Abstract
Phosphate (Pi) is an essential plant nutrient but by far the least mobile and least available for plants, therefore representing a limiting factor for their growth and development. It is well known that Pi deprivation triggers developmental adaptations in plants, including the modification of their root architecture. In white lupin (Lupinus albus), low Pi induces the formation of very specialized roots called cluster roots. These structures are made up of hundreds of short rootlets with a determinate growth, forming one or more very dense clusters. The spectacular ability of rootlets to improve Pi nutrition has been widely studied, though little information is known about the determinate development of these structures. My thesis project aims to unravel the molecular players involved in the determinate growth of white lupin rootlets.To this extent, an anatomical and physiological description of the rootlets, combined with transcriptomic approaches, allowed us to define 5 developmental stages, leading to the identification of a major transition change in rootlet development, both at the structural and functional level. Indeed, rootlets evolve from a growth phase, involving root meristem activity, to a functional phase fully dedicated to nutrition in which the meristem has disappeared.In order to unravel the molecular actors implied in rootlet determined growth, LaWOX5.1-like and LaWOX5.2-like, homologous genes of AtWOX5 known in the model plant Arabidopsis for being involved in the maintenance of the root meristem, were overexpressed in white lupin roots with hairy-root transformation. However, the obtained phenotypes did not allow us to confirm the involvement of LaWOX5.1-like and LaWOX5.2-like in the developmental transition of rootlets.It is known that rootlets emerge in a successive manner along the cluster root leading to the formation of a continuous spatial gradient of rootlet development. Abnormally long rootlets, differing from standard short rootlets by their longer size, caught our attention. Anatomical studies showed that long rootlet apex structure is very similar to the cluster root indeterminate apex. A transcriptomic data set was generated, allowing to compare: long rootlets, cluster root apexes and normal rootlets. The study of the transcriptomic data set revealed the high expression of a network of genes involved in the meristem maintenance in long rootlets compared to normal rootlets. In addition, its analysis allowed the identification of LaRGF2-like gene, homologous to AtRGF2 peptide, known to be involved in the meristem maintenance. Overexpression of LaRGF2-like in white lupin roots seems to maintain meristem activity in transformed rootlets, suggesting that this gene is a key gene in the molecular switch leading to rootlet determined growth.Finally, this thesis work has enabled the acquisition of knowledge and a better understanding of the molecular mechanisms involved in the determined growth of rootlets. In the long term, the understanding of the development of these special structures could make it possible to transfer the ability of producing cluster roots to other crop species, thereby improving their phosphate nutrition.