Jasmonates in root nodule development

Sammanfattning: Jasmonic acid (JA), its derivatives and its precursor form a group of phytohormones, the jasmonates, representing signal molecules involved in plant stress responses, in the defense against pathogens as well as in development. Elevated levels of JA have been shown to play a role in arbuscular mycorrhiza and in the induction of nitrogen-fixing root nodules. JA levels were analysed in roots and nodules of four different symbioses: two model legumes, Medicago truncatula and Lotus japonicus, and two actinorhizal species, Casuarina glauca and Datisca glomerata. JA levels in roots, nodules and leaves of all four plant species increased upon mechanical disturbance and wounding, thus any results on nodule JA levels were unreliable due to the harvesting procedure. Immunolocalization of two enzymes from JA biosynthes, lipoxygenase (LOX) and allene oxide cyclase (AOC), in nodules indicated the first intermediate in the pathway, 13-HPOT is produced by cytosolic LOXes and shuttled to the plastids.Detailed analyses of JA levels were performed for L. japonicus cultivated aeroponically. Levels were more or less stable over five weeks during nodulation or growth on nitrate or ammonium. However, L. japonicus nodules formed on aeroponically grown plants often showed patches of cells with reduced bacteroid density, presumably a stress symptom. These nodules seemed to contain less AOC protein than nodules of plants grown in perlite/vermiculite. Hence, aeroponically grown plants may have been habituated to stress which would have affected JA levels.The gene families of two enzymes of the JA biosynthetic pathway were characterized in L. japonicus, allene oxide synthase (AOS) and AOC. Transgenic plants carrying RNAi constructs for aos and for the aoc gene expressed in nodules, aoc1, were constructed. Preliminary results on F1 plants of different aoc1 RNAi lines indicated that a reduction in aoc1 transcript levels did not affect nodule induction or nitrogen fixation to a significant extent.