Jasmonates comprise a family of plant hormones that regulate gene expression to modulate diverse developmental and defensive processes. To screen a set of jasmonate-responsive Arabidopsis genes, we performed a microarray analysis using an Affymetrix GeneChip containing about 8,300 gene probes synthesized in situ. External treatment with 100 microM methyl jasmonate resulted in significant changes (more than twofold increases or decreases) in the expression levels of 137 genes in the rosette leaves of 5-week-old Arabidopsis plants. Of these, 74 genes were up-regulated, including those involved in jasmonate biosynthesis, defense responses, oxidative stress responses, senescence, and cell wall modification. In contrast, the expression of genes involved in chlorophyll constitution and photosynthesis was down-regulated. Most importantly, the jasmonate treatment significantly reduced transcripts of abscisic acid-responsive cold/drought-stress genes, which suggests that an antagonistic interaction occurs between the jasmonate and abscisic acid signaling pathways in abiotic stress responses. Northern blot analysis of some selected genes revealed that the jasmonate-responsive genes exhibited unique time-course expression patterns after the external jasmonate treatment. Based on the basic clustering of the genes, we established a likely regulation scenario: the genes induced early after treatment are involved in signaling mechanisms that activate or repress other genes, whereas intermediate- and late-accumulating genes are activated by the signaling mechanisms and are subsequently involved in the ultimate jasmonate-modulated cellular responses.

The Arabidopsis gene AtLEC (At3g15356) gene encodes a putative 30-kDa protein with a legume lectin-like domain. Likely to classic legume lectin family of genes, AtLEC is expressed in rosette leaves, primary inflorescences, and roots, as observed in Northern blot analysis. The accumulation of AtLEC transcript is induced very rapidly, within 30 min, by chitin, a fungal wall-derived oligosaccharide elictor of the plant defense response. Transgenic Arabidopsis carrying an AtLEC promoter-driven beta-glucuronidase (GUS) construct exhibited GUS activity in the leaf veins, secondary inflorescences, carpel heads, and silique receptacles, in which no expression could be seen in Northern blot analysis. This observation suggests that AtLEC expression is induced transiently and locally during developmental processes in the absence of an external signal such as chitin. In addition, mechanically wounded sites showed strong GUS activity, indicating that the AtLEC promoter responds to jasmonate. Indeed, methyl jasmonate and ethylene exposure induced AtLEC expression within 3-6 h. Thus, the gene appears to play a role in the jasmonate-/ethylene-responsive, in addition to the chitin-elicited, defense responses. However, chitin-induced AtLEC expression was also observed in jasmonate-insensitive (coi1) and ethylene-insensitive (etr1-1) Arabidopsis mutants. Thus, it appears that chitin promotes AtLEC expression via a jasmonate- and/or ethylene-independent pathway.