Genome-wide expression profiling and identification of genes under the control of the DROOPING LEAF gene during midrib development in rice.

The DROOPING LEAF (DL) gene has an important function in rice development. Loss-of-function mutation of DL fails to form midrib, a strong structure formed in the central region of the leaf, resulting in a drooping leaf phenotype. In addition, severe mutation in DL causes a floral homeotic change, i.e., replacement of the carpels by the stamens. Thus, DL regulates midrib formation in the leaf and carpel specification in the flower. DL encodes a putative transcription factor that belongs to the YABBY family, suggesting that it plays an essential role in these developmental processes by regulating many downstream genes, either directly or indirectly. As a first step to elucidate genetic networks involved in midrib formation, in this paper, we attempt to identify genes regulated by DL in the initial processes of midrib development. By comparing expression profiles in the shoot apex that involves the leaf primordia, we found 90 genes that are upregulated at least two-fold higher in the transgenic plants overexpressing DL (Act1:DL) than in the null dl mutant (dl-sup1), suggesting that these genes are involved in midrib formation under the control of DL. Subsequent RT-PCR analysis of arbitrarily chosen 18 genes suggests that some genes may be involved in only midrib formation, whereas others may be involved in both midrib formation and carpel development.