Abstract
The degree of shoot branching in Arabidopsis 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 BRANCHED1 (BRC1) has been proposed to integrate these regulatory signals. This idea is based on increased branching in brc1 mutants, the effects of bud-regulating hormones on BRC1 expression, and a general correlation between BRC1 expression and bud growth inhibition. These data demonstrate the important role of BRC1 in shoot branching, but here we show that in Arabidopsis this correlation can be broken. Buds lacking BRC1 expression can remain inhibited and sensitive to inhibition by strigolactone. Furthermore, buds with high BRC1 transcript levels can be active. Based on these data, we propose that BRC1 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 BRC1 transcription and the shoot auxin transport network.
Highlights
The degree of shoot branching is an excellent example of plant developmental plasticity
BRC1 transcript abundance decreases as buds activate Following floral transition, Arabidopsis buds activate in a basipetal sequence starting at the uppermost cauline node, proceeding down the shoot into the rosette nodes (Alvarez et al, 1992; Hempel and Feldman, 1994)
BRC1 is not necessary or sufficient for bud growth inhibition The Arabidopsis BRC1 gene, and its homologues in other species, are important regulators of shoot branching. Their loss of function results in highly branched phenotypes and their expression correlates with bud growth inhibition
Summary
The degree of shoot branching is an excellent example of plant developmental plasticity. Secondary shoots are formed by the activation of axillary buds, established in the axils of leaves produced by active shoot apical meristems. Differential activation of axillary buds allows a continuum of shoot forms, from a solitary unbranched stem to a highly ramified bush. Bud activity is regulated by diverse endogenous and environmental inputs. The integration of these inputs shapes the shoot system according to genotype and environment. The mechanisms by which buds integrate regulatory signals are poorly understood. There are two non-exclusive candidates for the integrating hub (Rameau et al, 2015).
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