Characterization of phytochrome C functions in the control of de-etiolation and agronomic traits in rice

Although phytochrome A (phyA) and phyB have been functionally characterized, functions of phyC in rice growth and development have remained elusive because of the functional dependency of phyC on the phyB protein. In this study, we introduced PHYB(C364A), in which the chromophore attachment site cysteine 364 was converted to alanine, into the phyAphyB double mutant (aabb) and the phyAphyBphyC triple mutant (aabbcc) to produce PHYB(C364A)/aabb lines and PHYB(C364A)/aabbcc lines, respectively. PHYB(C364A)/aabbcc lines were insensitive to red light (R) and far-red light (FR), suggesting that PHYB(C364A) protein was biologically inactive. Functions of phyC were characterized using the PHYB(C364A)/aabb lines, without the functional interference of phyA or phyB. Phytochrome C responded to R and FR to trigger de-etiolation in the very-low-fluence response and low-fluence response in the PHYB(C364A)/aabb lines. Compared with the aabb mutant, seedlings of PHYB(C364A)/aabb lines showed higher chlorophyll content and reduced leaf angle. The PHYB (C364A)/aabb lines also showed a delayed heading date under long-day conditions. Phytochrome C-regulated agronomic traits were measured at the mature stage. The PHYB(C364A)/aabb lines showed significantly increased plant height, panicle length, grain number per main panicle, seed-setting rate, grain size, and grain weight, compared with those of the aabb mutant. Taken together, the present findings confirm that phyC perceives R and FR, and plays an important role in photomorphogenesis and yield determination in rice.