Etude de la saccharose synthase dans la graine d'arabidopsis thaliana : localisation, regulations et fonctions

Sucrose synthase (SUS) is a key enzyme in sucrose metabolism. This enzyme catalysis the reversible conversion of sucrose and UDP to UDP-glucose and fructose, which are both used in multiple biochemical pathways. The Arabidopsis genome contains six sucrose synthase genes. A member of this family (SUS2) is strongly and specifically expressed in Arabidopsis seeds during the maturation phase, corresponding to reserve accumulation period. Total SUS activity reached a peak during seed maturation, at about 15 days after flowering (DAF), which corresponds approximately to the middle of this stage. This activity pattern fitted well with SUS2 mRNA abundance, although we cannot exclude the presence of other SUS isoform activities, such as SUS3, in seeds at late maturation. Using specific antibodies we have shown that, SUS2 is localised in the embryo, endosperm and seed coat with differential patterns. During the maturation phase, the SUS2 protein was mainly colocalised with plastids in the embryo. This result is consistent with the hypothesis that SUS2 association with the plastid may play a role in channelling of sucrose metabolism to G-6-P or PEP, both carbon compounds which enter the plastids enabling starch or lipid synthesis. Indeed, the regulation of SUS2 and SUS3 expression was investigated during seed development for wild type (WT) and for seed maturation mutants (i.e. lec2-4, fus3-4 and abi3). The accumulation patterns of SUS mRNA and protein, as well as total activity, were strongly affected in these mutants during seed development. We showed that the expression of the SUS2 promoter was induced in planta by sugar (glucose, sucrose and trehalose) and that the expressions of SUS2 and SUS3 promoters are redundantly regulated by LEC2 and FUS3 during the maturation phase. Last, a functional analysis was carried out in developing seeds, using biochemical characterisation and metabolite profiling of one sus1, two sus2 and two sus3 alleles. The results obtained outlined the importance of these SUS isoforms for sucrose metabolism in developing seeds of Arabidopsis