Like in animals, sexual reproduction in flowering plants relies upon the production of male (sperm) and female (eggs) gametes and their fusion upon fertilization. Despite the importance of plant fertility for seed production and agricultural productivity relatively little is known about the molecular processes underlying gamete development and function. Work in my lab is focused on using genetic and molecular approaches to characterize mutants that are defective in key stages of male gametophyte (pollen) development.
|An Arabidopsis pollen grain (male gametophyte) with a large vegetative cell (wall shown in red) containing a vegetative cell nucleus (vn) and two sperm cell nuclei (sn) labeled with a histone-GFP fusion protein.|
Polyploidy, the presence of more than two sets of chromosomes, has had a major impact upon the evolution of plants and the development of modern agricultural crop varieties. The major mechanism of polyploid formation is believed to be through the production of gametes that have not had their ploidy level reduced during meiosis and are thus termed unreduced. While mutants that produce unreduced gametes have been identified in a number of species, the underlying molecular mechanisms are largely unexplored.
I am investigating the mechanism through which a mutant, jason, makes a high frequency of unreduced male gametes in the model plant Arabidopsis thaliana. Meiosis in flowering plants is similar to that in animals and yeast in that it involves one round of DNA synthesis followed two rounds of chromosome separation to produce cells with half the chromosome content of the parent. In Arabidopsis, and many other flowing plants, meiotic cell division occurs once following both chromosome divisions. This means that the two chromosome divisions in meiosis II occur in a common cytoplasm. Mutations in jason result in defects in the positioning of the two meiosis II spindles relative to each other leading to the incorporation of two chromosome groups into a single cell during cytokinesis, and thus unreduced gametes. Current research involves looking at how JASON affects spindle position and the role of JASON homologs in other plant species.
|The second meiotic division in wild type (left) and jas (right). In wild type the two spindles(green, labeled with GFP-tubulin) with aligned chromosomes (red, stained with DAPI) are wellseparated, while in the jas mutant two spindle poles are in close physical proximity.|
In flowering plants the male germ line is not formed until late in development when a haploid microspore undergoes a highly asymmetric division. This forms a large vegetative cell and a smaller germ cell which is engulfed within the cytoplasm of the vegetative cell. The germ line then begins to express proteins required for sperm cell function and undergoes a single mitotic division. The MYB transcription factors, DUO1 and DUO3, are required for germ line specification and division. I am now interested in further exploring the mechanism through which these protein influence germ cell specification and, in particular, how DUO1 expression is limited to the germ cell.
Enquires about projects from prospective graduate students and postdoctoral fellows are welcome. Information about scholarships for New Zealand postgraduate students go to the University of Otago's website - Postgraduate & Scholarship Awards.
Benjamin Peters, Jonathan Casey, Jack Aidley, Stuart Zohrab, Michael Borg, David Twell, and Lynette Brownfield., A cis-regulatory module in the transcription factor DUO1 promoter., Plant Physiol 2016 p. pp.01192.2016., Link »
Andrew G Cridge, Peter K Dearden, and Lynette R Brownfield., Convergent occurrence of the developmental hourglass in plant and animal embryogenesis?, Ann Bot 2016 p. mcw024., Link »
Lynette Brownfield and David Twell, Plant Reproduction, Plant Reproduction John Wiley & Sons, Ltd, Chichester, UK., Link »
Lynette Brownfield, Jun Yi, Hua Jiang, Elena A Minina, David Twell, and Claudia Köhler., Organelles maintain spindle position in plant meiosis., Nat Commun 2015 vol. 6 p. 6492., Link »
Borg, M. *, Brownfield, L. *, Khatab, H., Sidorova, A., Lingaya, M. and Twell, D. (2011) The R2R3 MYB transcription factor DUO1 activates a male germline-specific regulon essential for sperm cell differentiation in Arabidopsis. Plant Cell, 23:534-549. * equal first author
Brownfield, L. and Köhler, C. (2011). Unreduced gamete formation in plants: mechanisms and prospects. Journal of Experimental Botany, 62:1659-1668.
Erilova, E., Brownfield, L., Exner, V., Rosa, M., Twell, D., Mittelsten Scheid, O., Hennig, L. and Köhler, C. (2009) Imprinting of the Polycomb Group gene MEDEA serves as a ploidy sensor in Arabidopsis. PLoS Genetics, 5:e1000663.
Brownfield, L. and Twell, T. (2009) A dynamic DUO of regulatory proteins coordinates gamete specification and germ cell mitosis in the angiosperm male germline. Plant Signaling and Behavior, 4:article 9950.
Brownfield, L., Hafidh, S., Durbarry, A., Khatab, H., Sidorova, A., Doerner, P. and Twell, T. (2009) Arabidopsis DUO POLLEN3 is a key regulator of male germline development and embryogenesis. Plant Cell, 21:1940-1956.
Borg, M., Brownfield, L. and Twell, D. (2009) Male gametophyte development; a molecular perspective. Journal of Experimental Botany, 60:1465-78.
Brownfield, L., Hafidh, S., Borg, M., Sidorova, A., Mori, T. and Twell, D. (2009) A plant germline-specific integrator of sperm specification and cell cycle progression. PLoS Genetics, 5:e1000430.
Kim, H.J., Oh, S-A., Brownfield, L., Ryu, H., Hwang, I., Twell, D. and Nam, H-G. (2008) Control of plant male germ cell division by SCFFBL17 mediated degradation of cell cycle inhibitors. Nature, 455:1134-1137.
Brownfield, L., Wilson, S., Newbigin, E., Bacic, A. and Read, S. (2008) Molecular regulation of the glucan synthase-like protein NaGSL1 and callose synthesis during growth of Nicotiana alata pollen tubes. Biochemical Journal, 414:43-52.
Töller, A*., Brownfield, L.*, Neu, C., Twell, D. and Schulze-Lefert, P. (2008) Dual function of Arabidopsis glucan synthase-like genes GSL8 and GSL10 in male gametophyte development and plant growth. Plant Journal, 54:911-923. * equal first author
Brownfield, L., Ford, K., Doblin, M.S., Newbigin, E., Read, S. and Bacic, A. (2007) Proteomic and biochemical evidence links the callose synthase in Nicotiana alata pollen tubes to the product of the NaGSL1 gene. Plant Journal, 52:147-156.