Photosynthetic photon flux density and phytochrome B interact to regulate branching in Arabidopsis

Abstract

Branching is regulated by environmental signals including phytochrome B (phyB)-mediated responses to the ratio of red to far red light. While the mechanisms associated with phytochrome regulation of branching are beginning to be elucidated, there is little information regarding other light signals, including photosynthetic photon flux density (PPFD) and how it influences phytochrome-mediated responses. This study shows that Arabidopsis (Arabidopsis thaliana) branching is modified by both varying PPFD and phyB status and that significant interactions occur between these variables. While phyB deficiency decreased branching when the PPFD was low, the effect was suppressed by high PPFD and some branching aspects were actually promoted. Photosynthesis measurements showed that PPFD may influence branching in phyB-deficient plants at least partially through a specific signalling pathway rather than directly through energy effects on the shoot. The expression of various genes in unelongated buds of phyB-deficient and phyB-sufficient plants grown under high and low PPFD demonstrated potential roles for several hormones, including auxin, cytokinins and ABA, and also showed imperfect correlation between expression of the branching regulators BRC1 and BRC2 and bud fate. These results may implicate additional undiscovered bud autonomous mechanisms and/or components contributing to bud outgrowth regulation by environmental signals.

Cytokinins Are Initial Targets of Light in the Control of Bud Outgrowth

“…10A). This reveals a strong light control over this central regulator, in accordance with previous observations whereby the expression of AtBRC1 in Arabidopsis buds is regulated by photosynthetic photon flux density (Su et al, 2011) and is increased in the presence of additional far-red light in WL (González-Grandío et al, 2013). BAP application in the dark led to a rapid inhibition of RhBRC1 (i.e.…”

“…A low light intensity or a low ratio of red/far-red (R/FR) light inhibits bud outgrowth and modulates the correlative inhibitions between buds in some species (Kebrom et al, 2006;Finlayson et al, 2010;Su et al, 2011;Demotes-Mainard et al, 2013;Furet et al, 2014;Drummond et al, 2015). In rose (Rosa hybrida) plants, darkness or far-red light (used as a single source of light) represses bud outgrowth (Girault et al, 2008).…”

“…In the control of branching, light acts both as a driver of photosynthesis for the supply of sugars and also as a photomorphogenic signal (Su et al, 2011). Such a signaling role for light is well described in the shadeavoidance syndrome, where perception by plant phytochromes (mainly phyB) of a low R/FR leads to bud inhibition even under a promoting photosynthetic photon flux density (Kebrom et al, 2006;Reddy et al, 2013).…”

“…In Arabidopsis (Arabidopsis thaliana), branching is promoted by the suppression of auxin signaling by phyB (Reddy and Finlayson, 2014) and by the negative transcriptional regulation of the SL signaling AtMAX2 gene by high R/FR light Su et al, 2011;González-Grandío et al, 2013). Evidence also was given that abscisic acid is involved in the bud response to R/FR light in this species (Reddy et al, 2013).…”

The timing of low R

“…1 An obvious architectural response of plants to reduced R:FR is to suppress bud outgrowth. [2][3][4][5][6] In Arabidopsis (Arabidopsis thaliana), buds formed near the top of the rosette have greater outgrowth potential than those further down, and the effects of the R:FR on outgrowth are very dependent on bud position and the timing of exposure.…”