A small subfamily of Arabidopsis RADIALIS-LIKE SANT/MYB genes: a link to HOOKLESS1-mediated signal transduction during early morphogenesis.

More than 1,600 genes encoding transcription factors have been identified in the Arabidopsis genome sequence, but their physiological functions are not yet fully understood. In this study, a small subfamily of single-MYB transcription factor genes, designated RSM1, RSM2, RSM3 and RSM4 (RADIALIS-LIKE SANT/MYB 1-4), was characterized. Here, we mainly examined the RSM1 gene, and found that it appears to play a role in close connection with the HLS1 (HOOKLESS 1) gene during early morphogenesis. Etiolated seedlings overexpressing RSM1 showed several phenotypes similar to those of hls1-1, viz., lack of apical hooks with short hypocotyls, and a defect in gravitropism. Furthermore, both RSM1-ox and hls1-1 seedlings were hypersensitive to red light during early photomorphogenesis, displaying shorter hypocotyls than wild-type seedlings. The histochemical profile of the DR5::GUS auxin-reporter in the RMS1-ox seedlings was considerably different from that in the wild-type seedlings. These results are discussed in the context of the possibility that RSM1 is implicated in HLS1-mediated auxin signaling, which is responsible for regulation of the early photomorphogenesis of Arabidopsis thaliana.