TISSUE LOCALIZATION OF EXPANSINS IN DEEP-WATER RICE

Expansins are a family of proteins capable of inducing stress relaxati on of isolated cell walls. In earlier studies, we showed that the expr ession of expansin genes in deepwater rice (Oryza sativa L.) is regula ted by developmental, hormonal and environmental stimuli. Here, we des cribe the spatial distribution pattern of expansin transcripts and pro teins in tissues and organs of deepwater rice using in situ mRNA hybri dization and immunohistochemical analysis. Expansin transcripts and pr oteins are present at high levels in the growing internodal epidermis, which has thick cell walls and acts, therefore, as a growth-limiting cell layer. Expansins are also concentrated in the differentiating vas cular bundles of internodes. In the primary root, expansins are predom inantly expressed in the tip region, particularly in the epidermis, di fferentiating vascular cylinder, and around the pericyle. Developing a dventitious roots and lateral root primordia also contain high levels of expansin mRNA. In the shoot apex, expansin transcripts are abundant in the emerging leaf primordia. Our results indicate that expansins p lay an important role in the expansion and differentiation of plant ti ssues and organs.