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.internodal elongation ͉ mutant ͉ phytochrome ͉ photoreceptor

Days to heading 7 , a major quantitative locus determining photoperiod sensitivity and regional adaptation in rice

“…Further, dozens of upstream negative regulators of Ehd1 have been identified. Among them, OsphyB executes transcriptional suppression of Ehd1 expression to inhibit flowering under LDs (15,16). Grain number, plant height, and heading date 7 (Ghd7), encoding a CO, CO-like, TOC1 (CCT) domain protein, and Days to heading 8 (DTH8), encoding a putative HAP3 subunit of the CCAAT box-binding transcription factor, act as LD-specific repressors of Ehd1 (5, 6).…”

Arabidopsis thaliana life without phytochromes

“…Thus, seed germination is the first developmental transition blocked by phytochrome absence in Arabidopsis. In contrast, modern rice cultivars might have lost alleles that are important for seed dormancy (39), and the rice phyA phyB phyC triple mutant does not require exogenous GA for seed germination (3).…”

“…Five phytochrome apoprotein genes are present in the reference plant Arabidopsis thaliana (PHYA-PHYE) (1), each with partially overlapping functions (2). Conversely, only three phytochrome genes are present in rice (3). Phytochromes bear a covalently attached linear tetrapyrrol chromophore, the phytochromobilin, which undergoes a cis-trans isomerization when photoconverted (4).…”

Evolutionary Studies Illuminate the Structural-Functional Model of Plant Phytochromes

“…each photoreceptor, although few data are available to test the conservation of gene function in seed plants other than Arabidopsis. Nonetheless, the available data from Brassica, cucumber (Cucumis sativus), pea (Pisum sativum), tobacco (Nicotiana tabacum), tomato (Solanum lycopersicum), maize (Zea mays), and rice (Oryza sativa) suggest that the functions of phyA and phyB are generally conserved and were established prior to the split of eudicots and monocots (Ballaré et al, 1991;Childs et al, 1991;Devlin et al, 1992;Weller et al, 2004;Sheehan et al, 2007;Takano et al, 2005Takano et al, , 2009, although independently duplicated phyB may subdivide functions differently than is seen in Arabidopsis phyB and phyD (e.g., Hudson et al, 1997;Kerckhoffs et al, 1999;Weller et al, 2000;Sheehan et al, 2007). Functional data from dicots that diverge deeper than the eudicot/monocot split in the angiosperm phylogenetic tree are needed to infer that their functions are conserved in all angiosperms, but this is a reasonable hypothesis.…”