Abstract
Cluster roots (CRs) are striking root developmental adaptations to soils with scarce nutrient avaibility. The development of these spectacular structures, made of dozen of short packed small roots named rootlets, is mainly triggered in phosphate-deprived conditions. These particular secondary root stuctures are dedicated to improve phosphate acquisition by the plant, and are of interest for plant nutrition. Surprisingly, even though white lupin (Lupinus albus) is a model species for the study of CRs, little information can be found about their formation and the molecular mechanisms behind it.To better understand rootlet formation in white lupin, an anatomical description was performed. Starting with a tissular study, 8 stages in rootlet primordium development were defined by analogy with the model plant Arabidopsis thaliana. Due to the major role of the phytohormone auxin in the formation of lateral roots, the work was next focussed on auxin response. The expression of the DR5:GUS auxin reporter showed that an auxin maximum was gradually established at the tip of the rootlet primordium.With the aim to improve the description of rootlet primordium development, cell divisions dynamic and tissue differentiation were studied. Using tissue-specific markers (AtCYCB1 ;1, LaWOL, LaSCR, LaPEP), cell divisions were observed not only in the pericycle, but also in the endodermis and the cortex, suggesting a contribution of these tissues to the formation of the rootlet primodium. Following these divisions, tissues started to differentiate to form a higly-organized meristem, whose cellular organization was described with the same set of molecular markers.Lastly, a transcriptomic approach was performed on two detailed transcriptomics datasets describing rootlet development in a spatial and temporal manner. Analysis of gene expression profiles during the early steps of rootlet formation enabled the identification of specific expression profiles and the selection of candidate genes for functional analyses. Inhibiting the activity of 9 transcription factors (by fusing them with the SRDX repressor domain) allowed the identification of three genes for which repression prevented rootlet formation, suggesting a crucial role for LaLBD16, LaERF12 and LaSTY1 during rootlet organogenesis.This thesis work suggests that a lateral root developmental program may have been recycled for the formation of cluster roots. The molecular mechanisms governing the massive induction of rootlets at the origin of the developmental curiosity that are cluster roots remain to be determined. In the future, this work may allow to transfer the ability to produce these structures to other crop species to expand their soil exploration capacity and improve their phosphate nutrition.