Cytokinin-induced bud outgrowth depends on sugar metabolism and signalling

Shoot branching is a key process of plant growth and development, finely controlled by cytokinin and sugars. However, cytokinin fails to induce bud outgrowth in the absence of sugar, and so far nothing is known about its ability to antagonize auxin when sugar availability is limited. Here we demonstrate in rose that cytokinin requires sugar metabolism and signalling to promote bud outgrowth, to down-regulate the expression of RhBRC1, a transcription factor gene that inhibits axillary bud growth, and to antagonize the inhibitory effect of auxin on bud outgrowth. Cytokinin regulation of bud sink strength was tightly associated with sugar metabolism, which was evidenced by the expression of genes involved in sugar metabolism [e.g. glycolysis, the tricarboxylic acid, and the oxidative pentose phosphate pathway (OPPP)], a metabolomic approach, and the quantification of total carbon and nitrogen in buds. Cytokinin supply is associated with a significant up-regulation of the OPPP and nitrogen accumulation. Meanwhile, sugar up-regulated bud sensitivity to cytokinin, associated with a significant down-regulation of the cytokinin signalling regulator RhARR1. These findings highlight the key role of sugar metabolism and signalling in cytokinin-induced bud outgrowth and provide new insights into the importance of nutrient-hormone crosstalk in the regulation of shoot branching.