| Tomato plants activate the synthesis of defense proteins such as proteinase inhibitors (PIs) in response to insect attack. Systemic accumulation of PIs in wounded plants is mediated by a long-distance signaling pathway that transmits a mobile signal from the wound site to distal undamaged leaves. Previous studies have established that the polypeptide signal systemin, which is derived from a precursor protein called prosystemin, regulates systemic expression of PI genes via the jasmonic acid (JA) signaling pathway. However, the precise role of JA and systemin in systemic wound signaling remains unclear. To address this question, two tomato mutants that are compromised in the systemic wound response were characterized.; The suppressor of prosystemin-mediated responses1 ( spr1) mutant was blocked in JA biosynthesis and subsequent PI gene expression in response to systemin. The systemin-insensitive phenotype of spr1 plants indicates that the Spr1 gene product plays a role in linking the perception of systemin to the activation of JA biosynthesis. Reciprocal grafting experiments between wild-type and spr1 plants demonstrated that this mutant is deficient in the production of the long-distance wound signal in damaged leaves, rather than the perception of that signal in neighboring undamaged leaves. This result indicates that systemin acts at or near the site of wounding to increase JA synthesis to a level that is required for the systemic response.; The role of JA in systemic wound signaling was investigated using the JL-1 mutant that is defective in both JA biosynthesis and wound-inducible expression of PI genes. Gas chromatography-mass spectrometry analysis showed that JL-1 plants are unable to produce JA from its precursor, 12-oxo-phytodienoic acid (OPDA), presumably because of a defect in fatty acid beta-oxidation. It was found that OPDA is not an active signal for PI expression in tomato. Analysis of reciprocal grafts between wildtype and JL-1 plants showed that JA biosynthesis is required for the generation of the systemic signal in wounded leaves. Taken together with the finding that JA perception is essential for systemic expression of PI in undamaged leaves, these results indicate that JA or a derivative of JA is the long-distance signal for systemic PI expression.; JA biosynthesis is regulated by substrate availability. Two related cytochrome P450s, allene oxide synthase (AOS) and hydroperoxide lyase (HPL), metabolize a common hydroperoxy fatty acid substrate to JA and volatile C6 aldehyde, respectively. (Abstract shortened by UMI.)... |
