<i>BRC1</i>expression regulates bud activation potential, but is not necessary or sufficient for bud growth inhibition in Arabidopsis

Abstract

The degree of shoot branching in <i>Arabidopsis</i> is determined by the activation of axillary buds. Bud activity is regulated by diverse environmental and developmental signals, often mediated via plant hormones, including auxin, strigolactone and cytokinin. The transcription factor <i>BRANCHED1</i> (<i>BRC1</i>) has been proposed to integrate these regulatory signals. This idea is based on increased branching in <i>brc1</i> mutants, the effects of bud-regulating hormones on <i>BRC1</i> expression, and a general correlation between <i>BRC1</i> expression and bud growth inhibition. These data demonstrate the important role of <i>BRC1</i> in shoot branching, but here we show that in <i>Arabidopsis</i> this correlation can be broken. Buds lacking <i>BRC1</i> expression can remain inhibited and sensitive to inhibition by strigolactone. Furthermore, buds with high <i>BRC1</i> transcript levels can be active. Based on these data, we propose that <i>BRC1</i> regulates bud activation potential in concert with an auxin transport-based mechanism underpinning bud activity. In the context of strigolactone-mediated bud regulation, our data suggest a coherent feed-forward loop in which strigolactone treatment reduces the probability of bud activation by parallel effects on <i>BRC1</i> transcription and the shoot auxin transport network.