Abstract
Small secondary metabolites, including glucosinolates and the major phytoalexin camalexin, play important roles in immunity in Arabidopsis thaliana. We isolated an Arabidopsis mutant with increased resistance to the powdery mildew fungus Golovinomyces cichoracearum and identified a mutation in the gene encoding cytochrome P450 83A1 monooxygenase (CYP83A1), which functions in glucosinolate biosynthesis. The cyp83a1-3 mutant exhibited enhanced defense responses to G. cichoracearum and double mutant analysis showed that this enhanced resistance requires NPR1, EDS1, and PAD4, but not SID2 or EDS5. In cyp83a1-3 mutants, the expression of genes related to camalexin synthesis increased upon G. cichoracearum infection. Significantly, the cyp83a1-3 mutant also accumulated higher levels of camalexin. Decreasing camalexin levels by mutation of the camalexin synthetase gene PAD3 or the camalexin synthesis regulator AtWRKY33 compromised the powdery mildew resistance in these mutants. Consistent with these observations, overexpression of PAD3 increased camalexin levels and enhanced resistance to G. cichoracearum. Taken together, our data indicate that accumulation of higher levels of camalexin contributes to increased resistance to powdery mildew.
Highlights
To protect themselves against pathogens, plants have evolved intricate immune responses that include accumulation of reactive oxygen species, deposition of callose, enhanced expression of pathogenesis-related (PR) genes, and biosynthesis of phytoalexins
We show that mutations in genes affecting camalexin production suppress the resistance of cyp83a1-3, indicating that higher accumulation of camalexin in cyp83a1-3 mutants contributes to their enhanced powdery mildew resistance
We used diaminobenzidine hydrochloride (DAB) staining to measure H2O2 accumulation at 2 dpi, but we found that the cyp83a1-3 and wild-type plants had similar levels of H2O2 (Figure 1C), suggesting that an enhanced oxidative burst is not responsible for the resistance phenotype of the cyp83a1-3 mutant
Summary
To protect themselves against pathogens, plants have evolved intricate immune responses that include accumulation of reactive oxygen species, deposition of callose, enhanced expression of pathogenesis-related (PR) genes, and biosynthesis of phytoalexins. The biosynthesis and regulation of camalexin in Arabidopsis remain only partially understood, and the full scope of camalexin functions remains to be defined. The Arabidopsis transcription factors WRKY33, WRKY18, and WRKY40 appear to be involved in the regulation of camalexin biosynthesis (Qiu et al, 2008; Pandey et al, 2010; Mao et al, 2011). Camalexin produces broad-spectrum resistance to many species of plant pathogens, how it functions remains unclear
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