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

ABSTRACT

Arabidopsis thaliana shoot branching is inhibited by a low red light to far red light ratio (RFR, 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 RFR promotes the growth of buds suppressed by low RFR 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 RFR. Increasing the RFR promoted the growth of buds inhibited by a low RFR within 6 h. Relative to a low RFR, bud ABA accumulation and signalling in plants given a high RFR 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 RFR or provided with a high RFR. Increasing the RFR promoted bud growth before decapitation, but decapitated plants eventually formed longer branches. The data suggest that rapid responses to an increased RFR may be mediated by changes in bud ABA physiology, although systemic auxin signalling is necessary for sustained bud repression under a low RFR.