FORMATION OF DI-ISODITYROSINE AND LOSS OF ISODITYROSINE IN THE CELL-WALLS OF TOMATO CELL-SUSPENSION CULTURES TREATED WITH FUNGAL ELICITORS OR H2O2

About 84% of the hydroxyproline residues in a cell culture of tomato ( Lycopersicon esculentum x Lycopersicon peruvianum) were present in phe nol-inextractable (i.e. covalently wall-bound) material. Treatment of the cells with any of three fungal elicitors (wall fragments from Phyt ophthora megasperma and Pythium aphanidermatum and xylanase from Aureo basidium pullulans) or with 1 mM H2O2 had little effect on the quantit y of phenol-inextractable hydroxyproline per milligram of freeze-dried cells. However, each treatment induced a decrease in the content of p henol-inextractable isodityrosine (Idt) residues. Each treatment, exce pt with the P. megasperma fragments, also induced an increase in pheno l-inextractable di- (Di-Idt). The increase in Di-Idt partly accounted for the loss of Idt. We conclude that the elicitors and H2O2 acted to reinforce the existing cross-linking of cell wall (glyco)proteins by e voking oxidative coupling reactions to convert Idt to Di-Idt plus unid entified products. The promotion of cross-linking by elicitor treatmen t is proposed to be a defensive response that restricts the penetratio n of pathogens.