Auxin-independent effects of apical dominance induce changes in phytohormones correlated with bud outgrowth.

Abstract

The inhibition of shoot branching by the growing shoot tip of plants, termed apical dominance, was originally thought to be mediated by auxin. Recently, the importance of the shoot tip sink strength during apical dominance has re-emerged with recent studies highlighting roles for sugars in promoting branching. This raises many unanswered questions on the relative roles of auxin and sugars in apical dominance. Here we show that auxin depletion after decapitation is not always the initial trigger of rapid cytokinin (CK) increases in buds that are instead correlated with enhanced sugars. Auxin may also act through strigolactones (SLs) which have been shown to suppress branching after decapitation, but here we show that SLs do not have a significant effect on initial bud outgrowth after decapitation. We report here that when sucrose or CK is abundant, SLs are less inhibitory during the bud release stage compared to during later stages and that SL treatment rapidly inhibits CK accumulation in pea (Pisum sativum) axillary buds of intact plants. After initial bud release, we find an important role of gibberellin (GA) in promoting sustained bud growth downstream of auxin. We are, therefore, able to suggest a model of apical dominance that integrates auxin, sucrose, SLs, CKs, and GAs and describes differences in signalling across stages of bud release to sustained growth.