BiP Functions in Two Distinct Steps During The Fusion of Polar Nuclei.

Abstract

Female gametophytes of angiosperms have a central cell containing two polar nuclei. In many species including Arabidopsis thaliana, the polar nuclei fuse during female gametogenesis. We have shown that BiP, an Hsp70 in the endoplasmic reticulum (ER), is essential for the membrane fusion step of this process. Hsp70 requires partner proteins to complete its functions. Hsp40 co-chaperones are the functional partners for Hsp70. The ER of Arabidopsis has three soluble Hsp40, AtP58^IPK, AtERdj3A and AtERdj3B. We found that double mutant ovules lacking AtERdj3A and AtP58^IPK (3a p58) or AtERdj3B and AtP58^IPK (3b p58) were defective in the fusion of polar nuclei. Electron microscopy showed that polar nuclei were in close contact but no membrane fusion was observed in the 3a p58 ovule. In contrast, the outer nuclear membrane of the polar nuclei was appeared to be connected via the ER in the 3b p58 remaining the inner membrane unfused. These results suggest that different sets of ER-resident Hsp40 function as partners for BiP to promote different steps of the fusion of polar nuclei.