Gene expression in the dimorphic sperm cells of Plumbago zeylanica: transcript profiling, diversity, and relationship to cell type

Plumbago zeylanica produces cytoplasmically dimorphic sperm cells that target the egg and central cell during fertilization. In mature pollen, the larger sperm cell contains numerous mitochondria, is associated with the vegetative nucleus (S-vn), and fuses preferentially with the central cell, forming endosperm. The other, plastid-enriched sperm cell (S-ua) fuses with the egg cell, forming the zygote and embryo. Sperm expressed genes were investigated using ESTs produced from each sperm type; differential expression was validated through suppression subtractive hybridization, custom microarrays, real-time RT-PCR and in situ hybridization. The expression profiles of dimorphic sperm cells reflect a diverse and broad complement of genes, including high proportions of conserved and unknown genes, as well as distinct patterns of expression. A number of genes were highly up-regulated in the male germ line, including some genes that were differentially expressed in either the S-ua or the S-vn. Differentially up-regulated genes in the egg-targeted S-ua showed increased expression in transcription and translation categories, whereas the central cell-targeted S-vn displayed expanded expression in the hormone biosynthesis category. Interestingly, the up-regulated genes expressed in the sperm cells appeared to reflect the expected post-fusion profiles of the future embryo and endosperm. As sperm cytoplasm is known to be transmitted during fertilization in this plant, sperm-contributed mRNAs are probably transported during fertilization, which could influence early embryo and endosperm development.