Effects of somatic cell fusion on zygotic development in rice

In angiosperms, a zygote is the initial totipotent cell that develops into an embryo. To reveal whether and how genomic and cytoplasmic contents from different cell types affect zygotic development, we analyzed developmental profiles of zygote-soma hybrid cells produced by rice in vitro fertilization system. When a leaf protoplast (2x) was fused with a zygote (fusion product between egg and sperm cells; 2x), the development of most of the leaf protoplast-fused zygotes with a 1:1 gamete:soma ratio was arrested at the one-cell stage. By contrast, when a leaf protoplast (2x) was fused with a polyploid zygote (3x or 4x), several fractions of the leaf protoplast-fused polyploid zygotes continued to develop. Furthermore, when a leaf protoplast (1x) isolated from a haploid rice plant was used for the fusion, more than half of the haploid leaf protoplast-fused zygotes developed into globular embryo-like structures and cell masses. Several of these calli then regenerated into plantlets, with the expected ploidy levels. Accordingly, leaf and zygote nuclei were integrated into the hybrid nucleus in zygote-soma hybrid cells. The resulting hybrid genome was maintained during the hybrid cell development and regeneration. These findings suggest that developmental profiles of zygote-soma hybrid cells are affected by the gamete:soma ratio. Furthermore, the hybrid cell fate shifts to the zygotic mode when the gamete:soma ratio changes from 1:1 to 2:1.