NUCLEAR FUSION DEFECTIVE1 Encodes the Arabidopsis RPL21M Protein and Is Required for Karyogamy during Female Gametophyte Development and Fertilization

Abstract

Karyogamy, or nuclear fusion, is essential for sexual reproduction. In angiosperms, karyogamy occurs three times: twice during double fertilization of the egg cell and the central cell and once during female gametophyte development when the two polar nuclei fuse to form the diploid central cell nucleus. The molecular mechanisms controlling karyogamy are poorly understood. We have identified nine female gametophyte mutants in Arabidopsis (Arabidopsis thaliana), nuclear fusion defective1 (nfd1) to nfd9, that are defective in fusion of the polar nuclei. In the nfd1 to nfd6 mutants, failure of fusion of the polar nuclei is the only defect detected during megagametogenesis. nfd1 is also affected in karyogamy during double fertilization. Using transmission electron microscopy, we showed that nfd1 nuclei fail to undergo fusion of the outer nuclear membranes. nfd1 contains a T-DNA insertion in RPL21M that is predicted to encode the mitochondrial 50S ribosomal subunit L21, and a wildtype copy of this gene rescues the mutant phenotype. Consistent with the predicted function of this gene, an NFD1-green fluorescent protein fusion protein localizes to mitochondria and the NFD1/RPL21M gene is expressed throughout the plant. The nfd3, nfd4, nfd5, and nfd6 mutants also contain T-DNA insertions in genes predicted to encode proteins that localize to mitochondria, suggesting a role for this organelle in nuclear fusion.

BiP-mediated polar nuclei fusion is essential for the regulation of endosperm nuclei proliferation in Arabidopsis thaliana

“…This result suggests that the function of BiP in promoting nuclear membrane fusion is conserved between yeasts and plants. Several A. thaliana mutants that are defective in the fusion of polar nuclei were also reported to be defective in other processes before fertilization (3,4,6). For example, pollen tube guidance into the ovule was affected in the maa mutants (3), and a role for the central cell in pollen tube guidance has been proposed (16).…”

“…The nfd1 mutant is defective in these nuclear fusion processes; sperm nuclei were found to persist within the egg and the central cell when nfd1/+ pistils were pollinated with wild-type pollen (6). To analyze sperm nuclear fusion in the bip mutants, we generated transgenic b1/+ b2/b2 plants producing histone H2B-GFP (17) in their female gametophytes and crossed with wild-type plants producing HTR10-monomeric red fluorescent protein 1 (mRFP1) (18) in their pollen.…”

“…Mutants affecting the fusion of polar nuclei have been isolated; however, little is known about the molecular mechanisms of this fusion process (3)(4)(5)(6). The mating of budding yeast involves one of the best-characterized nuclear fusion processes in eukaryotic cells.…”

Sperm entry is sufficient to trigger division of the central cell but the paternal genome is required for endosperm development in Arabidopsis

“…Failure of polar nuclei fusion during central cell differentiation can be associated with karyogamy failure but the mechanisms involved remain unclear (Maruyama et al, 2010;Portereiko et al, 2006). The fact that karyogamy with cdka;1 sperm fails preferentially in the central cell is intriguing.…”

Gene and Protein Expression Profiles in Rice Gametes and Zygotes: A Cue for Understanding the Mechanisms of Gametic and Early Zygotic Development in Angiosperms

“…Yeast mating is the most intensively investigated karyogamy event, and cytoskeleton-dependent nuclear congression and chaperone/ER-protein-dependent nuclear fusion have been well studied (Kurihara et al 1994 ;Melloy et al 2009 ;Tartakoff and Jaiswal 2009 ). However, in angiosperms, the mechanism in karyogamy is poorly understood, except that Bip, a chaperone in the lumen of endoplasmic reticulum, and NFD1, a component of the mitochondrial ribosome, function in polar nuclei fusion in Arabidopsis (Portereiko et al 2006 ;Maruyama et al 2010 ).…”