Photosynthetic assimilate determines branch size and biomass more than branch number in Arabidopsis

Shoot branching is a primary determinant of plant form and crop yield, yet whether auxin or carbon supply is the proximal regulator of branching remains contested. In Arabidopsis, an earlier report that exogenous auxin fails to restore apical dominance after decapitation has been taken to weaken the case for auxin, and several studies have proposed that sugars are the primary regulator. Resolving this has been difficult because most perturbations of sugar status also disturb auxin. Here, we revisit the control of Arabidopsis branching using well-controlled, largely unperturbed plants and approaches designed to isolate each pathway. Contrary to the earlier report, apically applied auxin restored the suppression of rosette branching after decapitation, placing Arabidopsis in line with other species. In a dataset of 718 plants, cauline and rosette branching were weakly but significantly negatively correlated, consistent with a polar-auxin-transport-based model and contrary to a previous conclusion of no relationship. Removing all rosette leaves at bolting slowed bud growth but did not alter the final number of branches. Varying photosynthetic photon flux density across six natural accessions, analyzed by piecewise structural equation modeling, showed that photoassimilate acted far more strongly on the mass deposited into branches than on whether a bud initiates a branch. We conclude that auxin remains a major regulator of apical dominance in Arabidopsis, and that photosynthetic assimilate, while required as a substrate for branch growth, contributes little to determining branch number but more to branch size and biomass.