Arabidopsis BRANCHED1Acts as an Integrator of Branching Signals within Axillary Buds

Abstract

Shoot branching patterns depend on a key developmental decision: whether axillary buds grow out to give a branch or whether they remain dormant in the axils of leaves. This decision is controlled by endogenous and environmental stimuli mediated by hormonal signals. Although genes involved in the long-distance signaling of this process have been identified, the genes responding inside the buds to cause growth arrest remained unknown in Arabidopsis thaliana. Here, we describe an Arabidopsis gene encoding a TCP transcription factor closely related to teosinte branched1 (tb1) from maize (Zea mays), BRANCHED1 (BRC1), which represents a key point at which signals controlling branching are integrated within axillary buds. BRC1 is expressed in developing buds, where it arrests bud development. BRC1 downregulation leads to branch outgrowth. BRC1 responds to developmental and environmental stimuli controlling branching and mediates the response to these stimuli. Mutant and expression analyses suggest that BRC1 is downstream of the MORE AXILLARY GROWTH pathway and that it is required for auxin-induced apical dominance. Therefore, BRC1 acts inside the buds as an integrator of signals controlling bud outgrowth and translates them into a response of cell growth arrest. The conservation of BRC1/tb1 function among distantly related angiosperm species suggests that a single ancestral mechanism of branching control integration evolved before the radiation of flowering plants.

Strigolactone Acts Downstream of Auxin to Regulate Bud Outgrowth in Pea and Arabidopsis

“…3). Growth of axr1 and tir1 afb1 afb2 afb3 auxin response mutant plants in medium containing GR24 also led to an inhibition of shoot bud outgrowth, whereas the tbl1/brc1 mutant, which is thought to act downstream of strigolactone response (Aguilar-Martínez et al, 2008;Finlayson, 2008), showed no response to strigolactone treatment, as expected (Fig. 3B).…”

“…GR24 again significantly reduced branching in axr1 and also in tir1 afb1 afb2 afb3 mutant plants (P , 0.001 by Student's t test). Branching in the Arabidopsis branched1/teosinte branched1-like1 (brc1/tbl1) mutant, which is mutated in a gene encoding a TCP transcription factor that is thought to function downstream of auxin and SMS perception (Aguilar-Martínez et al, 2008;Finlayson, 2008), was not reduced by GR24 (Fig. 3B).…”

A TCP domain transcription factor controls flower type specification along the radial axis of the Gerbera (Asteraceae) inflorescence

“…In addition to its sister gene HpTCP from sunflower, in the most-parsimonious and maximum likelihood trees based on nucleotide data, GhCYC1 groups (without statistical support) with Arabidopsis AtBRC1 and Lotus LjCYC5 (data not shown). AtBRC1 and AtBRC2 have been shown to be functionally related to maize TB1 (15). However, in our analyses, support for particular gene groupings with TB1 is lacking.…”

“…The name TCP derives from the three founding members of the family, TEOSINTE BRANCHED1 (TB1) of maize, CYC, and PROLIFERATING CELL FACTOR (PCF) of rice, all of which control meristem growth by affecting cell proliferation (14). Phylogenetic analysis based on the TCP domain has uncovered two subfamilies; PCF proteins form one clade (class I) whereas CYC/TB1 and CINCINNATA (CIN) group together to form the class II TCP proteins (15). The CYC/TB1 group is also called the ECE clade, within which three subclades, CYC1, -2, and -3, have been identified in core eudicots (16).…”

“…It acts as a repressor of growth by suppressing axillary branching, and thus promotes apical dominance in domesticated maize (17). TB1 homologs have also been shown to prevent the growth of axillary buds in rice and Arabidopsis (15,18). The two TCP genes CYC and DICHOTOMA (DICH) in Antirrhinum regulate flower symmetry by modulating celldivision related gene expression (12)(13)(14)19).…”