ChnagG Plays the Role of 5-Salicylate Hydroxylase in the Gentisic Acid Pathway of Salicylic Acid Metabolism in Cochliobolus heterostrophus.

Salicylic acid (SA) plays a crucial role in the defence strategies of plants against fungal pathogens. To circumvent plant immunity, pathogens use metabolic enzymes such as salicylate hydroxylase to degrade SA, thereby facilitating successful pathogenicity after infection. This phenomenon has not been previously reported in Cochliobolus heterostrophus. Our study demonstrates that high concentrations of SA can inhibit both growth and spore germination; however, at concentrations below 1 mM, SA does not significantly impact the growth and spore germination of C. heterostrophus, which is capable of metabolising exogenously supplied SA. Transcriptome and LC-MS analyses indicated that C. heterostrophus metabolises exogenous SA via the gentisic acid (GA) pathway, involving genes such as 5-salicylate hydroxylase (ChnagG). Prokaryotic expression of ChnagG confirmed its ability to convert SA into GA. Additionally, we created ChnagG gene deletion and complementation mutants, revealing that ChnagG influences melanin synthesis and the pathogenicity of C. heterostrophus. Analysis of the SA signalling pathway in plants during fungal infection indicated that the ChnagG knockout mutant did not alter the synthesis of SA in its host maize; however, it led to the upregulation of the downstream signalling pathway ZmPR1 gene compared to the wild type. These findings suggest that C. heterostrophus obstructs the immune signalling pathway of maize through SA metabolism, thereby enhancing its infection and pathogenicity. This study lays the groundwork for further elucidating the mechanisms underlying the interaction between maize and C. heterostrophus.