INTRODUCTION. Internal Organization of the Plant Body. Summary of Types of Cells and Tissues. General References. DEVELOPMENT OF THE SEED PLANT. The Embryo. From embryo to the Adult Plant. Apical Meristems and Their Derivatives. Differentiation, Specialization, and Morphogenesis. References. THE CELL. Cytoplasm. Nucleus. Plastids. Mitochondria. Microbodies. Vacuoles. Paramural Bodies. Ribosomes. Dictyosomes. Endoplasmic Reticulum. Lipid Globules. Microtubules. Ergastic Substances. References. CELL WALL. Macromolecular Components and Their Organization in the Wall. Cell Wall Layers. Intercellular Spaces. Pits, Primary Pit--Fields, and Plasmodesmata. Origin of Cell Wall During Cell Division. Growth of Cell Wall. References. PARENCHYMA AND COLLENCHYMA. Parenchyma. Collenchyma. References. SCLERENCHYMA. Sclereids. Fibers. Development of Sclereids and Fibers. References. EPIDERMIS. Composition. Developmental Aspects. Cell Wall. Stomata. Trichomes. References. XYLEM: GENERAL STRUCTURE AND CELL TYPES. Gross Structure of Secondary Xylem. Cell Types in the Secondary Xylem. Primary Xylem. Differentiation of Tracheary Elements. References. XYLEM: VARIATION IN WOOD STRUCTURE. Conifer Wood. Dicotyledon Wood. Some Factors in Development of Secondary Xylem. Identification of Wood. References. VASCULAR CAMBIUM. Organization of Cambium. Developmental Changes in the Initial Layer. Patterns and Causal Relations in Cambial Activity. References. PHLOEM. Cell Types. Primary Phloem. Secondary Phloem. References. PERIDERM. Structure of Periderm and Related Tissues. Development of Periderm. Outer Aspect of Bark in Relation to Structure. Lenticels. References. SECRETORY STRUCTURES. External Secretory Structures. Internal Secretory Structures. References. THE ROOT: PRIMARY STATE OF GROWTH. Types of Roots. Primary Structure. Development. References. THE ROOT: SECONDARY STATE OF GROWTH AND ADVENTITIOUS ROOTS. Common Types of Secondary Growth. Variations in Secondary Growths. Physiologic Aspects of Secondary Growth in Roots. Adventitious Roots. References. THE STEM: PRIMARY STATE OF GROWTH. External Morphology. Primary Structure. Development. References. THE STEM: SECONDARY GROWTH AND STRUCTURAL TYPES. Secondary Growth. Types of Stems. References. THE LEAF: BASIC STRUCTURE AND DEVELOPMENT. Morphology. Histology of Angiosperm Leaf. Development. Abscission. References. THE LEAF: VARIATIONS IN STRUCTURE. Leaf Structure and Environment. Dicotyledon Leaves. Monocotyledon Leaves. Gymnosperm Leaves. References. THE FLOWER: STRUCTURE AND DEVELOPMENT. Concept. Structure. Development. References. THE FLOWER: REPRODUCTIVE CYCLE. Microsporogenesis. Pollen. Male Gametophyte. Megasporogenesis. Female Gametophyte. Fertilization. References. THE FRUIT. Concept and Classification. The Fruit Wall. Fruit Types. Fruit Growths. Fruit Abscission. References. THE SEED. Concept and Morphology. Seed Development. Seed Coat. Nutrient Storage Tissues. References. EMBRYO AND SEEDLING. Mature Embryo. Development of Embryo. Classification of Embryos. Seedling. References. Glossary. Index.

Anatomy of Seed Plants

Lipid-transfer proteins (LTP) are basic, 9-kDa proteins present in high amounts (as much as 4% of the total soluble proteinss) in higher plants. LTPs can enhance the in vitro transfer of phospholipids between membranes and can bind acyl chains. On the basis of these properties, LTPs were thought to participate in membrane biogenesis and regulation of the intracellular fatty acid pools. However, the isolation of several cDNAs and genes revealed the presence of a signal peptide indicating that LTPs could enter the secretory pathway. They were found to be secreted and located in the cell wall. Thus, novel roles were suggested for plant LTPs: participation in cutin formation, embryogenesis, defense reactions against phytopathogens, symbiosis, and the adaptation of plants to various environmental conditions. The validity of these suggestions needs to be determined, in the hope that they will elucidate the role of this puzzling family of plant proteins.

Lipid-transfer proteins in plants

Chimaeric ribonuclease genes that are expressed in the anthers of transformed tobacco and oilseed rape plants were constructed. Chimaeric ribonuclease gene expression within the anther selectively destroys the tapetal cell layer that surrounds the pollen sac, prevents pollen formation, and leads to male sterility. These nuclear male sterility genes should facilitate the production of hybrid seed in various crop plants.

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Induction of male sterility in plants by a chimaeric ribonuclease gene

We identified Arabidopsis thaliana sterility mutants by screening T-DNA and EMS-mutagenized lines and characterized several male-sterile mutants with defects specific for different anther processes. Approximately 44 and 855 sterile mutants were uncovered from the T-DNA and EMS screens, respectively. Several mutants were studied in detail with defects that included the establishment of anther morphology, microspore production, pollen differentiation, and anther dehiscence. Both non-dehiscencing and late-dehiscencing mutants were identified. In addition, pollenless mutants were observed with either apparent meiotic defects and/or abnormalities in cell layers surrounding the locules. Two mutant alleles were identified for the POLLENLESS3 locus which have defects in functional microspore production that lead to the degeneration of cells within the anther locules. pollenless3–1 contains a T-DNA insertion that co-segregates with the mutant phenotype and pollenless3–2 has a large deletion in the POLLENLESS3 gene. The POLLENLESS3 gene has no known counterparts in the GenBank, but encodes a protein containing putative nuclear localization and protein-protein interaction motifs. The POLLENLESS3 gene was shown recently to be the same as MS5, a previously described Arabidopsis
thaliana male-sterility mutant. Three genes were identified in the POLLENLESS3 genomic region: GENEY, POLLENLESS3, and β9-TUBULIN. The segment of the Arabidopsis
thaliana genome containing the POLLENLESS3 and β9-TUBULIN genes is duplicated and present on a different chromosome. Analysis of the POLLENLESS3 expression pattern determined that the 1.3-kb POLLENLESS3 mRNA is localized specifically within meiotic cells in the anther locules and that POLLENLESS3 mRNA is present only during late meiosis.

Anther developmental defects in Arabidopsis thaliana male-sterile mutants

The ability to produce morphologically and developmentally normal embryos and, indeed, whole plants from undifferentiated somatic cells in culture, through the process of somatic embryogenesis, resides uniquely within the plant kingdom. Since the initial description of somatic embryo production from carrot callus cells more than 35 years ago (Steward et al., 1958), this unique developmental potential has been recognized both as an important pathway for the regeneration of plants from cell culture systems and as a potential model for studying early regulatory and morphogenetic events in plant embryogenesis. The last 5 to 10 years have witnessed an explosion in the number of species that can now be regenerated from cell culture into whole plants through somatic embryogenesis. The literature contains hundreds of references describing the specific manipulations required to effect somatic embryo development from a variety of agronomically and horticulturally important plants. Although this is obviously an extremely important application of the process of somatic embryogenesis, it is not the focus of this review. Rather, this review will focus on the use of somatic embryogenesis as a model system for understanding the regulation of gene expression required for the earliest developmental events in the life of a higher plant: the development of the fertilized zygote into a mature embryo. The events of fertilization and subsequent embryo development normally occur deep within maternal tissues. The early embryo is minute and is surrounded by both endosperm and maternal cells. Although the morphological description of embryo development has been extensively recorded through microscopy, molecular and biochemical analyses of early embryogenesis have been hampered significantly by this physical inaccessibility. As a consequence, we know very little about the genes that are necessary for early embryogenesis in higher plants and even less about their regulation. This is beginning to be remedied by recent intensive efforts to genetically identify genes required for early embryogenesis in model systems such as Arabidopsis and maize (see West and Harada, 1993, this issue); many interesting mutants have been identified that may provide entry points into molecular analyses of major morphogenetic events in embryogenesis. These analyses would be greatly enhanced by the availability of cell, tissue, and developmental stage-specific markers of important events in the differentiation of cells and the establishment of the major tissue systems of the plant, which occur early in embryogenesis. In addition, once genes have been identified that are essential for embryogenesis, the subsequent analysis of their regulation would be greatly facilitated by the availability of an appropriate in vitro model system that is not limited in tissue quantity or accessibility. The somatic embryo system represents just such a model system. This review will summarize the process of somatic embryogenesis and will address the strengths and limitations of somatic embryos as potential models for studying early events in plant embryo development.

https://www.jstor.org/stable/info/3869792

Somatic Embryogenesis: A Model for Early Development in Higher Plants.

We studied the temporal and spatial regulation of three mRNA sequence sets that are present exclusively, or at elevated levels, in the tobacco anther. One mRNA set accumulates in the tapetum and decays as the tapetum degenerates later in anther development. The second mRNA set accumulates after the tapetal-specific mRNAs, is localized within the stomium and connective, and also decays as these cell types degenerate during anther maturation. The third mRNA sequence set persists throughout anther development and is localized within most anther tissues. A tapetal-specific gene, designated as TA29, was isolated from a tobacco genome library. Runoff transcription studies and experiments with chimeric [beta]-glucuronidase and diphtheria toxin A-chain genes showed that the TA29 gene is regulated primarily at the transcriptional level and that a 122-base pair 5[prime] region can program the tapetal-specific expression pattern. Destruction of the tapetum by the cytotoxic gene had no effect on the differentiation and/or function of surrounding sporophytic tissues but led to the production of male-sterile plants. Together, our studies show that several independent gene expression programs occur during anther development and that these programs correlate with the differentiated state of specific anther cell types.

Different Temporal and Spatial Gene Expression Patterns Occur during Anther Development.

The sequence of a 6.2 kb region containing the tobacco antherspecific gene TA-29 is shown below. This gene was isolated from a Charon 32 library using the TA-29 cDNA as a probe, and is expressed in tapetal cells (R.B.Goldberg, Science 240, 1460-1467, 1988). TA-29 has no introns and a continuous coding region of 963 base pairs (underlined). The transcription start site is indicated by a dot, the putative TATA box at position 1446 is underlined, and the arrowhead indicates the transcriptional direction. SI mapping identified two 3' ends that are boxed. The sequence of a contiguous gene, designated as TSJT1, is also shown. This gene is transcribed in the opposite orientation, is expressed predominandy in the stem, and is active at lower levels in other organs. Three exons and a putative TATA box are underlined.

Jessie Truettner

Papers

Induction of male sterility in plants by a chimaeric ribonuclease gene

Different Temporal and Spatial Gene Expression Patterns Occur during Anther Development.

The nucleotide sequence of an anther-specific gene.