JIPs and RIPs: the regulation of plant gene expression by jasmonates in response to environmental cues and pathogens

After its first description as a fragrant constituent of the essential oil of Jasminum grandiflorum, (-)-jasmonic acid methyl ester (methyl cis-2-(2-penten-1-yl)-3-oxocyclopentenyl acetate, also known as methyl jasmonate [MeJA]), and related compounds were found to be widespread in the plant kingdom. In addition, jasmonates occur in fungi and algae. Interestingly, MeJA is also a component of female-attracting pheromones in certain moths. (-)-Jasmonic acid [(-)-JA] and its derivatives are cyclopentanone compounds that are synthesized from linolenic acid by a sequence of as yet poorly characterized reactions. The natural biosynthetic product of this pathway in most plant species appears to be (+)-7-iso-JA, which is rapidly converted into (-)-JA by isomerization. (-)-JA has been shown to undergo various modifications, giving rise to numerous metabolites with differing structures and biological activities. Major physiologically active compounds are (-)-JA, MeJA, and the amino acid conjugates of (-)-JA with isoleucine, valine, or leucine. (-)-JA and MeJA (which will be collectively referred to throughout this review as jasmonates) affect plant growth and development in a pleiotropic manner. Recently, jasmonates have been proposed to be stress-related compounds. MeJA, in particular, is a signal molecule that is released in plants in response to various stresses, such as wounding or pathogen attack treatment of plant tissues or plant cell cultures with fungal elicitors, or subjection of tissues to osmotica or desiccation. Jasmonates are thought to be transported both locally and systemically within wounded plants. Because MeJA is a volatile compound, it is also able to traverse the atmosphere and thus can reach neighboring plants, in which characteristic defense reactions may be induced. All of the different plant responses to jasmonates, whether applied externally or released internally, appear to be correlated with alterations in gene expression. At least three major jasmonate effects have been reported to be exerted at the gene expression level: (1) the induction of novel abundant polypeptides, designated jasmonate-induced proteins (JIPs), (2) the selective repression of synthesis of several polypeptides that are present before jasmonate or stress treatment, and (3) the temporally delayed general down-regulation of protein biosynthesis occurring in long-term MeJA-treated or long-term stressed leaf tissues.