Action of proteolysis-resistant systemin analogues in wound signalling.

In cultured cells of Lycopersicon peruvianum, the oligopeptide systemin which mediates systemic signalling in the tomato wound response is rapidly inactivated by proteolytic cleavage of the bond carboxy-terminal to Lys14. A systemin derivative in which this peptide bond had been modified by N-methylation was resistant to proteolytic inactivation. Systemin elicits a rapid, transient alkalinization of the growth medium in L. peruvianum cells. Consistent with its metabolic stability, the response elicited by the N-methylated peptide was found to be more sustained than that caused by systemin. In differentiated tomato plants, the stabilized peptide was found to be 3 times more active than systemin with respect to the induction of proteinase inhibitors I and II. This result indicates the possible physiological significance of the observed proteolytic degradation for systemin inactivation in planta. The activity of a protease capable of processing systemin carboxy-terminal of Lys14 was detected in tomato plasma membranes and may be responsible for the inactivation process. Two further peptides, N-methylated at the bonds carboxy-terminal of Gln3 and Arg10 had proteinase inhibitor inducing activities lower by a factor of 8 and 80, respectively, as compared to systemin. Correspondingly, the alkalinization response elicited by these two peptides in cultured cells was found to be more transient than the systemin response. The correlation between the duration of the alkalinization response and the proteinase inhibitor inducing activities of systemin analogues may be indicative of a casual relationship between ion fluxes and defense gene induction.