Different phototransduction kinetics of phytochrome A and phytochrome B in Arabidopsis thaliana

Abstract

The kinetics of phototransduction of phytochrome A (phyA) and phytochrome B (phyB) were compared in etiolated Arabidopsis thaliana seedlings. The responses of hypocotyl growth, cotyledon unfolding, and expression of a light-harvesting chlorophyll a/b-binding protein of the photosystem II gene promoter fused to the coding region of beta-glucuronidase (used as a reporter enzyme) were mediated by phyA under continuous far-red light (FR) and by phyB under continuous red light (R). The seedlings were exposed hourly either to n min of FR followed by 60 minus n min in darkness or to n min of R, 3 min of FR (to back-convert phyB to its inactive form), and 57 minus n min of darkness. For the three processes investigated here, the kinetics of phototransduction of phyB were faster than that of phyA. For instance, 15 min R h-1 (terminated with a FR pulse) were almost as effective as continuous R, whereas 15 min of FR h-1 caused less than 30% of the effect of continuous FR. This difference is interpreted in terms of divergence of signal transduction pathways downstream from phyA and phyB.

Figure 1

Schematic representation of the hourly experimental protocol. In hypocotyl-growth and cotyledon-unfolding experiments, 1-d-old seedlings were exposed to the indicated protocol for 3 d. In gene-expression experiments, 2-d-old seedlings were exposed to the indicated protocol for 20 h and harvested 24 h later.

Figure 2

Kinetics of the effects of phyA and phyB on hypocotyl growth. Top, One-day-old WT and mutant seedlings were exposed for 3 d to continuous FR (hatched bars), continuous R (white bars), and darkness (black bars). Bottom, WT seedlings were exposed to hourly FR (•) or R treatments (○; terminated with an FR pulse to remove active phyB) of different durations. Note that 60 min h⁻¹ is equivalent to continuous light. Data are means ± se of at least 5 (top) or 11 (bottom) replicate boxes.

Figure 3

Kinetics of the effects of phyA and phyB on cotyledon unfolding. Top, One-day-old WT and mutant seedlings were exposed for 3 d to continuous FR (hatched bars), continuous R (white bars), and darkness (black bars). Bottom, WT seedlings were exposed to hourly FR (•) or R treatments (○; terminated with an FR pulse to remove active phyB) of different durations. Data are means ± se of at least 5 (top) or 11 (bottom) replicate boxes.

Figure 4

Effects of different hourly light treatments on hypocotyl growth and cotyledon unfolding. One-day-old seedlings were exposed for 3 d to darkness; to hourly cycles of 5 min of R, 3 min of FR, and 52 min of darkness; to continuous R; to hourly cycles of 8 min of FR and 52 min of darkness; to hourly cycles of 5 min of FR at a fluence rate 12-fold higher than that of continuous FR (to equal hourly fluence) and 55 min of darkness; or to continuous FR. Data are means ± se of at least four replicate boxes. Different letters indicate significant differences (P < 0.05).

Figure 5

Effects of different durations of the hourly FR treatment in the phyB mutant (left) compared with the effects of R in the phyA mutant (right). One-day-old seedlings were exposed for 3 d to the indicated light or dark conditions. Data are means ± se of five replicate boxes.

Figure 6

Kinetics of the effects of phyA and phyB on the activity of the Arabidopsis Lhcb1*2 promoter fused to the gene of GUS. Top, Two-day-old WT and mutant seedlings were exposed for 20 h to FR (hatched bars), R (white bars), and darkness (black bars). Bottom, WT seedlings were exposed to FR (hatched bars) or R treatments (white bars; terminated with a FR pulse to remove active phyB) of different durations. Data are means ± se of at least 5 (top) or 11 (bottom) replicate boxes. 4-MU, 4-Methylumbelliferone.