Temperature and Phytochrome Effects On Branching Are Mediated by Auxin.

Environmental factors including light quality and quantity are known to regulate branching, a key determinant of plant architecture, biomass production, and grain yield.  Under dense populations, plants absorb red light (R) but reflect or transmit most far-red light (FR) and thus decrease the R:FR which evokes the shade avoidance response (SAR) including increased stem elongation, early flowering, and increased apical dominance (reduced branching).  Phytochrome B (phyB) is a specialized photoreceptor that plants use to perceive changes in light quality (R:FR) and to modulate the SAR.  Other environmental parameters, such as temperature, also likely control the branching process.  It is understood that basipetal transport of the phytohormone auxin from the shoot apex through polar auxin transport (PAT) suppresses outgrowth of lower branches.  We hypothesized that low R:FR perceived by phyB alters auxin flux via the PAT to suppress branch outgrowth and that high temperature would accentuate these responses in Arabidopsis.  Wild type (WT), phyB, pif4, and phyBpif4 mutant plants were grown in high R:FR (no shade signal) and in reduced R:FR (shade signal) under high (28° C), moderate (24° C), and low temperature (20° C) regimens.  Branching parameters were measured in response to temperature, shade signals and treatments designed to alter auxin levels (dissections and auxin transport inhibitor [TIBA] application).  Results revealed that under low R:FR conditions high temperatures suppressed branching more than under high R:FR.  Decapitation or TIBA treatment of WT under low R:FR or in phyB resulted in increased branching compared to WT under high R:FR, highlighting the role of phyB in limiting the PAT.  These results may offer opportunities to regulate biomass and grain production by altering crop branching responses to growth in high temperature and/or dense (shaded) populations.