A tomato peroxidase involved in the synthesis of lignin and suberin

The last step in the synthesis of lignin and suberin has been proposed to b e catalyzed by peroxidases, although other proteins may also be involved. T o determine which peroxidases are involved in the synthesis of lignin and s uberin, five peroxidases from tomato (Lycopersicon esculentum) roots, repre senting the majority of the peroxidase activity in this organ, have been pa rtially purified and characterized kinetically. The purified peroxidases wi th isoelectric point (pI) values of 3.6 and 9.6 showed the highest catalyti c efficiency when the substrate used was syringaldazine, an analog of ligni n monomer. Using a combination of transgenic expression and antibody recogn ition, we now show that the peroxidase pi 9.6 is probably encoded by TPX1, a tomato peroxidase gene we have previously isolated. In situ RNA hybridiza tion revealed that TPX1 expression is restricted to cells undergoing synthe sis of lignin and suberin. Salt stress has been reported to induce the synt hesis of lignin and/or suberin. This stress applied to tomato caused change s in the expression pattern of TPX1 and induced the TPX1 protein. We propos e that the TPX1 product is involved in the synthesis of lignin and suberin.