Tomato As A Model System To Study The Molecular Basis Of Plant Defense Responses To Insect Attack

Higher plant has established the complicated and elaborate defense mechanism against insect attack during the long-term evolution. Having deeper understanding of the molecular mechanism of plant resistance to herbivores will provide theoretical evidence to establish the environment-friendly agricultural herbivores management by employing the resistance ability of plant itself. We set up a research system to explore the mechanism of plant resistance to pest by forward chemical genetics approach with tomato as a model system. The past efforts demonstrated that phytohormone jasmonate plays pivotal role in plant systemic resistance to herbivores attack as a long-distance mobile signal. Base on this knowledge, we are initiating a screening project to isolate more tomato defenseless mutants at large-scale at the whole-genome level with some improvement on the basis of the traditional genetic screen.In this study, spr6, a new JA-signaling mutant, failed to accumulate proteinase inhibitors (PI-II) in response to mechanical wounding and exogenous systemin, but was fully responsive to exogenous JA in a dose-dependent manner. GC-MS assay indicates that even though the spr6 mutation does not affect the maintenance of basal JA levels in leaf tissues, it slightly damaged the wound-induced JA accumulation. Genetic characterization suggests that the mutant phenotype is defined by a single recessive nuclear gene. The results of Map-based cloning indicated that Spr6 located on the chromosome region defined by IL5-3. 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 CORONATIN-INSENSITIVE1 (COI1), an F-box protein that is required for JA-signaled processes in Arabidopsis. The spr6 which displays normal fertility and seed set, and responses to exogenous JA in a dose-dependent manner provides a valuable tool to dissect 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. As more genes and mutant loci are mapped, the probability for identifying genes corresponding to functionally characterized loci will increase. We speculate this candidate gene approach will be helpful to our long-term goal in genetic dissection of the systemin/JA-signaled defense responses in tomato.