RHA1, A GENE ENCODING A SMALL GTP-BINDING PROTEIN FROM ARABIDOPSIS, IS EXPRESSED PRIMARILY IN DEVELOPING GUARD-CELLS

The rha1 gene from Arabidopsis encodes a small GTP binding protein bel onging to the Ypt/Rab family. Transgenic Arabidopsis plants containing the promoter region of the rha1 gene fused to the beta-glucuronidase (gus) reporter gene revealed gus expression limited mainly to the guar d cells of stomata, the stipules, and the root tip of young plants. In flowering plants, expression was found predominantly in the receptacl e and in guard cells of the different flower organs. High GUS activity could also be seen in callus tissue and developing seeds. No detectab le activity was present in other plant tissues; activity could not be induced by various treatments. GUS activity was visualized histochemic ally using both 5-bromo-4-chloro-3-indolyl beta-D-glucuronide and a ne wly developed GUS substrate: Sudan II-beta-glucuronide, The latter pre cipitates as red crystals at the site of GUS activity. Results obtaine d by the gus analysis were confirmed by wholemount mRNA in situ hybrid ization. A hypothesis for the function of the Rha1 protein is discusse d.