An integrated view of changing nutrient availability in model speciesthe role of signaling in the plant response

Resumen

The present thesis is focused on rhe responses of (i) Arabidopsis tha!ia11a to phosphate starvation, and (ii) Medicago tru11catula to different N sources. In this sense, phosphate starvation compromises electron flow through the cytochrome pathway of the mitochondrial electron transport chain, and plants commonly respond to phosphate deprivation by increasing flow through the altemative oxidase (AOX). To test whether thís response is linked to the increase in nitric oxide (NO) production that also increases under phosphate starvation,Arabidopsis thaliana seedlings (wild type, WT and nitrare reductase double mutant nia) were grown for 15 days on media containing either O or 1lll\i1 inorganic phosphate. Phosphate deprivation increased NO production in WT roots, and the AOX level and the capacity of the alternative pathway to consume electrons in WT seedlings; whereas the same treatment failed to stimulate NO production and AOX expression in the nia mutant, and the plants had an altered growth phenotype. The NO donor S-nitrosoglutathione rescued the growth phenotype of the nía mutants under phosphate deprivation to sorne extent, and it also increased the respiratory capacity of AOX. It is concluded that NO is required for the induction of the AOX pathway when seedlings are grown under phosphate-limiting conditions. Regarding the second part of this thesis, ir was focused on the early identification of stress response and plant performance of Medicago truncat11/a growing in axenic medium with ammonium or urea as the sole source of nitrogen, with respect to nitrate-based nutrition. Both ammonium and ureic nutrition severely affected the root system architecture, resulting in changes in the main elongation rate, lateral root development, and insert position from the root base. The auxin content decreased in both urea- and ammonium-treated roots; however, only the ammonium-treated plants were affected at the shoot level. The analysis of chlorophyll a fluorescence transients showed that ammonium affected photosystem II, but urea did not impair photosynthetic activity. Superoxi.de dismutase isoenzymes in the plastids were moderately affected by urea and ammonium in the roots. Low <lose (1mM) of ammonium or urea did not affect the metabolism and signalling of N. Under this scenario, there was an up-regulation of aquaporins and the modulation of phenylpropanoids metabolism on roots, allowing an accurate sensing mechanism through which plants uptake and assimilated the N of the medium.