SALICYLIC-ACID IS NOT THE TRANSLOCATED SIGNAL RESPONSIBLE FOR INDUCING SYSTEMIC ACQUIRED-RESISTANCE BUT IS REQUIRED IN SIGNAL TRANSDUCTION

Infection of plants by necrotizing pathogens can induce broad-spectrum resistance to subsequent pathogen infection. This systemic acquired r esistance (SAR) is thought to be triggered by a vascular-mobile signal that moves throughout the plant from the infected leaves. A considera ble amount of evidence suggests that salicylic acid (SA) is involved i n the induction of SAR. Because SA is found in phloem exudate of infec ted cucumber and tobacco plants, it has been proposed as a candidate f ar the translocated signal. To determine if SA is the mobile signal, g rafting experiments were performed using transgenic plants that expres s a bacterial SA-degrading enzyme. We show that transgenic tobacco roo tstocks, although unable to accumulate SA, were fully capable of deliv ering a signal that renders nontransgenic scions resistant to further pathogen infection. This result indicated that the translocating, SAR- inducing signal is not SA. Reciprocal grafts demonstrated that the sig nal requires the presence of SA in tissues distant from the infection site to induce systemic resistance.