Identification of Two Feruloyl Esterases in Dickeya dadantii 3937 and Induction of the Major Feruloyl Esterase and of Pectate Lyases by Ferulic Acid

The plant-pathogenic bacteriumDickeya dadantii (formerlyErwinia chrysanthemi ) produces a large array of plant cell wall-degrading enzymes. Using anin situ detection test, we showed that it produces two feruloyl esterases, FaeD and FaeT. These enzymes cleave the ester link between ferulate and the pectic or xylan chains. FaeD and FaeT belong to the carbohydrate esterase family CE10, and they are the first two feruloyl esterases to be identified in this family. Cleavage of synthetic substrates revealed strong activation of FaeD and FaeT by ferulic acid. The genefaeT appeared to be weakly expressed, and its product, FaeT, is a cytoplasmic protein. In contrast, the genefaeD is strongly induced in the presence of ferulic acid, and FaeD is an extracellular protein secreted by the Out system, responsible for pectinase secretion. The product of the adjacent genefaeR is involved in the positive control offaeD in response to ferulic acid. Moreover, ferulic acid acts in synergy with polygalacturonate to induce pectate lyases, the main virulence determinant of soft rot disease. Feruloyl esterases dissociate internal cross-links in the polysaccharide network of the plant cell wall, suppress the polysaccharide esterifications, and liberate ferulic acid, which contributes to the induction of pectate lyases. Together, these effects of feruloyl esterases could facilitate soft rot disease caused by pectinolytic bacteria.