Phytochromes are the sole photoreceptors for perceiving red/far-red light in rice

Abstract

Phytochromes are believed to be solely responsible for red and far-red light perception, but this has never been definitively tested. To directly address this hypothesis, a phytochrome triple mutant (phyAphyBphyC) was generated in rice (Oryza sativa L. cv. Nipponbare) and its responses to red and far-red light were monitored. Since rice only has three phytochrome genes (PHYA, PHYB and PHYC), this mutant is completely lacking any phytochrome. Rice seedlings grown in the dark develop long coleoptiles while undergoing regular circumnutation. The phytochrome triple mutants also show this characteristic skotomorphogenesis, even under continuous red or far-red light. The morphology of the triple mutant seedlings grown under red or far-red light appears completely the same as etiolated seedlings, and they show no expression of the light-induced genes. This is direct evidence demonstrating that phytochromes are the sole photoreceptors for perceiving red and far-red light, at least during rice seedling establishment. Furthermore, the shape of the triple mutant plants was dramatically altered. Most remarkably, triple mutants extend their internodes even during the vegetative growth stage, which is a time during which wild-type rice plants never elongate their internodes. The triple mutants also flowered very early under long day conditions and set very few seeds due to incomplete male sterility. These data indicate that phytochromes play an important role in maximizing photosynthetic abilities during the vegetative growth stage in rice.

Fig. 1.

Traces of the shoot-top movement of growing rice seedlings. Five lines of rice seedlings were grown in the monitoring system either in the dark (Left of each pair, labeled “Dark”) or under the continuous red light (Right of each pair, labeled “Red”) for 1 week. Only the top point pixels of the growing seedlings were extracted from each individual time-lapse image and overlaid onto the final picture. Orange lines, movement of coleoptile tops; green lines, movement of the first/second leaf tops. WT, Nipponbare; phyB, phyB-1 mutant; phyBC, phyB-1phyC-1 mutant; phyABC, phyA-4phyB-1phyC-1 mutant; se5, se5 mutant.

Fig. 2.

Expression of light-induced genes in the triple mutants. Total RNA was extracted from the 1-week-old seedlings used in the experiments shown in Fig. 1. The expression of the LHCB and RBCS genes was examined by northern hybridization. D, dark-grown; Rc, grown under Rc; WT, Nipponbare; B, phyB-1 mutant; BC, phyB-1phyC-1 mutant; ABC, phyA-4phyB-1phyC-1 mutant.

Fig. 3.

Distinct shape of triple mutant juvenile plants. (A) The appearance of a WT and a triple mutant plant (Center, Left and Right, respectively) grown under long day conditions for 1 month. Left and Right are anatomical displays of the individual plants in the Center, respectively, in which all leaves were separated from the stem at the nodal portions. (B) The growing patterns of each portion of the WT and triple mutant plant during the first 3 weeks after germination. Green lines, leaf blade; blue, leaf sheath; orange, coleoptile; purple, internode. The mean ± SE, n = 3 to 5.

Fig. 4.

Distinctive features of the adult triple mutant plants. (A) From the Lower, lengths of panicle (P) and internodes (1∼7, numbered beginning at the Top) of WT, triple mutant and se5 mutant after heading is completed. WT, Nipponbare; phyABC, phyA-4phyB-1phyC-1 mutant; se5, se5 mutant. The mean ± SE obtained from 20 plants is displayed. Leaf lengths of Nipponbare (B), phyA-4phyB-1phyC-1 mutant (C), and se5 mutant (D). Gray bars, leaf sheath; open bars, leaf blade. The mean ± SE obtained from 20 plants is displayed. Leaves are numbered beginning at the Top (No. 1 is a flag leaf).