Abstract
The basidiomycetous fungus Sporisorium scitamineum causes a serious sugarcane smut disease in major sugarcane growing areas. Sexual mating is essential for infection to the host; however, its underlying molecular mechanism has not been fully studied. In this study, we identified a conserved farnesyltransferase (FTase) β subunit Ram1 in S. scitamineum. The ram1Δ mutant displayed significantly reduced mating/filamentation, thus of weak pathogenicity to the host cane. The ram1Δ mutant sporidia showed more tolerant toward cell wall stressor Congo red compared to that of the wild-type. Transcriptional profiling showed that Congo red treatment resulted in notable up-regulation of the core genes involving in cell wall integrity pathway in ram1Δ sporidia compared with that of WT, indicating that Ram1 may be involved in cell wall integrity regulation. In yeast the heterodimeric FTase is responsible for post-translational modification of Ras (small G protein) and a-factor (pheromone). We also identified and characterized two conserved Ras proteins, Ras1 and Ras2, respectively, and a MAT-1 pheromone precursor Mfa1. The ras1Δ, ras2Δ and mfa1Δ mutants all displayed reduced mating/filamentation similar as the ram1Δ mutant. However, both ras1Δ and ras2Δ mutants were hypersensitive to Congo red while the mfa1Δ mutant was the same as wild-type. Overall our study displayed that RAM1 plays an essential role in S. scitamineum mating/filamentation, pathogenicity, and cell wall stability.
Highlights
Sugarcane smut disease caused by the basidiomycetous fungus Sporisorium scitamineum is one of the most severe diseases in world-wide sugarcane growing areas
The predominant function of FTase composed of Ram1 and Ram2 is conserved among eukaryotes (Omer and Gibbs, 1994)
The ram1 mutant is viable, whereas the ram2 mutants could not be generated repeated attempts were made, which is consistent with what has been reported in S. cerevisiae, C. neoformans, and C. albicans that the RAM2 gene is essential while RAM1 is not (Alspaugh et al, 2000; Song and White, 2003; He et al, 2005)
Summary
Sugarcane smut disease caused by the basidiomycetous fungus Sporisorium scitamineum is one of the most severe diseases in world-wide sugarcane growing areas. The three-stage morphological transitions are most prominent during the smut fungus life cycle: the haploid sporidia switch to dikaryotic hyphal growth to infect the plant cane; in the later growth period, diploid teliospores form within the host stem; and after teliospore germination, haploid sporidia are generated. Switching from non-pathogenic yeast-like sporidia to pathogenic hypha through sexual mating between two compatible sporidia is a prerequisite for infection. The a locus encodes pheromone precursor MFA1/2 and receptor PRA1/2, controlling sporidial recognition and fusion during the sexual mating process. In S. scitamineum mfa knock-out mutants in MAT-2 background were mating-deficient (Lu et al, 2017), and b locus deletion mutants were defective in filamentous growth (Yan et al, 2016a), suggesting common molecular characters between S. scitamineum and U. maydis. The molecular mechanisms of S. scitamineum mating/filamentation and/or pathogenicity remain largely unclear
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