Role of RPT2 in Leaf Positioning and Flattening and a Possible Inhibition of phot2 Signaling by phot1

We investigated the roles of the blue light receptors phototropins (phot1 and phot2) and ROOT PHOTOTROPISM 2 (RPT2) in leaf positioning and flattening, and plant growth under weak, moderate and strong white light (10, 25 and 70 mu molm(-2) s(-1)). RPT2 mediated leaf positioning and flattening, and enhanced plant growth in a phot1-dependent manner. Under weak light, phot1 alone controls these responses. Under moderate and strong light, both phot1 and phot2 affect the responses. These results indicate that plants utilize a wide range of light intensities through phot1 and phot2 to optimize plant growth. The rpt2 single mutant generally exhibited phenotypes that resembled those of the phot1 phot2 double mutant. To our surprise, when the PHOT1 gene was disrupted in the rpt2 mutant, the resulting phot1 rpt2 double mutant showed the morphology of the wild-type plant under strong light, and additional disruption of PHOT2 in the double mutant abolished this recovery. This suggested that phot2 may function in the absence of phot1 and bypass RPT2 to transmit the signal to downstream elements. Expression and light-induced autophosphorylation of phot2 were not affected in the rpt2 mutant. We conclude that RPT2 mediates leaf flattening and positioning in a phot1-dependent manner, and that phot1 may inhibit the phot2 signaling pathways. We discuss the functional role of RPT2 in phototropin signaling.