GABA keeps nitric oxide in balance by regulating GSNOR to enhance disease resistance of harvested tomato against Botrytis cinerea

Gamma-aminobutyric acid (GABA), a widely distributed metabolite in prokaryotes and eukaryotes, has many functions for plants in stress responses. In this study, hypotonic treatment with 10 mmol L-1 GABA in cherry tomato induced resistance to Botrytis cinerea with markedly lower disease incidence and lesion diameter, led to endogenous nitric oxide (NO) tansient accumulation before inoculation the pathogen then decrease after inoculation, and enhanced the content of arginine (Arg) and glutamic acid (Glu). The resistance of fruit treated with a NO scavenger, carboxy-2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO), was significantly reduced. Moreover, the enzyme activity and gene expression of S-nitrosoglutathione reductase (GSNOR) were enhanced following endogenous NO increased. The endogenous NO level was excessively high after treatment with a GSNOR scavenger, N6022, making the fruit more susceptible to pathogen. Similarly, after break down of SlGSNOR, fruit had much higher endogenous NO and lower disease resistance. However, overexpression of SlGSNOR exhibited opposite consequences. These results suggest that a suitable level of NO is beneficial for enhancing disease resistance, and GABA can help tomatoes maintain NO equilibrium by regulating GSNOR.