Title : Polyploidy and adaptations to environmental stress: physiological, biochemical and genomic determinants of tolerance in citrus polyploid rootstocks
The Mediterranean basin is one of the main citrus production areas in the world. It ranks third behind China and Brazil. In the heart of the Mediterranean, Corsica is located at the northern limit of the citrus production area (40° North). It benefits from pedoclimatic conditions favorable to the cultivation of citrus fruits. However, this citrus cultivation can be endangered by the emergence of strong abiotic constraints. Indeed, the current climate changes result in an increase in the frequency and intensity of stress episodes in plants. In Corsica, these climatic changes can be characterized by episodes of intense drought that force producers to irrigate their crops more intensively. This results in an increase in soil salinity. The increase in salinity leads to a decrease in growth, tree productivity and citrus quality. The selection of rootstocks adapted to these emerging abiotic constraints seems to be a key criterion to facilitate the adaptation of citrus crops. Work carried out by the University of Corsica, CIRAD and INRAE for about ten years now, has shown that tetraploid citrus rootstocks (4x) would be more resistant to nutritional stress, cold and water deficit compared to diploid rootstocks (2x) which are the most commonly used. However, the molecular and genetic basis of this adaptation is not well known, so this thesis aims to study the mechanisms promoting the response to environmental stresses of 2x and 4x citrus rootstocks. It is highly probable that 4x rootstocks show a better tolerance to salt stress but the reasons for this better adaptation remains to be elucidated. To understand and identify the mechanisms involved, we will develop an integrative approach including the study of physiological (photosynthetic capacity, stomatal conductance, chlorophyll fluorescence), biochemical (oxidation markers, specific activity of enzymes involved in the management of oxidative stress), genetic (transcriptome analysis by RNA-seq) and epigenetic (methylome analysis by Bs-seq and Me-Dip-seq) mechanisms. At the end of this work, we will be able to provide new evidence to explain the advantages of tetraploidy over diploidy. We will be able to determine if there is a constitutive preadaptation linked to tetraploidy or if tetraploidy improves the response to stress through phenotypic plasticity.