Concurrent overactivation of the cytosolic glutamine synthetase and the GABA shunt in the ABA‐deficient sitiens mutant of tomato leads to resistance against Botrytis cinerea

Summary Deficiency of abscisic acid (ABA) in the sitiens mutant of tomato ( S olanum lycopersicum ) culminates in increased resistance to B otrytis cinerea through a rapid epidermal hypersensitive response ( HR ) and associated phenylpropanoid pathway‐derived cell wall fortifications. This study focused on understanding the role of primary carbon : nitrogen ( C : N ) metabolism in the resistance response of sitiens to B . cinerea . How alterations in C : N metabolism are linked with the HR ‐mediated epidermal arrest of the pathogen has been also investigated. Temporal alterations in the γ‐aminobutyric acid ( GABA ) shunt, glutamine synthetase/glutamate synthase ( GS / GOGAT ) cycle and phenylpropanoid pathway were transcriptionally, enzymatically and metabolically monitored in both wild‐type and sitiens plants. Virus‐induced gene silencing, microscopic analyses and pharmacological assays were used to further confirm the data. Our results on the sitiens– B . cinerea interaction favor a model in which cell viability in the cells surrounding the invaded tissue is maintained by a constant replenishment of the tricarboxylic acid ( TCA ) cycle through overactivation of the GS / GOGAT cycle and the GABA shunt, resulting in resistance through both tightly controlling the defense‐associated HR and slowing down the pathogen‐induced senescence. Collectively, this study shows that maintaining cell viability via alterations in host C : N metabolism plays a vital role in the resistance response against necrotrophic pathogens.