Capsella embryogenesis: the early embryo

Arabidopsis mitochondrial protein slow embryo development1 is essential for embryo development

The plant seeds formation are crucial parts in reproductive process in seed plants as well as food source for humans. Proper embryo development ensure viable seed formation. Here, we showed an Arabidopsis T-DNA insertion mutant slow embryo development1 (sed1) which exhibited retarded embryogenesis, led to aborted seeds. Embryo without SED1 developed slower compared to normal one and could be recognized at early globular stage by its white appearance. In later development stage, storage accumulated poorly with less protein and lipid body production. In vitro culture did not rescue albino embryo. SED1 encoded a protein targeted to mitochondria. Transmission electron microscopic analysis revealed that mitochondria developed abnormally, and more strikingly plastid failed to construct grana in time in sed1/sed1 embryo. These data indicated that SED1 is indispensable for embryogenesis in Arabidopsis, and the mitochondria may be involved in the regulation of many aspects of seed development.

Chloroembryos: a unique photosynthesis system

The embryos of some angiosperm taxa contain chlorophyll and this chlorophyllous stage is persisting until the embryo matures (further referred as chloroembryos). Besides being chlorophyllous, these embryos seem to have the ability to photosynthesize. This suggests that the chlorophyllous state of the embryo has an important role in seed development. The photosynthesis of chloroembryos is highly shade adaptive in nature as it is embedded within the supporting tissues (several layers of pod wall, seed coat and endosperm). Moreover, these chloroembryos are developing in a highly osmotic environment, and contain various components of the photosynthetic machinery. Detailed studies were performed in these chloroembryos in order to elucidate the structure of the chloroplasts, pigment composition, the photochemical activities, the rate of carbon assimilation and also the shade adaptive features. It has been shown that the respired CO2 within these chloroembryos is recycled by the efficient photosynthetic components of the chloroembryos and thus potentially influences the seed's carbon economy. Thus, the major role of embryonic photosynthesis is to produce both energy-rich molecules and oxygen, of which the former can be directly used for biosynthesis. During embryogenesis oxygen production is especially important, in a situation wherein the oxygen is limited within the enclosed seed. As these chloroembryos grow in an environment of a sugar rich endosperm, it requires some adaptive mechanisms in this high osmotic environment. The additional polypeptides found in the thylakoids of chloroembryo chloroplasts in comparison to the thylakoids of leaf chloroplast have been suggested to have a role in protecting the photosynthetic components in the chloroembryos in an environment of high osmotic strength. An attempt to understand osmotic stress tolerance existing in these chloroembryos may lead to a better understanding of tolerance of photosynthesis to osmotic stress.

EMB1211 is required for normal embryo development and influences chloroplast biogenesis in Arabidopsis

Chloroplast biogenesis is tightly linked with embryogenesis and seedling development. A growing body of work has been done on the molecular mechanisms underlying chloroplast development; however, the molecular components involved in chloroplast biogenesis during embryogenesis remain largely uncharacterized. In this paper, we show that an Arabidopsis mutant carrying a T-DNA insertion in a gene encoding a multiple membrane occupation and recognition nexus (MORN)-containing protein exhibits severe defects during embryogenesis, producing abnormal embryos and thereby leading to a lethality of young seedlings. Genetic and microscopic studies reveal that the mutation is allelic to a previously designated Arabidopsis embryo-defective 1211 mutant (emb1211). The emb1211 +/- mutant plants produce approximately 25% of white-colored ovules with abnormal embryos since late globular stage when primary chloroplast biogenesis takes place, while the wild-type plants produce all green ovules. Transmission electron microscopic analysis reveals the absence of normal chloroplast development, both in the mutant embryos and in the mutant seedlings, that contributes to the albinism. The EMB1211 gene is preferentially expressed in developing embryos as revealed in the EMB1211::GUS transgenic plants. Taken together, the data indicate that EMB1211 has an important role during embryogenesis and chloroplast biogenesis in Arabidopsis.

Plant embryogenesis: zygote to seed

Most differentiation events in higher plants occur continuously in the postembryonic adult phase of the life cycle. Embryogenesis in plants, therefore, is concerned primarily with establishing the basic shoot-root body pattern of the plant and accumulating food reserves that will be used by the germinating seedling after a period of embryonic dormancy within the seed. Recent genetics studies in Arabidopsis have identified genes that provide new insight into how embryos form during plant development. These studies, and others using molecular approaches, are beginning to reveal the underlying processes that control plant embryogenesis.

The Arabidopsis BE1 gene, encoding a putative glycoside hydrolase localized in plastids, plays crucial roles during embryogenesis and carbohydrate metabolism

Carbohydrate metabolism is central to plant growth and development. However, little is known about its role in embryogenesis. Here, we report the characterization of multiple alleles of the BRANCHING ENZYME1 (BE1) gene (also known as EMB2729). The weak allele of be1-3, characterized by positional cloning, carries a single-nucleotide substitution in an exon-intron junction and shows various developmental defects during post-germination growth. This mutation causes a reduced level of BE1 mRNA that, likely generated from cryptically spliced pre-mRNA, contains a Glu-to-Lys substitution at codon 366. In four null alleles, BE1 is disrupted by T-DNA insertions, causing embryo developmental arrests at the heart stage. Light microscopy reveals reduced cell divisions and abnormal cell differentiation, thereby leading to defects in setting up the shoot apical meristem, embryonic vascular tissues and cotyledons. Overexpression of BE1 results in a pleiotropic phenotype, indicating that the fine-tuned BE1 level is crucial for plant growth and development. BE1 encodes a putative glycoside hydrolase that is highly conserved in higher plants. A BE1-GFP fusion protein, which is fully functional in complementing be1 mutants, is localized in plastids. The be1-3 phenotype can be partially rescued by glucose, fructose or sucrose, implying the involvement of BE1 in carbohydrate metabolism in plastids.

The Arabidopsis embryo mutant schlepperless has a defect in the chaperonin-60alpha gene

We identified a T-DNA-generated mutation in the chaperonin-60alpha gene of Arabidopsis that produces a defect in embryo development. The mutation, termed schlepperless (slp), causes retardation of embryo development before the heart stage, even though embryo morphology remains normal. Beyond the heart stage, the slp mutation results in defective embryos with highly reduced cotyledons. slp embryos exhibit a normal apical-basal pattern and radial tissue organization, but they are morphologically retarded. Even though slp embryos are competent to transcribe two late-maturation gene markers, this competence is acquired more slowly as compared with wild-type embryos. slp embryos also exhibit a defect in plastid development-they remain white during maturation in planta and in culture. Hence, the overall developmental phenotype of the slp mutant reflects a lesion in the chloroplast that affects embryo development. The slp phenotype highlights the importance of the chaperonin-60alpha protein for chloroplast development and subsequently for the proper development of the plant embryo and seedling.

Temporal and spatial pattern of plastid gene expression during crucifer seedling development and embryogenesis

Several genes which are located close together on mustard (Sinapis alba L.) chloroplast DNA have been found to differ in their temporal mode of expression throughout seedling development. One predominant expression program, exemplified by thepsbA gene, is characterized by an early (light-independent) rise in transcript levels, followed by subsequent further accumulation to levels which are much higher in the light than in darkness (development of 'competence' for photocontrol). Other genes located next to thepsb A gene show transient or constitutive modes of expression, with no light-dark difference in transcript levels throughout seedling development. The characteristics of light-responsive expression were shown for the nuclearrbcS gene family inBrassica napus L. andSinapis alba L. cotyledons. The spatial distribution ofrbcS andpsbA transcripts across sections of crucifer cotyledons appeared to be relatively uniform, but restricted to photosynthetically active cells. Finally, assessment of these transcripts in immature seeds and embryos ofCapsella bursa-pastoris has provided in-situ evidence for tissuespecific gene expression during early development.

Light microscopic histochemistry on plastic sections

As compared with conventional paraffin, celloidin, and frozen sections, semithin plastic sections offer a superior quality of the light microscopic image in terms of better resolution, absence of distortion and shrinkage artifacts, and suitability for calcified tissues. Application of histochemical methods to such sections often encounters, however, serious difficulties resulting from a considerably reduced reactivity of plastic-embedded biological material. Factors involved include a poor penetration of reagents into plastic embedding media due to a steric or hydrophobic hindrance, as well as a blockade of the reactive chemical groups in the sample due to interactions with fixatives and plastics. Embedding in polar (hydrophilic) plastics, such as glycol methacrylate, permits carrying out a large number of histochemical reactions, including the demonstration of enzymatic activities, directly on sections, but is less suitable for combined light/electron microscopic studies because of an imperfect ultrastructural preservation of tissues. Embedding in nonpolar epoxy resins, particularly if combined with a double aldehyde-osmium fixation, results in a high quality ultrastructure but almost fully inhibits the histochemical reactivity of the embedded material. In order to restore this reactivity, i.e. to unmask chemical groups bound by the polymerized resin, semithin epoxy sections require the removal of the embedding matrix by alkoxides prior to the histochemical procedure. Additional steps are also often necessary: treatment of osmium-fixed sections with oxidative agents, e.g., hydrogen peroxide or periodate which reoxidize the bound osmium and remove it from tissue, and a controlled proteolytic digestion, especially useful in immunocytochemical studies, which probably cleaves the bonds between the primary aldehyde fixative, and the reactive sites. This article reviews histochemical methods which have been successfully applied to plastic-embedded material. Using polar methacrylates and/or nonpolar epoxy resins as embedding media, it has been possible to demonstrate proteins and aminoacid residues, carbohydrates, lipids, nucleic acids, biogenic amines, inorganic ions, and some enzymes, although the spectrum of methods found as suitable for plastic-embedded material is far narrower than that available for paraffin or frozen sections.(ABSTRACT TRUNCATED AT 400 WORDS)

Observations on the selective demonstration of nucleolar material by protein staining techniques in epon thick sections

The effect of different protein stainings on thick sections of Epon embedded salivary glands of Chironomus pallidivittatus is described. Pretreatment of sections with several metallic salts affected the affinity for aniline blue black in different structures. The uranyl and aluminium ions followed by aniline blue black resulted in a highly selective nucleolar staining which proved also efficient in Allium cepa meristematic cells and which seems useful for the preferential demonstration of nucleolar material in thick sections.

The development of protein and oil bodies in the seed of Sinapis alba L

The cotyledons of Sinapis alba L. seed are the storage organs and first photosynthetic organs. The development of the cotyledon cell contents was studied using electron and light microscopy. From the heart shaped embryo (11 days from petal fall) to the mature seed, nine stages were examined.Both types of protein grains (designated aleurone grains and myrosin grains) were found to form within vacuoles, but the mode of protein accumulation differed with each type of grain.Oil bodies were apparent with the EM from 18 days onwards, but could not be seen to arise from the ER. They were granular in appearance at early stages, but later became electron transparent.

Embryogenesis and germination in rye (Secale cereale L.) : 1. Fine structure of the developing embryo

Cells of the young embryo contain highly differentiated organelles. During maturation and dehydration, complexity is reduced, the many layers of endoplasmic reticulum associated with electron lucent bodies become reduced to a few residual crescents, lipid droplets distributed in the cytoplasm migrate to and become closely appressed to the plasmalemma, mitochondrial cristae are reduced in number and dictyosomes are compacted.