Analyses of the Hsp70 System in the Endoplasmic Reticulum Involved in Female Gametogenesis in Arabidopsis thaliana.

Abstract

BiP is an Hsp70 in the endoplasmic reticulum (ER), and Arabidopsis thaliana has three BiP genes, BiP1, BiP2 and BiP3. We found that simultaneous deletion of the BiP1 and BiP2 genes resulted in defect in the polar nuclei fusion during female gametophyte formation. Transmission electron microscopy revealed that the defect was at the nuclear membrane fusion step. Since BiP has shown to function in the nuclear membrane fusion during mating in budding yeast, our results suggests that the BiP-mediated nuclear fusion is conserved from yeast to plant. In yeast, two J domain-containing co-chaperones (J proteins) in the ER, Sec63p and Jem1p, function in nuclear membrane fusion during mating as partners for BiP. While Arabidopsis has two orthologs of the SEC63 and one ortholog of JEM1, single mutants of these J protein genes did not show female gametophyte formation defect. We are now analyzing double mutants of these genes.