SYSTEMIN ACTIVATES SYNTHESIS OF WOUND-INDUCIBLE TOMATO LEAF POLYPHENOL OXIDASE VIA THE OCTADECANOID DEFENSE SIGNALING PATHWAY

Tomato plants overexpressing a prosystemin gene that encodes the precu rsor of a mobile wound signal called systemin have been shown previous ly to constitutively synthesize extraordinarily high levels of two def ensive proteinase inhibitor proteins in leaves in the absence of wound ing. We herein report that leaves of these transgenic plants possess e nhanced levels of another defensive protein, polyphenol oxidase (PPO) at levels that are up to 70-fold higher than levels found in leaves of wild-type plants. Supplying young wild-type tomato plants with system in through cut stems induced PPO activity in leaves, and wounding lowe r leaves of young tomato plants induced PPO activity in both wounded a nd unwounded leaves to levels equal to those induced by systemin. Expo sing young tomato plants to methyl jasmonate vapor caused an increase in PPO activity equivalent to levels found in plants overexpressing th e prosystemin gene. The data indicate that PPO and proteinase inhibito r genes are coactivated systemically by wounding via the octadecanoid signal transduction pathway and that systemin has a much broader role in signaling plant defensive genes than was previously known.