The Coupling Between Cell Wall Integrity Mediated by MAPK Kinases and SsFkh1 Is Involved in Sclerotia Formation and Pathogenicity of Sclerotinia sclerotiorum

The plant pathogenic fungus Sclerotinia sclerotiorum can survive on a wide range of hosts and cause significant losses on crop yields. FKH, a forkhead box (FOX)-containing protein, functions to regulate transcription and signal transduction. As a transcription factor (TF) with multiple biological functions in eukaryotic organisms, little research has been done on the role of FKH protein in pathogenic fungi. SsFkh1 encodes a protein which has been predicted to contain FOX domain in S. sclerotiorum. In this study, the deletion mutant of SsFkh1 resulted in severe defects in hyphal development, virulence, and sclerotia formation. Moreover, knockout of SsFkh1 lead to gene functional enrichment in mitogen-activated protein kinase (MAPK) signaling pathway in transcriptome analysis and SsFkh1 was found to be involved in the maintenance of the cell wall integrity (CWI) and the MAPK signaling pathway. Yeast two-hybrid and bimolecular fluorescence complementation assays showed that SsFkh1 interacts with SsMkk1. In addition, we explored the conserved MAPK signaling pathway components, including Bck1, Mkk1, Pkc1, and Smk3 in S. sclerotiorum. ΔSsmkk1, ΔSspkc1, ΔSsbck1, and ΔSssmk3knockout mutant strains together with ΔSsmkk1^com, ΔSspkc1^com, ΔSsbck1^com, and ΔSssmk3^com complementation mutant strains were obtained. The results indicated that ΔSsmkk1, ΔSspkc1, ΔSsbck1, and ΔSssmk3 displayed similar phenotypes to ΔSsfkh1 in sclerotia formation, compound appressorium development, and pathogenicity. Taken together, SsFkh1 may be the downstream substrate of SsMkk1 and involved in sclerotia formation, compound appressorium development, and pathogenicity in S. sclerotiorum.

An atypical forkhead-containing transcription factor SsFKH1 is involved in sclerotial formation and is essential for pathogenicity in Sclerotinia sclerotiorum

Sclerotinia sclerotiorum (Lib.) de Bary is a necrotrophic plant pathogen with a worldwide distribution. The sclerotia of S. sclerotiorum are pigmented multicellular structures formed from the aggregation of vegetative hyphae. These survival structures play a central role in the life and infection cycles of this pathogen. Here, we characterized an atypical forkhead (FKH)-box-containing protein, SsFKH1, involved in sclerotial development and virulence. To investigate the role of SsFkh1 in S. sclerotiorum, the partial sequence of SsFkh1 was cloned and RNA interference (RNAi)-based gene silencing was employed to alter the expression of SsFkh1. RNA-silenced mutants with significantly reduced SsFkh1 RNA levels exhibited slow hyphal growth and sclerotial developmental defects. In addition, the expression levels of a set of putative melanin biosynthesis-related laccase genes and a polyketide synthase-encoding gene were significantly down-regulated in silenced strains. Disease assays demonstrated that pathogenicity in RNAi-silenced strains was significantly compromised with the development of a smaller infection lesion on tomato leaves. Collectively, the results suggest that SsFkh1 is involved in hyphal growth, virulence and sclerotial formation in S. sclerotiorum.