Spatial and temporal expression of endosperm transfer cell-specific promoters in transgenic rice and barley

Two putative endosperm-specific rice genes, OsPR602 and OsPR9a, were identified from database searches. The promoter regions of these genes were isolated, and transcriptional promoter:beta-glucuronidase (GUS) fusion constructs were stably transformed into rice and barley. The GUS expression patterns revealed that these promoters were active in early grain development in both rice and barley, and showed strongest expression in endosperm transfer cells during the early stages of grain filling. The GUS expression was similar in both rice and barley, but, in barley, expression was exclusively in the endosperm transfer cells and differed in timing of activation relative to rice. In rice, both promoters showed activity not only in the endosperm transfer cells, but also in the transfer cells of maternal tissue and in several floral tissues shortly before pollination. The expression patterns of OsPR602 and OsPR9a in flowers differed. The similarity of expression in both rice and barley suggests that these promoters may be useful to control transgene expression in the transfer cells of cereal grains with the aim of altering nutrient uptake or enhancing the barrier against pathogens at the boundary between maternal tissue and the developing endosperm. However, the expression during floral development should be considered if the promoters are used in rice.