Jasmonate biosynthesis in legume and actinorhizal nodules.

Jasmonic acid (JA) is a plant signalling compound that has been implicated in the regulation of mutualistic symbioses. In order to understand the spatial distribution of JA biosynthetic capacity in nodules of two actinorhizal species, Casaurina glauca and Datisca glomerata, and one legume, Medicago truncatula, we determined the localization of allene oxide cyclase (AOC) which catalyses a committed step in JA biosynthesis. In all nodule types analysed, AOC was detected exclusively in uninfected cells. The levels of JA were compared in the roots and nodules of the three plant species. The nodules and noninoculated roots of the two actinorhizal species, and the root systems of M. truncatula, noninoculated or nodulated with wild-type Sinorhizobium meliloti or with mutants unable to fix nitrogen, did not show significant differences in JA levels. However, JA levels in all plant organs examined increased significantly on mechanical disturbance. To study whether JA played a regulatory role in the nodules of M. truncatula, composite plants containing roots expressing an MtAOC1-sense or MtAOC1-RNAi construct were inoculated with S. meliloti. Neither an increase nor reduction in AOC levels resulted in altered nodule formation. These data suggest that jasmonates are not involved in the development and function of root nodules.

GH3::GUS reflects cell-specific developmental patterns and stress-induced changes in wood anatomy in the poplar stem.

GH3 genes related to the auxin-inducible Glycine max (L.) Merr. GmGH3 gene encode enzymes that conjugate amino acids to auxin. To investigate the role of GH3 enzymes in stress responses and normal wood development, Populus x canescens (Ait.) was transformed with the promoter-reporter construct GH3::GUS containing a GH3 promoter and the 5' UTR from soybean. beta-Glucuronidase (GUS) activity was present in the vascular tissues of leaves and in developing lateral roots and was inducible in silent tissues by external auxin application. A decrease in GUS activity from the stem apex to the bottom corresponded to decreases in auxin concentrations in these tissues. High auxin concentration and high GH3::GUS activity were present in the pith tissue, which may provide storage for auxin compounds. GH3 reporter was active in ray cells, paratracheal parenchyma cells, maturing vessels and in cells surrounding maturing phloem fibers but not in the cambium and immature phloem, despite high auxin concentrations in the latter tissues. However, the GH3 promoter in these tissues became active when the plants were exposed to abiotic stresses, like bending or salinity, causing changes in wood anatomy. We suggest that adjustment of the internal auxin balance in wood in response to environmental cues involves GH3 auxin conjugate synthases.

Phosphoinositide and inositolpolyphosphate signalling in defense responses of Arabidopsis thaliana challenged by mechanical wounding.

Various biochemical signals are implicated in Arabidopsis wound signalling, including jasmonic acid (JA), salicylic acid, auxin, and Ca2+. Here, we report on cross-talk of phytohormones with phosphoinositide signals not previously implicated in plant wound responses. Within 30 min of mechanical wounding of Arabidopsis rosette-leaves, the levels of the lipid-derived soluble inositolpolyphosphate, inositol 1,4,5-trisphosphate (InsP(3)), increased four to five-fold. Concomitantly, the precursor lipids, phosphatidylinositol 4,5-bisphosphate, phosphatidylinositol 4-phosphate and phosphatidylinositol transiently depleted, followed by re-synthesis after 30-60 min of stimulation. Increased InsP(3) levels with wounding coincided with JA increases over the first hours of stimulation. In dde2-2-mutant plants deficient in JA biosynthesis, no InsP(3) increase was observed upon wounding, indicating that JA was required for InsP(3) formation, and InsP(3) levels increased in wild-type plants challenged with sorbitol, increasing endogenous JA levels. In InsP 5-ptase plants with attenuated phosphoinositide signalling, the induction of wounding-inducible genes was diminished compared with wild-type plants, suggesting a role for phosphoinositide signalling in mediating plant wound responses. The gene-expression patterns suggest that phosphoinositides contribute to both JA-dependent and JA-independent aspects of wound signalling. Weight gain of Plutella xylostella caterpillars feeding on InsP 5-ptase plants was increased compared with that of caterpillars feeding on wild-type plants. The ecophysiological relevance of phosphoinositide signals in plant defense responses to herbivory is discussed in light of recent findings of inositolpolyphosphate involvement in phytohormone-receptor function.