Mecanismes fisiològics, moleculars i genètics d’adaptació a sòls salins i alcalins en poblacions d'arabidopsis thaliana

Resumen

Saline-alkaline soils produce more harmful effects in plants than neutral salts, severely reducing plant productivity. This reduction in fitness is opposite to the agronomic objective of this century, which consists of increasing agricultural productions to feed a growing population. This study aims to explore the physiological, molecular, and genetic mechanisms underlying the differences in plant performance under alkaline-saline conditions using natural populations of A. thaliana from Catalonia, on one hand, and the worldwide distributed stock center collection on the other. In chapter I, physiological responses from a moderate-carbonate tolerant A1 (c+) and a sensitive T6(c-) A. thaliana deme (small strands) from Catalonia were determined in plants submitted to alkaline treatments (high pH vs bicarbonate) under hydroponic conditions. In both demes the NaHCO3 treatment was more harmful than alkaline pH alone achieved by organic buffer. The tolerant deme translocated more nutrients, maintained higher chlorophyll levels, had better growth performance and consequently had a higher siliqua production under alkalinity than the sensitive deme. A quick activation of genes related to iron uptake was found in the roots of the tolerant line. Differences in physiological traits under alkaline treatment indicate differences at the transcriptomic level. In the second chapter of this dissertation, RNA-sequencing in leaves and microarray in roots were performed in plants exposed for 3h or 48h to either pH stress alone (pH 5.9 vs pH 8.3) or pH and alkalinity (10 mM NaHCO3 at pH 8.3). Differences in plant responses to high pH vs bicarbonate were detected. Our results indicate that leaves of carbonate-tolerant plants do not sense iron deficiency as fast as sensitive ones. In A1(c+) leaves, the activation of other genes related to stress perception, signal transduction, glucosinolates, sulfur acquisition, and cell cycle yield an efficient response to bicarbonate stress and precedes the induction of iron homeostasis mechanisms. Across the Catalonia populations, plant tolerance to calcareous soils was found to be driven by the native soil CaCO3 content. To see whether this also applies to a worldwide distribution scale, in Chapter III, 360 A. thaliana populations were sown in natural soils with contrasted CaCO3 concentrations. Plants with higher relative growth on calcareous soil were able to take up more sulfur, zinc and phosphorous. These phenotypes were used to perform a Genome Association Study. Candidate gene validation was performed using T-DNA lines in Col-0 background grown again in contrasted calcareous soils from the study area. The Zn phenotype, points to a malate vacuolar transporter, TDT, as an important negative regulator of mineral nutrient content and pH regulation on calcareous soils. Chapter IV addresses the fact that the disruption of A. thaliana distribution along the Catalan coast overlaps with the presence of alkaline-saline soil spots, suggesting that tolerance mechanisms to saline calcareous soils are not present in this plant species. To test this hypothesis, local A. thaliana populations were tested in multiyear small-scale common gardens and greenhouse experiments using natural alkaline-saline soil. Overall, germination was severely inhibited, especially in plants with low CaCO3 content in their native soil. Nutrition status and silique number were higher in demes from regions with moderate levels of both salinity and bicarbonate in their native soils. In conclusion, differences in plant responses were found between high pH and bicarbonate treatments. Under bicarbonate exposure, shoots of sensitive plants detect nutrient deficiency earlier than tolerant ones as tolerants maintain more efficient transport mechanisms. Alkaline-saline stress produces more deleterious effects than the single stress factors. Germination is severely reduced by alkaline salinity, especially in coastal populations adapted to saline siliceous soils. This could be a possible explanation for absence of Arabidopsis thaliana on coastal alkaline soils in NE Catalonia.