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

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.