| Synthesized from polyunsaturated fatty acids via the octadecanoid pathway, jasmonic acid (JA) and its cyclic precursors and derivatives, collectively called jasmonates, play critical roles in regulating many plant defensive and developmental processes. Extensive studies of the wound signaling pathways in tomato have led to a proposed model in which systemin, an 18-amino-acid polypeptide, acts as a mobile signal to evoke de novo synthesis of JA, which in turn activates the expression of defense-related genes. A major gap in our understanding of the function of jasmonates concerns how jasmonate perception is coupled to transcriptional activation of jasmonate-inducible genes in response to developmental and environmental cues. The focus of this dissertation research was to dissect the role of jasmonates in tomato defense and developmental processes by isolating and characterizing mutants with impaired responses to exogenous JA. To this end, a fast neutron-mutagenized tomato population was screened for plants that were deficient in methyl-JA-induced accumulation of polyphenol oxidase and proteinase inhibitor-II, two jasmonate-regulated defensive proteins. One recessive mutant (called JA- insensitive1-1) was isolated that was completely defective in jasmonate signaling in roots, leaves, and flowers. Failure of jai1-1 plants to express jasmonate-regulated genes was correlated with increased susceptibility to herbivores. Reciprocal grafting experiments using jai1 and spr2, a tomato mutant defective in JA biosynthesis, showed that spr2 plants are defective in the production, but not recognition, of a graft-transmissible wound signal, whereas jai1 plants are compromised in the recognition but not the production of this signal. These results indicate that JA or a related jasmonate species is an essential component of the long-distance wound signal. Plants homozygous for the jai1-1 mutation exhibited several novel development phenotypes, including female sterility and impaired glandular trichome development. These findings extend the role of the jasmonate signaling pathway to developmental processes in tomato that have not been previously associated with jasmonates. In a separate screen for ethyl methane sulfonate-induced mutations that suppress prosystemin-mediated responses, a mutant (called jai1-2) that was unresponsive to wounding and methyl-JA was shown to be allelic to jai1-1. The jai1 mutants were determined to harbor mutations in a gene that is homologous to the Arabidopsis CORONATINE INSENSITIVE1 ( COI1), which encodes an F-box protein involved in ubiquitin-dependent protein degradation. Stable transformation of jai1-1 plants with the tomato JAI1/COI1 cDNA restored jasmonate-induced expression of defense genes, fertility, and trichome development. We conclude that JAI1/COI1 is a key regulator of the jasmonate signaling pathway in tomato and anticipate that the jai1 mutants will be useful for future investigations aimed at elucidating in greater details the function of the jasmonate signaling pathway in defense and development. |
