Phytochrome D acts in the shade-avoidance syndrome in Arabidopsis by controlling elongation growth and flowering time

Abstract

Shade avoidance in higher plants is regulated by the action of multiple phytochrome (phy) species that detect changes in the red/far-red ratio (R/FR) of incident light and initiate a redirection of growth and an acceleration of flowering. The phyB mutant of Arabidopsis is constitutively elongated and early flowering and displays attenuated responses to both reduced R/FR and end-of-day far-red light, conditions that induce strong shade-avoidance reactions in wild-type plants. This indicates that phyB plays an important role in the control of shade avoidance. In Arabidopsis phyB and phyD are the products of a recently duplicated gene and share approximately 80% identity. We investigated the role played by phyD in shade avoidance by analyzing the responses of phyD-deficient mutants. Compared with the monogenic phyB mutant, the phyB-phyD double mutant flowers early and has a smaller leaf area, phenotypes that are characteristic of shade avoidance. Furthermore, compared with the monogenic phyB mutant, the phyB-phyD double mutant shows a more attenuated response to a reduced R/FR for these responses. Compared with the phyA-phyB double mutant, the phyA-phyB-phyD triple mutant has elongated petioles and displays an enhanced elongation of internodes in response to end-of-day far-red light. These characteristics indicate that phyD acts in the shade-avoidance syndrome by controlling flowering time and leaf area and that phyC and/or phyE also play a role.

Figure 1

Effect of R/FR on flowering time in La-er, phyD, phyB, phyB-phyD, phyA-phyB, and phyA-phyB-phyD. Seedlings were grown in continuous light of either high R/FR (solid bars) or low R/FR (hatched bars). Flowering time was measured as the number of rosette leaves produced at bolting. Error bars represent se.

Figure 2

. Phenotypes of La-er, phyD, phyB, phyB-phyD, phyA-phyB, and phyA-phyB-phyD grown in continuous light of high R/FR for 30 d. Scale bar = 1 cm.

Figure 3

. Appearance of internodes in phyA-phyB and phyA-phyB-phyD in response to EOD far-red-light treatment. Seedlings were grown for 60 d in either 8 h of light/16 h of dark (control) or with the same photoperiods plus 15 min of EOD far-red light.

Figure 4

. Responses to EOD far-red light in phyA-phyB and phyA-phyB-phyD mutants. Seedlings were grown with 8-h light/16-h dark photoperiods (control, black bars) or in the same photoperiods plus 15 min of EOD far-red light (hatched bars). a, Effect of EOD far-red light on flowering time in phyA-phyB and phyA-phyB-phyD measured as the number of rosette leaves produced at bolting. Error bars represent se. b, Effect of EOD far-red light on internode elongation in phyA-phyB and phyA-phyB-phyD. The length of the internode between “rosette” leaves 5 and 6 was measured after bolting had occurred. Error bars represent se. c, Effect of EOD far-red light on petiole length in La-er, phyD, phyB, phyB-phyD, phyA-phyB, and phyA-phyB-phyD. Petiole length from the largest fully grown leaf was measured after bolting had occurred. Error bars represent the se.

Figure 5

Effect of R/FR on leaf area. La-er, phyD, phyB, phyB-phyD, phyA-phyB, and phyA-phyB-phyD were grown in continuous light of either high R/FR (black bars) or low R/FR (hatched bars). Leaf area of the largest fully grown leaf was measured after bolting had occurred. Error bars represent the se.