*4.6. Increasing Ascorbic Acid Can Alter Pathogen Defense Responses*

The role of Asc in pathogen defense has received only limited attention. In an early study, the reduced Asc content of *Arabidopsis vtc1* or *vtc2* mutants resulted in reduced growth of the bacterial pathogen *Pseudomonas syringae* pv *maculicola* ES4326 and hyphal growth of the fungal pathogen *Peronospora parasitica* pv *Noco* [100]. The reduction in growth of *P. syringae* in *vtc1* plants correlated with a greater induction of the pathogenesis-related proteins PR-1 and PR-5, increased expression from some senescence-associated gene (SAG) genes and higher levels of salicylic acid. The reduced Asc content in these mutants resulted in the premature senescence of uninfected plants with an accompanying increase in salicylic acid [100]. These observations suggest that reducing Asc content predisposed *Arabidopsis* to induce defense responses faster upon pathogen attack. Whether an increase in Asc content would have had the opposite effect on these pathogens was not examined in this study. Quite different results were observed in a more recent study that also employed the same mutants. In this study, *Arabidopsis vtc1* and *vtc2* were more susceptible to the pathogenic ascomycete *Alternaria brassicicola* and Asc strongly inhibited growth of fungal cultures [101]. Asc levels decreased following *A. brassicicola* infection with an increase in DHA, suggesting that Asc is being consumed during infection [101]. Given the limited number of studies focusing on the

relationship between Asc content and pathogen defense, it is not possible at this point to conclude how increasing Asc content will affect defense responses. These two studies do suggest, however, that changes in Asc content may affect defense responses in a very pathogen-specific manner.
