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

Abstract

SUMMARYFertilization in flowering plants involves two sperm cells and two female gametes, the egg cell and the central cell, progenitors of the embryo and the endosperm, respectively. The mechanisms triggering zygotic development are unknown and whether both parental genomes are required for zygotic development is unclear. In Arabidopsis, previous studies reported that loss-of-function mutations in CYCLIN DEPENDENT KINASE A1 (CDKA;1) impedes cell cycle progression in the pollen leading to the production of a single sperm cell. Here, we report that a significant proportion of single cdka;1 pollen delivers two sperm cells, leading to a new assessment of the cdka;1 phenotype. We performed fertilization of wild-type ovules with cdka;1 mutant sperm cells and monitored in vivo the fusion of the male and female nuclei using fluorescent markers. When a single cdka;1 sperm was delivered, either female gamete could be fertilized leading to similar proportions of seeds containing either a single endosperm or a single embryo. When two cdka;1 sperm cells were released, they fused to each female gamete. Embryogenesis was initiated but the fusion between the nuclei of the sperm cell and the central cell failed. The failure of karyogamy in the central cell prevented incorporation of the paternal genome, impaired endosperm development and caused seed abortion. Our results thus support that the paternal genome plays an essential role during early seed development. However, sperm entry was sufficient to trigger central cell mitotic division, suggesting the existence of signaling events associated with sperm cell fusion with female gametes.

Loss of centromeric histone H3 (CENH3) from centromeres precedes uniparental chromosome elimination in interspecific barley hybrids

“…On the other hand, a reduced CENH3 amount can trigger centromere inactivity in neocentromeres of maize (64). Nevertheless, if a zygotic resetting of CENH3, as demonstrated for GFPtagged CENH3 in A. thaliana (65,66), also exists in grasses, active removal of both parental CENH3s before the first zygote division would occur and the reactivation of H. bulbosum centromeres in unstable hybrids would be diminished.…”

The Emerging Importance of Type I MADS Box Transcription Factors for Plant Reproduction

“…These authors compared genome-wide transcription profiles of seeds fertilized with the wild type or a mutant in the cell cycle regulator CYCLIN DEPENDENT KINASE A;1 (CKDA;1) (Iwakawa et al, 2006;Nowack et al, 2006). In cdka;1 mutant pollen, the second mitosis is missing or severely delayed, and these pollen can successfully fertilize the egg cell, whereas karyogamy does not take place in the central cell (Nowack et al, 2006;Aw et al, 2010). Although not properly fertilized, the majority of the central cells in cdka;1 fertilized ovules are triggered to initiate endosperm proliferation.…”

Live and let die: a REM complex promotes fertilization through synergid cell death in Arabidopsis

“…To confirm these results, we used pollen from plants carrying the HTR10-RFP transgene, which marks the two sperm nuclei, to visualize the location of the sperm cells in the two mutants and to enable gametic fusion to be monitored (Aw et al, 2010;Hamamura et al, 2011). Importantly, in vdd-1/+ (Fig.…”

“…The Arabidopsis stk and stk, shp1, shp2 mutants were provided by M. Yanofsky (Pinyopich et al, 2003). The transgenic lines: pGFA2::GUS and pMYB98::GFP were provided by Prof. Gary Drews (Kasahara et al, 2005;Christensen et al, 2002); HTR10_RFP was provided by Frederic Berger (Aw et al, 2010); FER_GFP by Prof. Ueli Grossniklaus and LAT52::GUS by Prof. Ravi Palanivelu.…”