Ovule morphogenesis in Zea mays (Poaceae)

摘要

A comparative structural-histochemical study of the maize mutants pam1 and mac1, and cultivar "White night" has been realized. The ovule of Z. mays develops on the basal placenta. Periclinal divisions of the subepidermal placenta cells form the ovular primordium. The ovule is ortho-campylotropous, bitegmic, afunicular (placento-chalaza is formed). The postament, podium and nucellar cap are differentiated in the nucellus; sometimes separation of parietal cells or parietal tissue formation takes place. The hypostase is formed at the nucellus and integument base on the border of chalaza. A mature ovule consists of 2-3 layers of flattened cells with dense cytoplasm. The integuments of epidermal origin arise from the common initial zone, but their differentiation is separated in time. In both mutants and normal plants the formation of female generative organs is generally similar. A group of cells, namely the complex of initial cells (4-5 in the "White night" and pam1, up to 10-15 in mac1), is distinguished in the subepidermal layer of the primordium apical part. Some of the cells, located in centre of the complex, start differentiation as the archesporial cells (1-4 archesporial cells in pam1 and "White night", from 2-5 to 10-15 cells in mac1). In normal plants and pam1 one archesporial cell (in the centre of the group) transforms into the megasporocyte, in mac1 up 6-8 cells. The most of the megasporocytes in pam1 do not undergo the meiosis and stop at the prophase I, while surrounding ovule structures continue their development. In "White night" and mac1 meiosis occurs without defections. The megasporocytes of mac1 undergo meiosis asynchronously; different stages of megasporo- and megagametogenesis are observed in the same ovule. The embryo sac is formed by the polygonum-type, and additional embryo sacs of aposporial and diplosporial origin are found in mac1 mutants. A comparative study of protein and polysaccharide accumulation and utilization in the ovule of mac1, pam1 and "White night" has revealed similarity in space-time coordination of distribution of plastic substances. The differences are quantitative and determined by the number of sinks (megaspores, embryo sacs) using these substances.