Abstract
During pathogenesis on sunflower cotyledons, Botrytis cinerea and Sclerotinia sclerotiorum show a striking resemblance in symptom development. Based on pH change profiles, the colonization process of both fungi can be divided into two stages. The first stage is associated with a pH decrease, resulting from an accumulation of citric and succinic acids. The second stage is correlated with a pH increase, resulting from an accumulation of ammonia. In this article, we also report that oxalic acid is produced at the late stage of the colonization process and that ammonia accumulation is concomitant with a decrease in free amino acids in decaying tissues. Sclerotinia sclerotiorum produces eight-fold more oxalic acid and two-fold less ammonia than B. cinerea. Consequently, during sunflower cotyledon colonization by B. cinerea, pH dynamics differ significantly from those of S. sclerotiorum. In vitro assays support the in planta results and show that decreases in pH are linked to glucose consumption. At different stages of the colonization process, expression profiles of genes encoding secreted proteases were investigated. This analysis highlights that the expression levels of the B. cinerea protease genes are higher than those of S. sclerotiorum. This work suggests that the overt similarities of S. sclerotiorum and B. cinerea symptom development have probably masked our recognition of the dynamic and potentially different metabolic pathways active during host colonization by these two necrotrophic fungi.