REGULATION BY CYTOKININS OF ENDOGENOUS LEVELS OF JASMONIC AND SALICYLIC ACIDS IN MECHANICALLY WOUNDED TOBACCO PLANTS

Plants respond differentially to wounding and pathogens using distinct signaling pathways, so that wound signals are transmitted to jasmonic acid (JA) which induces basic pathogenesis-related (PR) proteins, whe reas pathogenic signals cause, in addition to JA, accumulation of sali cylic acid (SA) which stimulates production of acidic PR proteins. Tra nsgenic tobacco plants expressing a gene for a small GTP binding prote in respond abnormally to mechanical wounding to produce SA and consequ ently acidic PR proteins, suggesting that wound signals cross with pat hogen signaling pathways [Sane et al. (1994) Proc. Natl. Acad. Sci. US A 91: 10556]. This unusual signal crossing is associated with a highly sensitive wound-response of transgenic plants which, upon wounding, p roduce JA at least eighteen hours earlier than wild-type plants. When wildtype plants are wounded in the presence of the synthetic cytokinin , benzylaminopurine, production of JA begins six hours earlier than in untreated samples, and also SA begins to accumulate. The cytokinin an tagonist, -chloro-4-cyclohexylamino-6-ethylamino-s-triazine, erases th ese effects. Because transgenic plants constitutively produce four- to six-fold higher amounts of endogenous cytokinins than wild-type plant s, it is concluded that cytokinins are indispensable for control of en dogenous levels of SA and JA.