Ovule Development in Higher Plants

小行星9304998 植物生活史的一个独特方面是二倍体孢子体和单倍体配子体之间的世代交替。 在高等植物中，孢子体是优势世代，雄配子体和雌配子体的大小和复杂性大大减少，都依赖于孢子体。雌配子体在胚珠中形成，胚珠由特化组织、珠被和珠心组成。珠心产生大孢子母细胞，大孢子母细胞减数分裂形成四个单倍体大孢子。三个大孢子退化，而与珠心相邻的大孢子生长并成为雌配子体，胚囊。在胚囊内，由有丝分裂产生的单倍体核迁移到不同的细胞质位置，形成结构和功能不同的细胞，包括卵细胞。胚珠珠被包围在珠心和胚囊周围，提供营养并在受精过程中形成花粉管的开口。虽然大量的信息胚珠和雌配子体形态是可用的，控制其发展的分子机制是未知的。它们的发育协调的程度，胚珠和配子体的显著极性是如何产生的，以及它们的细胞组织对胚胎随后发育的影响，还不清楚。Fischer博士将用来解决这些问题的策略包括研究拟南芥中降低雌性生育力的突变。他在通过插入根癌农杆菌的T-DNA诱变的品系中分离出胚珠突变体。突变表型包括珠被不能启动、珠被发育异常、珠心内细胞增殖异常、所有孢子退化、胚囊形成抑制和胚败育。这些突变体的分子遗传学分析将提供有关珠被起始和大孢子发生的早期事件的信息，珠被对胚囊发育的作用，珠心和胚囊之间的竞争，胚珠内的生长，以及母体对胚胎发生的影响。 这些实验的意义在于，它们将确定调控胚珠和配子体发育的基因和过程，并将提供对单倍体和二倍体世代之间相互作用的深入了解。此外，它们将促进我们对细胞和组织如何分化和交流的理解，并可能导致有关高等植物胚胎起源过程的新概念信息。 ***

9304998 Fischer A unique aspect of the plant life cycle is the alternation of generations between diploid sporophytes and haploid gametophytes. In higher plants, the sporophyte is the dominant generation, and the male and female gametophytes, greatly reduced in size and complexity, are dependent upon the sporophyte. The female gametophyte is formed in the ovule that is composed of specialized tissues, integuments and nucellus. The nucellus gives rise to a megasporocyte that undergoes meiosis to form four haploid megaspores. Three megaspores degenerate, whereas the megaspore adjacent to the nucellus grows and becomes the female gametophyte, the embryo sac. Within the embryo sac, haploid nuclei generated by mitotic divisions migrate to different cytoplasmic positions and form structurally and functionally distinct cells, including the egg cell. Ovule integuments surround the nucellus and embryo sac, providing nutrients and form the opening for the pollen tube during fertilization. Although a wealth of information about ovule and female gametophyte morphology is available, the molecular mechanisms that control their development are not known. The extent to which their development is coordinated, how the striking polarity of the ovule and gametophyte is generated, and the effect of their cellular organization on subsequent development of the embryo, is not understood. The strategy Dr. Fischer will use to address these questions involves studying mutations in Arabidopsis thaliana that reduce female fertility. He has isolated ovule mutants among lines mutagenized by insertion of the T-DNA from Agrobacterium tumefaciens. Mutant phenotypes include failure to initiate integuments, abnormal integument development, aberrant cell proliferation within the nucellus, degeneration of all spores, repression of embryo sac formation, and embryo abortion. Molecular genetic analysis of these mutants will provide information about early events involving integument initiation and megasporogenesis, the role of integuments on embryo sac development, competition between the nucellus and embryo sac for growth within the ovule, and maternal effects on embryogenesis. %%% The significance of these experiments is that they will identify genes and processes that regulate ovule and gametophyte development and will provide insight into interactions between the haploid and diploid generations. Moreover, they will advance our understanding about how cells and tissues differentiate and communicate, and may result in new conceptual information about processes leading to the inception of the embryo in higher plants. ***