State models of phyC and phyB in rice seedlings.

<p><b>A.</b> In the cells of WT seedlings grown in the dark, most of the phyC exists as phyB/phyC heterodimers (B/C) and, probably, to a smaller degree as phyC monomers (C?), while phyB exist as phyB/phyC heterodimers (B/C) and phyB/phyB homodimers (B/B). In etiolated seedlings, phyB stabilizes phyC in the B/C conformation. Consequently, phyC levels are quite low in the <i>phyB</i>-deficient mutants. <b>B.</b> When seedlings are exposed to light, including W, R, and FR, phyC subunits in phyB/phyC heterodimers (B/C) are light-labile and biologically active to participate in the multiple processes of rice development (inhibition of coleoptile growth, induction of light-regulated genes, and chlorophyll accumulation). By contrast, phyC monomers (C) are light-stable but do not participate in the de-etiolation of rice seedlings. phyB in its heterodimeric (B/C) and homodimeric (B/B) forms probably play different roles in the responses to light.