The wound response mutant suppressor of prosystemin-mediated responses6 (spr6) is a weak allele of the tomato homolog of CORONATINE-INSENSITIVE1 (COI1).
The systemic defense response of tomato plant in response to insect attack and wounding is regulated by the 18 amino acid peptide systemin and the phytohormone jasmonic acid (JA). Recent genetic analyses based mainly on spr (suppressors of prosystemin-mediated responses) mutant screens have led to the hypothesis that systemin acts at, or near, the site of wounding to amplify the production of JA, which in turn functions as a mobile signal to promote the systemic defense response. In order to identify more components involved in the systemin/JA-signaled defense response, we carried out a larger scale screen for new spr mutants in tomato. Here we describe the characterization of spr6, a mutant impaired in wound- and systemin-induced defense gene expression. Using a candidate gene approach based on genetic linkage, we demonstrate that spr6 is allelic to jai1-1, which is a loss-of-function allele of the tomato homolog of CORONATINE-INSENSITIVE1 (COI1), an F-box protein that is required for JA-signaled processes in Arabidopsis. We show several aspects of the spr6 mutant phenotype distinct from that of jai1-1. First, the responsiveness of spr6 plants to exogenous JA shows a dosage dependency, i.e. it is more sensitive to JA than jai1-1 while less sensitive to JA than the wild-type. Secondly, unlike the sterile jai1-1, the spr6 plant displays normal fertility and seed set and thus can be maintained as a pure line and does not require selection. Therefore, spr6 provides a valuable tool, which can complement the limitations of jai1-1, to study JA signaling in tomato. The gene identification process of Spr6 we described herein represents an example showing the convenience of a candidate gene approach, based on genetic linkage, to identify gene functions of genetic loci defined by tomato wound response mutants.