The life cycle of plants involves the alternation between a diploid sporophytic generation and a haploid gametophytic generation. The female gametophyte (FG) in most angiosperms consists of just 7 cells, and is embedded within the ovule, but it produces the female gametes essential for plant reproduction. The initiation of the FG starts with formation of a single megaspore mother cell (MMC) that divides meiotically to produce four haploid megaspores in the nucellus of the ovule. One of these cells, called the functional megaspore (FM), survives and undergoes mitotic divisions, cellularization and cell specification to give rise to the mature FG containing one egg cell and one central cell. The restriction of one germ line precursor cell at each step leads to formation of a typical seed harboring a single embryo.
In this dissertation, I have summarized studies on multiple aspects of FG development: (1) FG cell types at maturity; (2) Cytokinin signaling as an important molecular regulator of FG development; (3) FG development at two key steps can be altered by manipulation of cytokinin levels.
Using diverse FG specific molecular reporters, in combination with confocal microscopy, we found the persistence of antipodal cells at FG mature stage, and even after fertilization, until the 16-nuclei endosperm stage of the developing seed. This result shows that the Arabidopsis female gametophyte at maturity should be considered as a seven-celled organism with four cell types, rather than the previously prevailing view of four-cells and three cell types at maturity.
We manipulated a cytokinin inactivation enzyme (CKX1, cytokinin oxidase1) within the female gametophyte and found that reduced CK signaling levels trigger the reprogramming of FG development at two critical developmental events. The first occurred at the FM formation stage. Excess FMs were specified, and further developed into twin mature FGs. The second was at the syncytial nuclear division stage of the developing FG, resulting in nuclei over-proliferation and supernumerary functional egg cell formation.
The two types of novel FG phenotypes revealed cytokinin regulation of two developmental checkpoints during germline formation, and illustrated potential developmental plasticity for manipulation of the reproductive lineage. Notably, both the aberrant FGs successfully underwent fertilization and formed viable twin embryos, showing possibilities for engineering plant reproduction to benefit agriculture. We further found that the haploid megaspore fates are regulated in a non-cell-autonomous manner, with the chalazal-most megaspore as the likely source of regulation. Formation of a single FM per ovule is a key characteristic shared by both gymnosperms and angiosperms for the sexual reproduction. Thus, the developmental mechanism identified in this study may be essential for evolution of this feature among all seed plants.
|Commitee:||Gasser, Charles, Liu, Bo|
|School:||University of California, Davis|
|School Location:||United States -- California|
|Source:||DAI-B 77/08(E), Dissertation Abstracts International|
|Subjects:||Genetics, Plant sciences, Developmental biology|
|Keywords:||Cytokinin, Excess functional megaspore, Non-autonomy, Reproduction, Twin embryo, Twin female gametophyte|
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