The Phytochrome B/Phytochrome C Heterodimer Is Necessary for Phytochrome C-Mediated Responses in Rice Seedlings

<div><p>Background</p><p>PhyC levels have been observed to be markedly lower in <i>phyB</i> mutants than in <i>Arabidopsis</i> or rice wild type etiolated seedlings, but the mechanism of this phenomenon has not been fully elucidated.</p><p>Results</p><p>In the present study, we investigated the mechanism by which phyB affects the protein concentration and photo-sensing abilities of phyC and demonstrated that rice phyC exists predominantly as phyB/phyC heterodimers in etiolated seedlings. PHYC-GFP protein was detected when expressed in <i>phyA phyC</i> mutants, but not in <i>phyA phyB</i> mutants, suggesting that phyC requires phyB for its photo-sensing abilities. Interestingly, when a mutant <i>PHYB</i> gene that has no chromophore binding site, <i>PHYB</i>(<i>C364A</i>), was introduced into <i>phyB</i> mutants, the phyC level was restored. Moreover, when <i>PHYB(C364A)</i> was introduced into <i>phyA phyB</i> mutants, the seedlings exhibited de-etiolation under both far-red light (FR) and red light (R) conditions, while the <i>phyA phyB</i> mutants were blind to both FR and R. These results are the first direct evidence that phyC is responsible for regulating seedling de-etiolation under both FR and R. These findings also suggest that phyB is indispensable for the expression and function of phyC, which depends on the formation of phyB/phyC heterodimers.</p><p>Significance</p><p>The present report clearly demonstrates the similarities and differences in the properties of phyC between <i>Arabidopsis</i> and rice and will advance our understanding of phytochrome functions in monocots and dicots.</p>