The systemic production of proteinase inhibitors in young tomato plants is triggered by a complex sequence of events (Figure 23.20):
Wounded tomato leaves synthesize prosystemin, a large (200 amino acids) precursor protein.
Prosystemin is proteolytically processed to produce the short (18 amino acids) polypeptide DAMP called systemin.
Systemin is released from damaged cells into the apoplast.
In adjacent intact tissue (phloem parenchyma), systemin binds to a pattern recognition receptor on the plasma membrane.
The activated systemin receptor becomes phosphorylated and activates a phospholipase A2 (PLA2).
The activated PLA2 generates the signal that initiates JA biosynthesis.
JA is then transported through the phloem to systemic parts of the plant by an unknown mechanism.
JA is taken up by target tissues and activates the expression of genes that encourage proteinase inhibitors.
"Plant Physiology and Development" int'l 6e - Taiz, L., Zeiger, E., Møller, I.M., Murphy, A.
