Bud break is a key developmental process controlling plant architecture. Buds are sink organs which require to import sugars to outgrowth. Very little is known about the way this sugars are imported and the role of sugar transporters in this process. In rosebush (Rosa hybrida), bud break requires light to occur, even in the absence of correlative inhibitions such as apical dominance. Bud break photocontrol involves a stimulation of sugar metabolism under light, indicating a mobilization of sucrose from the stem. The main goal of this work was to determine the role of sugar transporters in rose bud break photocontrol. In vitro experiments showed that rose buds need to import metabolisable sugars (sucrose, glucose or fructose) to outgrowth under light. Supplying buds with these sugars also promote bud break under total darkness (inhibitory condition). After beheading (apical dominance release), assays with radiolabelled sugars showed that bud break photocontrol is correlated with an uptake of hexose and then of sucrose. Furthermore, these uptake require both passive and active components. They could therefore involve symplasmic connections and sugar transporter activity. Phloem sap labelling with a fluorescent tracer demonstrated that dormant bud are not symplasmically connected to the surrounding stem and that connections take place at late stages of bud break. To study the role of sugar transporters into bud break photocontrol we isolated 10 putative sugar transporters (3 for sucrose and 7 for hexose). RT-QPCR analysis suggest that the high a nity sucrose/H+ symporter RhSUC2, as well the putative hexose transporters RhSTP1 and RhTMT2 are involved in bud break photocontrol. Functional validation of RhSUC2 in yeast expression system confirmed it is a sucrose/H+ symporter. All together, results allowed us to get insights of the mechanisms involved in bud break photocontrol by sugar metabolism.
