Jasmonic acid (JA) is a fatty acid-derived signaling molecule that regulates a broad range of plant defense responses against herbivores and some microbial pathogens. Molecular genetic studies in Arabidopsis have established that JA also performs a critical role in anther and pollen development but is not essential for other developmental aspects of the plant's life cycle. Here, we describe the phenotypic and molecular characterization of a sterile mutant of tomato ( jasmonic acidinsensitive1 [ jai1 ]) that is defective in JA signaling. Although the mutant exhibited reduced pollen viability, sterility was caused by a defect in the maternal control of seed maturation, which was associated with the loss of accumulation of JAregulated proteinase inhibitor proteins in reproductive tissues. jai1 plants exhibited several defense-related phenotypes, including the inability to express JA-responsive genes, severely compromised resistance to two-spotted spider mites, and abnormal development of glandular trichomes. We demonstrate that these defects are caused by the loss of function of the tomato homolog of CORONATINE-INSENSITIVE1 (COI1), an F-box protein that is required for JA-signaled processes in Arabidopsis. These findings indicate that the JA/COI1 signaling pathway regulates distinct developmental processes in different plants and suggest a role for JA in the promotion of glandular trichome-based defenses.

A number of studies have noted that nucleotide substitution rates at the chloroplast-encoded rbcL locus violate the molecular clock principle. Substitution rate variation at this plastid gene is particularly pronounced between palms and grasses; for example, a previous study estimated that substitution rates in rbcL sequences are -5-fold faster in grasses than in palms. To determine whether a proportionate change in substitution rates also occurs in plant nuclear genes, we characterized nucleotide substitution rates in palm and grass sequences for the nuclear gene Adh. In this article, we report that palm sequences evolve at a rate of 2.61 x 10-9 substitution per synonymous site per year, a rate which is slower than most plant nuclear genes. Grass Adh sequences evolve -2.5-fold faster than palms at synonymous sites. Thus, synonymous rates in nuclear Adh genes show a marked decrease in palms relative to grasses, paralleling the pattern found at the plastid rbcL locus. This shared pattern indicates that synonymous rates are correlated between a nuclear and a plastid gene. Remarkably, nonsynonymous rates do not show this correlation. Nonsynonymous rates vary between two duplicated grass Adh loci, and nonsynonymous rates at the palm Adh locus are not markedly reduced relative to grasses.As a corollary to neutral theory, Kimura (1) predicted that nucleotide substitution rates should be a linear function of clock time. This strict molecular clock hypothesis predicts that nucleotide substitution rates should be equal in different evolutionary lineages. A great many studies have rejected the strict molecular clock hypothesis. Deviations from linearity have been demonstrated between rodents and primates (2, 3), sharks and whales (4), primates and humans (5, 6), and echinoids and vertebrates (7), to name a few examples. However, an alternative molecular clock prediction that posits a correlation of substitution rates with rates of germ-line cell division is also consistent with neutral theory (1, 8). According to this hypothesis, nucleotide substitution rates should be higher in species with shorter generation times, because the rate of cell division per unit time (e.g., per year) is greater (2). This prediction appears to hold true in some mammalian lineages (2,3,5,6).The molecular clock hypothesis has not been tested extensively in plants, although it is clear that a strict molecular clock does not hold in some cases. For example, nuclear heat-shock proteins (9) and nuclear rDNA sequences (10) exhibit marked rate heterogeneity between evolutionary lineages. There are also deviations from a strict molecular clock at the plastid locus rbcL (11-13). Gaut et al. (13) examined rate heterogeneity among rbcL sequences from 35 monocotyledonous taxa and found that rates of evolution are hierarchical, wherein sequences from members of the grass family evolve more rapidly than sequences from the orchid, lily, pineapple, and palm families. This rate hierarchy is correlated with minimum generation time (measured as time from...

Completed genome sequences have made it clear that multicopper oxidases related to laccase are widely distributed as multigene families in higher plants. Laccase-like multicopper oxidase (LMCO) sequences culled from GenBank and the Arabidopsis thaliana genome, as well as those from several newly cloned genes, were used to construct a gene phylogeny that clearly divided plant LMCOs into six distinct classes, at least three of which predate the evolutionary divergence of angiosperms and gymnosperms. Alignments of the predicted amino acid sequences highlighted regions of variable sequence flanked by the highly conserved copper-binding domains that characterize members of this enzyme family. All of the predicted proteins contained apparent signal sequences. The expression of 13 of the 17 LMCO genes in A. thaliana was assessed in different tissues at various stages of development using RT-PCR. A diversity of expression patterns was demonstrated with some genes being expressed in a constitutive fashion, while others were only expressed in specific tissues at a particular stage of development. Only a few of the LMCO genes were expressed in a pattern that could be considered consistent with a major role for these enzymes in lignin deposition. These results are discussed in the context of other potential physiological functions for plant LMCOs, such as iron metabolism and wound healing.

In mammalian cells, induced expression of arginase in response to wound trauma and pathogen infection plays an important role in regulating the metabolism of Larginine to either polyamines or nitric oxide (NO). In higher plants, which also utilize arginine for the production of polyamines and NO, the potential role of arginase as a control point for arginine homeostasis has not been investigated. Here, we report the characterization of two genes (LeARG1 and LeARG2) from Lycopersicon esculentum (tomato) that encode arginase. Phylogenic analysis showed that LeARG1 and -2, like all other plant arginases, are more similar to agmatinase than to arginases from vertebrates, fungi, and bacteria. Nevertheless, recombinant LeARG1 and -2 exhibited specificity for L-arginine over agmatine and related guanidino substrates. The plant enzymes, like mammalian arginases, were inhibited (K i ϳ 14 M) by the NO precursor N G -hydroxy-L-arginine. These results indicate that plant arginases define a distinct group of ureohydrolases that function as authentic L-arginases. LeARG1 and LeARG2 transcripts accumulated to their highest levels in reproductive tissues. In leaves, LeARG2 expression and arginase activity were induced in response to wounding and treatment with jasmonic acid (JA), a potent signal for plant defense responses. Wound-and JA-induced expression of LeARG2 was not observed in the tomato jasmonic acid-insensitive1 mutant, indicating that this response is strictly dependent on an intact JA signal transduction pathway. Infection of wild-type plants with a virulent strain of Pseudomonas syringae pv. tomato also up-regulated LeARG2 expression and arginase activity. This response was mediated by the bacterial phytotoxin coronatine, which exerts its virulence effects by co-opting the host JA signaling pathway. These results highlight striking similarities in the regulation of arginase in plants and animals and suggest that stress-induced arginase may perform similar roles in diverse biological systems.L-Arginine is one of the most functionally diverse amino acids in living cells. In addition to serving as a constituent of proteins, arginine is a precursor for the biosynthesis of polyamines, agmatine, and proline, as well as the cell-signaling molecules glutamate, ␥-aminobutyric acid, and nitric oxide (1-3). Two of the most intensively studied pathways of arginine metabolism are those catalyzed by arginase and nitric-oxide synthase (NOS).1 Arginase hydrolyzes arginine to urea and ornithine, the latter of which is a precursor for polyamine biosynthesis. Recent studies in animal systems indicate that increased arginase expression stimulates the production of polyamines that promote tumor cell proliferation (4), wound healing (5), and axonal regeneration following injury (6). Juxtaposed to the growth-promoting effects of polyamines are the cytostatic effects of NO produced by activated macrophages. The switch between the arginase and NOS branches of arginine metabolism is controlled by various inflammatory signals that regulate ar...