The silent information regulator protein (Sir2) and its homologs are NAD

homologs are NAD + -dependent deacetylase enzymes that play important roles in a variety of physiological processes. However, the functions of the Sir2 family in plants are poorly understood. Here, we report that Arabidopsis AtSRT2, a homolog of yeast Sir2, negatively regulates plant basal defense against the pathogen Pseudomonas syringae pv. tomato DC3000 ( Pst DC3000). In response to Pst DC3000 infection, the expression of AtSRT2 was down-regulated in a salicylic acid (SA)-independent manner. In addition, knock-out of AtSRT2 ( srt2 ) enhanced resistance against Pst DC3000 and increased expression of pathogenesis-related gene 1 ( PR1 ). Conversely, overexpression of AtSRT2 resulted in hypersusceptibility to Pst DC3000 and impaired PR1 induction. Consistent with this phenotype, expression of PAD4 , EDS5 and SID2 , three essential genes in the SA biosynthesis pathway, were increased in the srt2 mutant and decreased in AtSRT2 -overexpressing plants. Taken together, these results demonstrate that AtSRT2 is a negative regulator of basal defense, possibly by suppressing SA biosynthesis.