Figure 9.

Enhanced Availability of HY5 under FR Enrichment Inhibits Lateral Root Emergence.

Model depicting our hypothesis for the stabilization of HY5 in WL+FR (supplemental FR light, low R:FR ratio) and its subsequent action in the LRP leading to decreased LR emergence. Top part illustrates the shoot. In WL+FR, phyB is converted to the inactive form (Pr) by FR light (dark T-bar), which releases repression of PIFs, leading to shoot elongation. In WL, phyB indirectly promotes HY5 degradation (dashed T-bar), which is relieved in WL+FR, where phyA indirectly promotes HY5 stabilization (dashed arrow). HY5 indirectly represses shoot elongation in both WL and WL+FR (dashed T-bar). The size of the boxes reflects the protein amounts. Bottom part illustrates the root. HY5 is small enough to be transported to the root through the phloem and in the root HY5 can induce its own transcription (circular arrow). HY5 has a negative effect on PIN3 and LAX3 levels in the cortex overlaying the LRP, although it is not clear if this is a direct effect (dashed T-bar). One way of achieving this is to reduce the expression of ARF19 (dark T-bar and arrow). Lower PIN3 and LAX3 abundance leads to reduced auxin concentrations in the overlaying cortex cell (red box, black arrows), which is necessary for IDA induction and cell separation. Ultimately this leads to a reduced LR emergence. Red boxes are the PHYs, green boxes are auxin signaling and transport components, and HY5 is blue.