A new study using high-resolution imaging during wheat sperm cell development reveals the way that chloroplasts are passed from one generation to another via only the maternal parent.
Chloroplasts are important structures in plant cells that perform photosynthesis and mature from small precursors called plastids. Wheat, like many other plants, inherit their chloroplasts only from their mother via the egg cell. However, the mechanism that leads to this was not known. Scientists at Rothamsted Research and colleagues at the University of Manchester labelled plastids in wheat with a green fluorescent protein (GFP) and observed them in developing pollen grains. They show for the first time that plastids are degraded in mature sperm cells just prior to fertilization.
All chloroplasts present in a plant arise, by division, from the plastids originally inherited from the egg and/or sperm cell at fertilization depending on the plant species. It has been hypothesised that in wheat this occurs by ‘cytoplasmic stripping’ at fertilization, whereby all the cellular material except the DNA of the sperm cell stays outside the egg cell at the point of fertilisation.
Dr Anil Day, University of Manchester explained: “This is very interesting work and provides a possible explanation for something that has remained a matter of open discussion in the community. We have shown that plastids are actually degraded in mature sperm cells just prior to fertilization rather than simply being stripped away with the rest of the cytoplasm.”
Dr Lucia Primavesi, Rothamsted Research scientist who did the detailed microscopy work said: “We labelled the plastids with GFP and observed them in developing pollen grains. We saw labelled plastids in immature sperm cells but they were undetectable in mature sperm cells. Instead, we observed green fluorescence from a differently shaped structure that we speculate is a candidate for an organelle involved in the terminal degradation of plastids in pollen, possibly a lytic vacuole, in other words a compartment that contains molecules that degrade biological material in cells.”
Professor Huw Jones, senior investigator at Rothamsted Research, added: “In contrast to chromosomes of sperm and egg cells that combine during sexual reproduction, the DNA in specialised small organelles in the cell such as mitochondria and chloroplasts is almost exclusively inherited from only one parent or the other. This fundamental observation applies to both plants and animals (including humans). There is considerable uncertainty over the evolutionary pressures that have led to this situation and over the precise methods that various species use to achieve it. This paper finally elucidates the mechanism for wheat chloroplasts and offers opportunities to exploit this understanding in future plant breeding”.
Publication Primavesi et al., 2016. Visualisation of plastid degradation in sperm cells of wheat pollen. Protoplasm - See more at: http://www.rothamsted.ac.uk/news-views/unravelling-mechanism-chloroplast...