Biologists discover 'control centre' for sperm production
Issued by University of Leicester Press Office 1 February 2011
Biologists at the University of Leicester have published results of a new study into the intricacies of sex in flowering plants.
They have found that a gene in plants, called DUO1, acts as a master switch to ensure twin fertile sperm cells are made in each pollen grain.
The research identifies for the first time that DUO1 switches on a battery of genes that together govern sperm cell production and their ability to produce seeds.
The findings have implications for plant fertility, seed production – and could be used to help produce improved crops to help meet food shortages. This work also formed part of one of the author’s PhD thesis (Dr Michael Borg, University of Leicester).
The new study is reported in the journal The Plant Cell and was funded by the Biotechnology and Biological Sciences Research Council (BBSRC).
Professor David Twell and colleagues in the Department of Biology at the University of Leicester previously reported the discovery of a master regulator protein called DUO1 that has a critical role in allowing precursor reproductive cells to divide once to form twin sperm cells. The discovery of a battery of genes governed by DUO1 has shed light on the mechanisms by which plants control sperm cell formation and fertility.
Professor Twell said: “Unlike animals, flowering plants require not one, but two sperm cells for successful reproduction. These two sperm cells are housed within pollen grains, which act as a vehicle to deliver the sperm cells to the female sex cells within a flower.
“One sperm cell will join with the egg cell to produce the future plant or embryo, whilst the other will join with a second cell deep within the flower (the central cell) to produce a nutrient-rich tissue called the endosperm. Together these two structures make up the seeds and grains that form the staple food of humans and livestock across the globe.
“A mystery in this ‘double fertilisation’ event was how each pollen grain could produce the pair of sperm cells needed to make seeds. We now report that the regulatory gene DUO1 switches on a battery of genes that together govern sperm cell production and their ability to fuse with the egg and central cells. So in effect DUO1 acts as a master switch to ensure twin fertile sperm cells are made. “
Their new study expands on their previous work on pollen development and has identified a battery of new genes that collectively ensure male fertility in flowering plants.
The study of genes active within plant sperm is technically challenging because their sperm cells are not only tiny, but they are encased within tough pollen grains and as such are difficult to isolate. “We overcame this problem by genetically forcing plants to make DUO1 in plant roots, a place it is not normally found because DUO1 is normally restricted to sperm cells. By studying these genetically modified plants, we were able to survey the target genes switched on by DUO1.”
The researchers also report on the mechanism by which DUO1 switches on its target genes. Being a regulatory protein, DUO1 was shown to bind to short DNA sequences near the genes that it targets, which in turn allows DUO1 to control a wide variety of processes needed for sperm cell production.
“This work provides insight into the genetic mechanisms by which fertile gamete production is achieved in flowering plants. Such knowledge will also be helpful in devising strategies for the targeted manipulation of sperm cells, enabling plant breeders to control crossing behaviour in crop plants.” This work also provides new molecular tools for the manipulation of plant fertility and hybrid seed production as well the means to control gene flow in transgenic crops where the male contribution may need to be eliminated.
Professor Twell added that the study is timely given the challenges of breeding improved crops to meet the demands of food shortage and food price inflation the world is currently facing.
NOTES TO EDITORS:
FOR INTERVIEWS CONTACT PROFESSOR DAVID TWELL, DEPARTMENT OF BIOLOGY, UNIVERSITY OF LEICESTER ON 0116 252 2281 and email email@example.com
Full text of the study is available here.
The male germline in flowering plants arises through asymmetric division of a haploid microspore. Each divided microspore produces a large, non-germline vegetative cell and a single germ cell that divides once to produce the sperm cell pair required for double fertilisation. Despite the importance of sperm cells in plant reproduction, relatively little is known about the genetic hierarchy controlling plant sperm formation.
Here, we investigate the role of the Arabidopsis male germline-specific MYB protein DUO POLLEN1 (DUO1) as a positive regulator of male germline development. We show that DUO1 plays an essential role in sperm cell specification by activating a germline-specific differentiation programme. We show that ectopic expression of DUO1 in roots upregulates a significant number (~63) of germline-specific or enriched genes and validated 14 previously unknown DUO1 target genes by demonstrating DUO1-dependent promoter activity in the male germline. DUO1 is shown to directly regulate its target promoters through binding to canonical MYB sites, suggesting that the DUO1 target genes validated thus far are likely to be direct targets.
This work advances knowledge of the DUO1 regulon that encompasses genes with a range of cellular functions including transcription, protein fate, signalling and transport. Thus the DUO1 regulon has a major role in shaping the germline transcriptome and functions to commit progenitor germ cells to sperm cell differentiation.
TPC Image 1 - DUO1 expression in Arabidopsis roots
Pictured is an image of an Arabidopsis root showing expression of GFP-tagged DUO POLLEN 1 (DUO1) transcription factor along with a DIC image of the same root showing the different cell files. DUO1 is normally restricted to sperm cells because its expression has been shown to be germline-specific. It is normally technically challenging to study genes active within plant sperm cells because plant sperm are tiny and encased within tough pollen grains. Using these genetically modified plants, the researchers were able to survey the target genes switched on by DUO1.
Image Credit: Image generated by Lynette Brownfield (University of Leicester)
TPC Image 2 - Illustrating sperm cell-specific expression in Arabidopsis pollen
Pictured is a fluorescence image of an Arabidopsis pollen grain showing sperm cell-specific expression of a GFP-tagged marker of the IMPa-8 promoter along with a corresponding DAPI-stained image of the same pollen grain. The DAPI image shows the two densely-stained sperm cell nuclei and the diffusely-stained vegetative cell nuclei. In the GFP image, fluorescence is confined to the sperm cell nuclei, illustrating sperm cell-specific expression. The authors show that sperm cell specification of several genes, including IMPa-8, is regulated by the germline-specific transcription factor DUO POLLEN 1 (DUO1). Thus DUO1 has a major role in shaping the germline transcriptome and functions to commit progenitor germ cells to sperm cell differentiation.
Image Credit: Image generated by Hoda Khatab (University of Leicester)
BBSRC is the UK funding agency for research in the life sciences and the largest single public funder of agriculture and food-related research.
Sponsored by Government, in 2010/11 BBSRC is investing around £470 million in a wide range of research that makes a significant contribution to the quality of life in the UK and beyond and supports a number of important industrial stakeholders, including the agriculture, food, chemical, healthcare and pharmaceutical sectors.
BBSRC provides institute strategic research grants to the following:
The Babraham Institute, Institute for Animal Health, Institute for Biological, Environmental and Rural Studies (Aberystwyth University), Institute of Food Research, John Innes Centre, The Genome Analysis Centre, The Roslin Institute (University of Edinburgh) and Rothamsted Research.
The Institutes conduct long-term, mission-oriented research using specialist facilities. They have strong interactions with industry, Government departments and other end-users of their research.
For more information see: http://www.bbsrc.ac.uk