rha1, a gene encoding a small GTP binding protein from Arabidopsis, is expressed primarily in developing guard cells.

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Abstract
The rha1 gene from Arabidopsis encodes a small GTP binding protein belonging 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 guard cells of stomata, the stipules, and the root tip of young plants. In flowering plants, expression was found predominantly in the receptacle and in guard cells of the different flower organs. High GUS activity could also be seen in callus tissue and developing seeds. No detectable activity was present in other plant tissues; activity could not be induced by various treatments. GUS activity was visualized histochemically using both 5-bromo-4-chloro-3-indolyl beta-D-glucuronide and a newly developed GUS substrate: Sudan II-beta-glucuronide. The latter precipitates as red crystals at the site of GUS activity. Results obtained by the gus analysis were confirmed by whole-mount mRNA in situ hybridization. A hypothesis for the function of the Rha1 protein is discussed.