SsMet1 is a critical gene in methionine biosynthesis in Sclerotinia sclerotiorum

Methionine, a key sulfur-containing amino acid, is involved in various important functions in cellular metabolism. Genes that encode enzymes to catalyze steps of the methionine biosynthesis pathway are essential for survival of fungi. The <i>SsMet1</i> (SS1G_11000) gene in <i>Sclerotinia sclerotiorum</i> is an orthologue of <i>BcStr2</i>, a gene characterized in <i>Botrytis cinerea</i> that plays a key role in methionine biosynthesis. In this study, we characterized <i>SsMet1</i> in <i>S. sclerotiorum</i> by creating <i>SsMet1</i>-deletion mutants, <i>Met1-2</i> and <i>Met1-4</i>, using a split marker technique. The <i>SsMet1</i>-deletion mutants were unable to grow on minimal medium and did not produce sclerotia. Supplementation with methionine and homocysteine rescued the defects in mycelial growth, but not sclerotial development of the <i>SsMet1-</i>deletion mutants. These results indicate that <i>SsMet1</i>-deletion mutants are auxotrophic for methionine. In addition, the <i>SsMet1</i>-deletion mutants exhibited increased sensitivity to osmotic and oxidative stresses, cell wall-damaging agents, and thermal stress. The mutants were avirulent on detached bean leaves, but virulence was also restored with methionine supplementation in minimal media. All the defects were restored by genetic complementation of the mutant with wildtype <i>SsMet1</i> allele. The results of this study indicate that <i>SsMet1</i> plays a critical role in the regulation of various cellular processes in <i>S. sclerotiorum</i>.