Embryo differentiation in Arabidopsis thaliana follows the classical Capsella variation of the Onagrad type. Fertilization occurs approximately 3 h after flowering, whereupon vacuolar organization in the zygote changes and the cell elongates rapidly to approximately three times its original length. Cytoplasmic polarization is maintained. During the first two division steps there is very little increase in total cell volume, and during subsequent divisions vacuole number increases, with a concomitant decrease in size. Plastids remain undifferentiated up to the late globular stage, after which grana begin to develop. Ribosomal concentration increases significantly after fertilization. Differences between embryo proper cells become evident by the heart stage; vacuole, plastid, and mitochondrial abundance, size, and complexity vary within the embryo. There are no plasmodesmatal connections with the endosperm or integuments. Suspensor development is complete by the early globular stage, when it consists of seven to nine highly vacuolate cells, each linked by end wall plasmodesmata. Ribosome and volume densities of plastids and mitochondria are significantly lower than in the embryo proper organelles, and dictyosomes are infrequent. Embryo sac wall projections proliferate throughout the micropylar chamber, especially adjacent to the filiform apparatus and zygote base, and ingrowths form on the basal cell proximal wall. Key words: Arabidopsis, embryogenesis, embryo differentiation, wall ingrowths.
Cotyledon cell development in Arabidopsis thaliana L. during reserve deposition has been analyzed qualitatively and quantitatively. Development has been related to the previously defined time scale for Arabidopsis, hours after flowering. Between 144 and 216 h after flowering the major cell changes in the cotyledon are an increase in the cell volume, a decrease in the volume fraction of cytoplasm and plastids, and an increase in lipid and vacuole volume fractions. The endoplasmic reticulum and dictyosome volume fractions are high during early reserve formation (144 – 168 h after flowering) but decrease significantly thereafter. Evidence as to the origin of the storage lipid is inconclusive, although a dual involvement of plastids and rough endoplasmic reticulum is a likely theory. The 3-nm lipid body membrane, which allows the bodies to retain their individuality during accumulation, is probably a half-unit biological membrane, derived from closely associated rough endoplasmic reticulum cisternae. Much of the evidence obtained in this study indicates that both the endoplasmic reticulum and dictyosomes are involved in protein synthesis and transport to the vacuole. The accumulation of reserves occurs in a well-defined and relatively short period during late embryogenesis (144–216 h after flowering). Key words: Arabidopsis, cotyledons, embryogenesis, reserve deposition, stereology.
Five new recessive male-sterile mutants of Arabidopsis thaliana were isolated following seed mutagenesis by X-rays and ethyl methanesulfonate. The cytology of plants homozygous for the msY and msW mutations suggested that pollen development in these lines became abnormal at or before meiosis. The msK mutation caused faulty timing of synthesis or turnover and distribution of callose. In plants homozygous for the msZ mutation, pollen development failed at a late stage. In wild-type plants, the stamen filament elongated just prior to anther dehiscence. In contrast, in the msZ mutant stamen elongation did not occur. Pollen in msH homozygotes was fertile, but anthers failed to dehisce. The msI mutant of J.H. Van der Ween and P. Wirtz (1968. Euphytica 17: 371 – 377) was included in the present study. Pollen development in this mutant failed shortly after microspore release from tetrads. Complementation tests confirmed that the ms mutations were at different loci. Reduced transmission of certain ms genes was observed. Key words: Arabidopsis thaliana, male sterile mutants, anther dehiscence, callose, inheritance.
Arabidopsis thaliana has a seven-celled eight-nucleate megagametophyte of the Polygonum type; each cell type displays a different form of structural specialization. The egg apparatus cells are highly polarized; the egg has a large micropylar vacuole and chalazally sited nucleus, whereas the opposite is true for the synergids. At the chalazal region of the egg apparatus cells there are no cell wall boundaries, although their plasmalemmas are in intimate contact. The common wall between the two synergids is thin and irregular and contains plasmodesmatal connections. The synergid cytoplasm is rich in organelles; profiles of rough endoplasmic reticulum appear in masses of parallel stacked cisternae, and large accumulations of mitochondria occur adjacent to the filiform apparatus. The egg cell cytoplasm is quiescent; ribosome concentration and frequencies of dictyosomes and endoplasmic reticulum are noticeably lower and plastids are poorly differentiated. The central cell is long and vacuolate with a large diploid nucleus; fusion of the polar nuclei occurs prior to embryo sac maturity. The cytoplasm contains numerous starch-containing plastids accumulated in a shell around the nucleus. A high ribosome concentration and the absence of vacuoles and dictyosomes typifies the antipodal cell cytoplasm. All antipodal cells are interconnected by plasmodesmata as well as being connected to the nucellus and central cell. Key words: Arabidopsis, embryo sac, embryogenesis, cell specializations, stereology.
The structure of the vascular tissues of nitrogen-fixing nodules of 27 genera of legumes and some non-legumes has been investigated by light microscopy. Pisum and Trifolium nodules have been examined by electron microscopy.Attention is directed to the presence of a pericycle in the vascular bundles of the nodules. In 7 of the legumes the pericycle cells possess a wall labyrinth consisting of branched filiform protuberances. The ultrastructure of the pericycle cell cytoplasm is described: its most striking feature is its abundant rough endoplasmic reticulum. These cells surround the xylem and phloem of the bundles, and are in turn surrounded by a layer of endodermal cells with Casparian strips. The pericycle cells develop their wall labyrinth in the levels of the nodule at which the bacterial tissue becomes pigmented; in nodule senescence their cytoplasm is disrupted level with the breakdown of the bacterial tissue.A pathway for symplastic lateral transfer of assimilates exists, from the sieve elements through the pericycle, endodermis and cortex to the bacterial tissue. The apoplast within the endodermis consists largely of the pericycle wall labyrinth and the xylem. The ultrastructure of the Casparian strip resembles that of roots.Intact, detached nodules can be induced to bleed a fluid from their severed vascular tissue. This fluid is exceptionally rich in organic nitrogen, particularly amides, but does not appear to contain sugars. Comparison between its amino acid composition and that of other parts of the nodule suggests that an active uptake or secretion of nitrogenous compounds precedes export from the nodule. Special functions are suggested for the nodule endodermis and the pericycle cells in this export process.
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