ABSTRACT
Megasporogenesis, megagametogenesis and embryogenesis of Liparis elliptica (family Orchidaceae, tribe Malaxideae, subtribe Malaxidinae) have been studied. It was shown that the L. elliptica embryo sac is monosporic and develops from the chalazal cell of the megaspore triad according to the modified Polygonum type. The embryo sacs are reduced to four-six nuclei. The suspensor is unicellular, spherical in shape, originating from the basal cell (cb). A unique feature of L. elliptica is the unitegmal ovule, which distinguishes this species from other members of the tribe Malaxideae. The seed coat is formed by an outer layer of the single internal integument. Reduction of the outer integument is a rare feature for epiphytic orchid species with photosynthetic leaves.
Subject(s)
Gametogenesis, Plant , Orchidaceae , Ovule , Seeds , Embryonic DevelopmentABSTRACT
Embryological features were used for the first time as a taxonomic attribute to confirm the difference between closely related taxa in the subtribe Malaxidinae. It was shown that the branched shape of the suspensor in Liparis elegans and L. parviflora from the section Blepharoglossum distinguishes these species from the inflated suspensor without a neck, which is characteristic of the embryos of L. viridiflora and L. dendrochiloides from the section Cestichis. Differentiation of the development and shape of the suspensor is an additional embryological criterion in favor of separating the genus Blepharoglossum from the genus Liparis.
Subject(s)
Orchidaceae , Seeds , Embryonic DevelopmentABSTRACT
Maxillaria crassifolia (Lindl.) Rchb.f. belongs to the polyphyletic genus Maxillaria Ruiz & Pav., which currently is the subject of several taxonomic research. There are conflicting descriptions of megasporogenesis, megagametogenesis, and embryogenesis in orchids from the tribe Cymbidieae, in general, and in the genus Maxillaria, in particular. In the present report, all stages of embryonic development of M. crassifolia were examined using confocal fluorescence microscopy. Some features of the development of the ovule and embryo, which distinguish M. crassifolia from other species of the tribe Cymbidieae were identified. The T-shaped arrangement of megaspores is formed by dividing the micropylar megaspore of the dyad. The megagametophyte develops according to the modified Polygonum-type with an unstable number of nuclei in the embryo sacs. The nucleus of the central cell varies in composition and may include unfused micropylar and chalazal nuclei and daughter nuclei formed during their division. The sequence of embryonal divisions is strictly structured. A special variant of embryogenesis, the Cymbidium-type Maxillaria-variant, has been described. Its characteristic features are the strictly apical nature of embryonic divisions, the absence of basal cell (cb) division, the formation of one to three pairs of tubular suspensor cells, and the localization of all suspensor cells within the inner integument.
Subject(s)
Orchidaceae , Embryonic Development , Flowers , Gametogenesis, Plant , OvuleABSTRACT
The orchid reproductive strategy, including the formation of numerous tiny seeds, is achieved by the elimination of some stages in the early plant embryogenesis. In this study, we documented in detail the formation of the maternal tissues (the nucellus and integuments), the structures of female gametophyte (megaspores, chalazal nuclei, synergids, polar nuclei), and embryonic structures in Dendrobium nobile. The ovary is unilocular, and the ovule primordia are formed in the placenta before the pollination. The ovule is medionucellate: the two-cell postament and two rows of nucellar cells persist until the death of the inner integument. A monosporic eight-nucleated embryo sac is developed. After the fertilization, the most common central cell nucleus consisted of two joined but not fused polar nuclei. The embryogenesis of D. nobile is similar to the Caryophyllad-type, and it is characterized by the formation of all embryo cells from the apical cell (ca) of a two-celled proembryo. The only exception is that there is no formation of the radicle and/or cotyledons. The basal cell (cb) does not divide during the embryogenesis, gradually transforming into the uninuclear suspensor. Then the suspensor goes through three main stages: it starts with an unbranched cell within the embryo sac, followed by a branched stage growing into the integuments, and it ends with the cell death. The stage-specific development of the female gametophyte and embryo of D. nobile is discussed.
Subject(s)
Dendrobium/chemistry , Embryonic Development/genetics , Gametogenesis, Plant/genetics , Plants/chemistryABSTRACT
Dose-response relationship between resistance of wheat seedlings (Triticum aestivum, cultivar Zarya) to Erysiphe graminis f. sp. tritici Marchal. (Syn. Blumeria graminis), a causal organism of wheat powdery mildew and exogenous zeatin has been investigated. Two-week-old seedlings were inoculated with the pathogen. Zeatin or zeatinriboside were added to the nutrient solution immediately after inoculation. The dose-response curve of cytokinin in the most cases was multiphasic, with peaks of increased susceptibility occurring at 0.25-1.5 and 1.5-9microM cytokinin, separated by a region of increased resistance at 0.5-3microM cytokinin. The change in mineral nutrition or simultaneous treatment with thidiazuron revealed alterations of the dose-response curve ranging from a curve with maximum of resistance to a curve with maximum of susceptibility. Both multiphase nature of dose-response and its variability were proposed as possible explanations for earlier observed discrepancies in experimental data on modification of disease resistance by cytokinins. A mathematical model for two metabolic processes with substrate inhibition connected in-series was suggested to explain the multiphase dose-response. In this model, the product of the first reaction was used as substrate for the second reaction. Numerical experiments showed the changes in the shape of dose-response curve with changes in parameters dependent of cytokinin metabolism.
