Monocarboxylate permease gene SsMP1 regulates Sporisorium scitamineum mating, pathogenicity and absorption and transport of citric acid

Abstract

Sugarcane is an important sugar crop, and smut disease caused by Sporisorium scitamineum poses a severe threat with limited control measures. This study is based on transcriptome sequencing data from two distinct pathogenic strains of Sporisorium scitamineum generated in our laboratory. Among the significantly differentially expressed genes, we identified a gene predicted to encode a monocarboxylate permease, which we designated SsMP1. To investigate its biological function, SsMP1 knockout and complemented mutants were generated using PEG-mediated protoplast transformation. Our results showed that SsMP1 had little to no effect on sporidial morphology, colony morphology, growth rate, or tolerance to abiotic stresses. In the knockout mutant, SsMP1 expression was undetectable, whereas it was readily detected in both the wild-type and complemented strains, Moreover, the expression level in the complemented mutant was restored to that of the wild-type strain. Notably, sexual mating ability was almost completely abolished in the knockout mutant but was fully restored in the complemented mutant. Interestingly, supplementation with exogenous signaling molecules, including cAMP or tryptophol, largely rescued the mating defect of the knockout mutant. Consistently, the expression levels of Uac1, a gene involved in cAMP biosynthesis, and Aro8, a gene associated with tryptophol biosynthesis, were significantly lower in the knockout mutant than in the wild-type and complemented strains. In addition, the knockout mutant exhibited more than a 70% reduction in pathogenicity compared with the wild-type and complemented strains. Furthermore, citric acid transport and uptake were markedly impaired in the knockout mutant. Taken together, these findings suggest that SsMP1 may indirectly regulate the expression of key genes involved in the cAMP and tryptophol biosynthetic pathways, thereby modulating signaling molecule production and subsequently affecting mating and pathogenicity. In addition, SsMP1 appears to positively regulate citric acid uptake and transport in haploid sporidia. This study provides new molecular insights into the pathogenic mechanisms of S. scitamineum.