Salicylic acid is a systemic signal and an inducer of pathogenesis-related proteins in virus-infected tobacco.

Abstract

Systemic induction of pathogenesis-related (PR) proteins in tobacco, which occurs during the hypersensitive response to tobacco mosaic virus (TMV), may be caused by a minimum 10-fold systemic increase in endogenous levels of salicylic acid (SA). This rise in SA parallels PR-1 protein induction and occurs in TMV-resistant Xanthi-nc tobacco carrying the N gene, but not in TMV-susceptible (nn) tobacco. By feeding SA to excised leaves of Xanthi-nc (NN) tobacco, we have shown that the observed increase in endogenous SA levels is sufficient for the systemic induction of PR-1 proteins. TMV infection became systemic and Xanthi-nc plants failed to accumulate PR-1 proteins at 32 degrees C. This loss of hypersensitive response at high temperature was associated with an inability to accumulate SA. However, spraying leaves with SA induced PR-1 proteins at both 24 and 32 degrees C. SA is most likely exported from the primary site of infection to the uninfected tissues. A computer model predicts that SA should move rapidly in phloem. When leaves of Xanthi-nc tobacco were excised 24 hr after TMV inoculation and exudates from the cut petioles were collected, the increase in endogenous SA in TMV-inoculated leaves paralleled SA levels in exudates. Exudation and leaf accumulation of SA were proportional to TMV concentration and were higher in light than in darkness. Different components of TMV were compared for their ability to induce SA accumulation and exudation: three different aggregation states of coat protein failed to induce SA, but unencapsidated viral RNA elicited SA accumulation in leaves and phloem. These results further support the hypothesis that SA acts as an endogenous signal that triggers local and systemic induction of PR-1 proteins and, possibly, some components of systemic acquired resistance in NN tobacco.

Characterization and Expression of Caffeoyl-Coenzyme A 3-O-Methyltransferase Proposed for the Induced Resistance Response of Vitis vinifera L

“…The communication of tissues in the SAR requires a systemic signaling mechanism, the nature of which has not been elucidated. SA was suggested as an endogenous signal substance in this process (Malamy et al, 1990;Metraux et al, 1990;Rasmussen et al, 1991;Yalpani et al, 1991;Raskin, 1992), and recent reports corroborated the local requirement of SA at the site of the challenge inoculation (Gaffney et al, 1993). However, grafting experiments with transgenic tobacco conceivably ruled out SA as the mobile signal for SAR transmission, and its exact role is still under controversial discussion (Vernooij et al, 1994;Shulaev et al, 1995).…”

Genetic Elucidation of Nitric Oxide Signaling in Incompatible Plant-Pathogen Interactions

“…The observed induction of pal in the Pseudomonas-Arabidopsis pathosystem might then feed alternative processes like lignification or the production of other phenylpropanoid compounds to a significant extent. If SA is the causative agent of PR-1 induction (Yalpani et al, 1991;Uknes et al, 1992), the rather weak attenuation of SA induction might explain the fact that in these cases the expression of PR-1 is not affected (Fig. 5).…”

Salicylic Acid in Rice (Biosynthesis, Conjugation, and Possible Role)

“…Tissue samples were analyzed for SA as previously described (Yalpani et al, 1991;Enyedi et al, 1992). Conjugates were quantified after either chemical (base followed by acid) hydrolysis or enzymatic hydrolysis with P-glucosidase as previously described (Enyedi et al, 1992).…”