenous auxins(IAA,IBA)and synthetic auxins(NAA,2,4-D)could induce resistance of pear fruits against Penicillium expansum at the almost same level of GABA.IAA at 100-500 ?g mL-1 significantly enhanced the resistance of pear fruit against blue mold rot when the treating interval was between 24 and 48 h,while it had no direct antifungal activities towards P.expansum in vitro.Besides,IAA induced a significant increase on the activities of superoxide dismutase(SOD),?-1,3-glucanase(GLU),chitinase(CHI)and phenylalanine ammonia-lyase(PAL),and improved the expressions of endoglu9,CHI4,PR1,PR4 and PAL in pear fruit,suggesting that the defence mechanisms may be closely related to the elicitation of defense-related enzymes and genes.Moreover,IAA had no influence on firmness,total soluble solids,titratable acidity and ascorbic acid of pear fruits.(2)IAA induced resistance against Alternaria alternata in tomato fruits,and the effect was similar as GABA's.The determination of resistance-related physiological and biochemical indexes in tomato fruits showed that IAA could significantly stimulate the activities of SOD,catalase(CAT),peroxidase(POD),polypheol oxidase(PPO),and reduce the content of malondialdehyde(MDA),indicating that the physiological mechanisms of induced resistance by IAA were related to the clear of excessive reactive oxygen species(ROS)and the accumulation of quinones.Meanwhile,IAA stimulated the activities of GLU and CHI as well,suggesting that the defence mechanisms may be closely related to the elicitation of defense-related enzymes.(3)IAA had a strong influence on the biosynthetic and signaling pathways of the classical resistance-related homones salicylic acid(SA),jasmonic acid(JA)and ethylene(ET)in tomato fruits.IAA significantly increased the transcriptional level of PAL5 and PAL7,but had no effect on the gene expression of ICS1,indicating that IAA might stimulate the PAL pathway at the transcriptional level.In SA signaling pathway,NPR1,TGA1,TGA2 and WRKY70 were significantly up-regulated after IAA treatment,demostrating that IAA might activate the SA signal signaling pathway.Besides,IAA could enhance the gene expression of AOS2 and MYZ2,but inhibited the gene expression of LOX1 and sharply stimulated the gene expression of the negative regulator JAZ1,revealing that IAA may play a major role in inhibiting JA biosynthetic and signaling pathways at the transcriptional level.Moreover,IAA significantly increased the gene expression levels of ACS1,ETR1,MPK3 and MPK6,indicating that IAA might play a major role in activating ET biosynthetic and signaling pathways at the transcriptional level.(4)IAA had an interaction with GABA.Exogenous IAA treatment increased the content of GABA in tomato fruits,and stimulated the gene expression of key enzymes in GABA shunt:IAA could enhance the gene expression of GAD1,GAD2,GAD3 and GABA-TP1,but inhibited the gene expression of SSADH.This result indicated that IAA-induced defence mechanisms may be closely related to GABA and GABA shunt.In addition,exogenous GABA treatment improved the IAA content in tomato fruit.