The ratio of red light to far red light alters Arabidopsis axillary bud growth and abscisic acid signalling before stem auxin changes

Arabidopsis thaliana shoot branching is inhibited by a low red light to far red light ratio (R:FR, an indicator of competition), and by loss of phytochrome B function. Prior studies have shown that phytochrome B deficiency suppresses bud growth by elevating systemic auxin signalling, and that increasing the R:FR promotes the growth of buds suppressed by low R:FR by inhibiting bud abscisic acid (ABA) accumulation and signalling. Here, systemic auxin signalling and bud ABA signalling were examined in the context of rapid bud responses to an increased R:FR. Increasing the R:FR promoted the growth of buds inhibited by a low R:FR within 6 h. Relative to a low R:FR, bud ABA accumulation and signalling in plants given a high R:FR showed a sustained decline within 3 h, prior to increased growth. Main stem auxin levels and signalling showed a weak, transient response. Systemic effects and those localised to the bud were further examined by decapitating plants maintained either under a low R:FR or provided with a high R:FR. Increasing the R:FR promoted bud growth before decapitation, but decapitated plants eventually formed longer branches. The data suggest that rapid responses to an increased R:FR may be mediated by changes in bud ABA physiology, although systemic auxin signalling is necessary for sustained bud repression under a low R:FR.