IMMUNOLOCALIZATION OF THE G-PROTEIN ALPHA-SUBUNIT ENCODED BY THE GPA1 GENE IN ARABIDOPSIS

Heterotrimeric GTP binding proteins (G proteins) are important signal transducers in lower eukaryotes and in animal cells. In plants, the occurrence of GTP binding proteins has been reported, but their biological function remains unclear. Two genes coding for G protein a subunits have been cloned: GPA1 in Arabidopsis and TGA1 in tomato. To gain some insights into the function of GPA1, we describe an extensive immunolocalization of GP alpha 1, the gene product of GPA1, during Arabidopsis development. Our results show that GP alpha 1 is present through all stages of development and in all organs examined, with the exception of mature seeds. It is expressed in roots, floral stem, rosette leaves, cauline leaves, flowers, and seedpods. Interestingly, the level of GP alpha 1 protein is higher in immature organs than in mature organs. GP alpha 1 is present at a high level in the root meristem and elongation zone, in the shoot and floral meristems, and in the leaf primordium and floral organ (sepal, petal, stamen, and gynoecium) primordia. During flower development, dividing microspores, but not mature pollen, show high levels of GP alpha 1. During pollination, GP alpha 1 is present in the growing pollen tubes. The protein is also present in nectaries and developing ovules and, after fertilization, in developing embryos. In mature tissue, GP alpha 1 is preferentially found in the vascular system but is also present in other cell types. The complexity of the GP alpha 1 localization pattern suggests that GP alpha 1 might be involved in different signaling pathways depending on the developmental stage.