Low Red Light:Far Red Light Inhibits Branching by Promoting Auxin Signaling

The branching trait is influenced by various environmental signals, including the Red light:Far Red light (R:FR), an indicator of competition and an inhibitor of axillary bud growth. Branch development is influenced by an array of hormones, including auxin which indirectly suppresses bud growth as a consequence of auxin transport and signaling in the main stem. The suppressive effect of auxin sourced from the shoot apex and transported basipetally is a major mechanism contributing to apical dominance, a form of correlative inhibition where superior growing points restrict the growth of those lower on the plant. The current study shows that increased apical dominance is a mechanism that suppresses the branching of plants grown in a low R:FR. The elevated apical dominance was not due to increased levels of the natural auxin IAA, but was associated with enhanced expression of auxin-induced genes both in the presence and absence of exogenous auxin. A direct test of bud sensitivity to auxin confirmed that a low R:FR promoted auxin responsiveness leading to reduced bud growth. Thus, the low R:FR enhancement of auxin sensitivity is a mechanism that contributes to the reduced branching phenotype of plants growing in competitive environments.