Development of male gametes in flowering plants

The male gametes of angiosperms consist of two sperm cells within a pollen grain or a pollen tube. They are derived from a single generative cell, which is formed as the smaller cell by unequal cell division in the microspore after meiosis. Limited information is available about these male gametic cells, beyond observations by electron microscopy, because each is surrounded by the cytoplasm of a larger vegetative cell. Recently, large quantities of generative cells and sperm cells have been isolated from pollen grains or pollen tubes of various plant species, and their physiological, biochemical and molecular characterization is now possible. Although almost all the available results are still preliminary, it is evident that the male gametic cells are peculiar in terms both of cell structure and composition. For example, they are rich in axial microtubules which maintain the spindle-like shape of each cell. However, they lack plastids which are DNA-containing cytoplasmic organelles. Biochemical characterization of their proteins indicates the presence of male gamete-specific polypeptides. These findings suggest, not unexpectedly, the possibility of male gamete-specific gene expression and of a strict genetic mechanism that controls the formation of male gametes.     

The resurgence of haploids in higher plants

The life cycle of plants proceeds via alternating generations of sporophytes and gametophytes. The dominant and most obvious life form of higher plants is the free-living sporophyte. The sporophyte is the product of fertilization of male and female gametes and contains a set of chromosomes from each parent; its genomic constitution is 2n. Chromosome reduction at meiosis means cells of the gametophytes carry half the sporophytic complement of chromosomes (n). Plant haploid research began with the discovery that sporophytes can be produced in higher plants carrying the gametic chromosome number (n instead of 2n) and that their chromosome number can subsequently be doubled up by colchicine treatment. Recent technological innovations, greater understanding of underlying control mechanisms and an expansion of end-user applications has brought about a resurgence of interest in haploids in higher plants.     

The biosynthesis of sterols in higher plants

1. [2-(14)C]Mevalonate was incorporated into squalene and the major phytosterols of pea and maize leaves; it was also incorporated into compounds belonging to the 4,4-dimethyl and 4alpha-methyl steroid groups and which may be possible phytosterol intermediates. 2. l-[Me-(14)C]Methionine was incorporated into the major sterols and also into the 4,4-dimethyl and 4alpha-methyl steroid groups. No radioactivity was detected in squalene. 3. Under anaerobic conditions incorporation of [2-(14)C]-mevalonate into the non-saponifiable lipid of pea leaves was drastically decreased but radioactive squalene was accumulated. 4. Cycloartenol, 24-methylenecycloartanol, 24-methylenelophenol, 24-ethylidenelophenol, fucosterol, beta-sitosterol, stigmasterol and campesterol have been identified by gas-liquid chromatography in pea leaves. 5. The significance of these results in connexion with phytosterol biosynthesis and the introduction of the alkyl group at C-24 into phytosterols is discussed.     

The cytoskeleton and gravitropism in higher plants

The cellular and molecular mechanisms underlying the gravitropic response of plants have continued to elude plant biologists despite more than a century of research. Lately there has been increased attention on the role of the cytoskeleton in plant gravitropism, but several controversies and major gaps in our understanding of cytoskeletal involvement in gravitropism remain. A major question in the study of plant gravitropism is how the cytoskeleton mediates early sensing and signal transduction events in plants. Much has been made of the actin cytoskeleton as the cellular structure that sedimenting amyloplasts impinge upon to trigger the downstream signaling events leading to the bending response. There is also strong molecular and biochemical evidence that the transport of auxin, an important player in gravitropism, is regulated by actin. Organizational changes in microtubules during the growth response phase of gravitropism have also been well documented, but the significance of such reorientations in controlling differential cellular growth is unclear. Studies employing pharmacological approaches to dissect cytoskeletal involvement in gravitropism have led to conflicting results and therefore need to be interpreted with caution. Despite the current controversies, the revolutionary advances in molecular, biochemical, and cell biological techniques have opened up several possibilities for further research into this difficult area. The myriad proteins associated with the plant cytoskeleton that are being rapidly characterized provide a rich assortment of candidate regulators that could be targets of the gravity signal transduction chain. Cytoskeletal and ion imaging in real time combined with mutant analysis promises to provide a fresh start into this controversial area of research.     

The cell biology of lignification in higher plants

Background Lignin is a polyphenolic polymer that strengthens and waterproofs the cell wall of specialized plant cell types. Lignification is part of the normal differentiation programme and functioning of specific cell types, but can also be triggered as a response to various biotic and abiotic stresses in cells that would not otherwise be lignifying.Scope Cell wall lignification exhibits specific characteristics depending on the cell type being considered. These characteristics include the timing of lignification during cell differentiation, the palette of associated enzymes and substrates, the sub-cellular deposition sites, the monomeric composition and the cellular autonomy for lignin monomer production. This review provides an overview of the current understanding of lignin biosynthesis and polymerization at the cell biology level.Conclusions The lignification process ranges from full autonomy to complete co-operation depending on the cell type. The different roles of lignin for the function of each specific plant cell type are clearly illustrated by the multiple phenotypic defects exhibited by knock-out mutants in lignin synthesis, which may explain why no general mechanism for lignification has yet been defined. The range of phenotypic effects observed include altered xylem sap transport, loss of mechanical support, reduced seed protection and dispersion, and/or increased pest and disease susceptibility.     

The photoprotective role of carotenoids in higher plants

Carotenoids have two important roles in photosynthetic organisms. First, they act as accessory light-harvesting pigments, effectively extending the range of light absorbed by the photosynthetic apparatus. Secondly, they perform an essential photoprotective role by quenching triplet state chlorophyll molecules and scavenging singlet oxygen and other toxic oxygen species formed within the chloroplast. Only recently an additional, novel, protective role has been proposed for the carotenoid zeaxanthin, involving the dissipation of harmful excess excitation energy under stress conditions. Zeaxanthin may be formed through de novo synthesis in response to long-term environmental stress, and through the rapid enzymic de-epoxidation of the carotenoid violaxanthin (the xanthophyll cycle) in response to short-term alterations in the plant's light environment. Interspecific differences occur in the ability of plants and algae to produce zeaxanthin under stress conditions, and hence the ability to photoprotect the photosynthetic apparatus through this means varies from species to species. The ability of a plant to respond to light-mediated environmental stress by producing zeaxanthin may therefore affect, at least in part, the ability of that plant to inhabit or colonise certain habitats (e.g. sun or shade conditions).     

高等植物体内的肌动蛋白

植物肌动蛋白是植物细胞生物学领域发展起来的热点,它涉及植物细胞的分裂机制。细胞运动,细胞器运动,细胞的极性,细胞空间形状的维持,物质运输等,对高等植物肌动蛋白的性质,功能,研究方法及分子生物学领域的研究作了综述。并对该领域存在的问题及应用前景作了展望。     

Rhodanese in higher plants

Rhodanese activity was detected in crude leaf extracts of 12 randomly selected plant species consisting of 9 non-cyanophoric and 3 cyanophoric species. In each case, the enzyme exhibited high activity at pH 10·4 and 55°. There appeared to be no correlation between rhodanese activity and the cyanophoric nature of the plant.     

Endoreduplication in higher plants

Cell polyploidisation can be achieved by endoreduplication, which consists of one or several rounds of DNA synthesis in the absence of mitosis. As a consequence, chromosomes with 2ⁿ chromatids are produced without change in the chromosome number. Endoreduplication is the most common mode of polyploidisation in plants and can be found in many cell types, especially in those undergoing differentiation and expansion. Although accumulating data reveal that this process is developmentally regulated, it is still poorly understood in plants. At the molecular level, the increasing knowledge on plant cell cycle regulators allows the acquisition of new tools and clues to understand the basis of endoreduplication control and, in particular, the switch between cell proliferation and cell differentiation.     

The uptake of mannitol by higher plants

Experiments with youngHordeum sativum andHelianthus annus plants showed that in the excretion of mannitol in the guttation liquid observed byGroenewegen andMills (1960) after uptake by the root system of plants, the osmotic concentration of mannitol in the nutrient medium and the temperature are significant. The beginning of mannitol excretion during guttation is accelerated considerably by the increase of the osmotic concentration of mannitol in the nutrient medium and the rising temperature. The osmotic concentration of mannitol is also important for the duration of mannitol excretion in the guttation liquid after transfer of the plants into a nutrient medium without mannitol. In the presence of mannitol in the nutrient medium water uptake by the root system and growth are inhibited and the tissues of the organs above ground and of the root system are dehydrated. The inhibitory effect of mannitol on the water uptake by the root system is immediate.     

The physiological significance of cyanide-resistant respiration in higher plants

Abstract A brief survey of the biochemistry of the alternative oxidative pathway (‘cyanide-resistant respiration’) and its occurrence in vivo is given. Several hypotheses about the physiological significance of the alternative chain are discussed. These include a role in (1) heat production, (2) fruit ripening, (3) respiration of plants that contain high levels of cyanogenic glycosides, producing HCN upon wounding, (4) oxidation of NADH that is produced by various causes in excess of that required for ATP production, (5) ion uptake, and (6) osmoregulation. In intact roots of higher plants, the activity of the alternative pathway is more active when less carbohydrate is required for assimilation of N (NH+₄ NO-₃ or N₂) and is less active in those when carbohydrates are being stored in a storage organ or when the availability of photosynthate is reduced. An increase in carbohydrate requirement for osmoregulation is also correlated with decreased alternative chain activity. It is concluded that the alternative pathway in roots plays an important role in oxidation of sugars which are not required for carbon skeletons, energy production for growth and maintenance processes, osmoregulation or storage. However, the significance of this role may vary in different tissues and physiological states.     

The molecular biology of plastid division in higher plants

Plastids are essential plant organelles vital for life on earth, responsible not only for photosynthesis but for many fundamental intermediary metabolic reactions. Plastids are not formed de novo but arise by binary fission from pre-existing plastids, and plastid division therefore represents an important process for the maintenance of appropriate plastid populations in plant cells. Plastid division comprises an elaborate pathway of co-ordinated events which include division machinery assembly at the division site, the constriction of envelope membranes, membrane fusion and, ultimately, the separation of the two new organelles. Because of their prokaryotic origin bacterial cell division has been successfully used as a paradigm for plastid division. This has resulted in the identification of the key plastid division components FtsZ, MinD, and MinE, as well as novel proteins with similarities to prokaryotic cell division proteins. Through a combination of approaches involving molecular genetics, cell biology, and biochemistry, it is now becoming clear that these proteins act in concert during plastid division, exhibiting both similarities and differences compared with their bacterial counterparts. Recent efforts in the cloning of the disrupted loci in several of the accumulation and replication of chloroplasts mutants has further revealed that the division of plastids is controlled by a combination of prokaryote-derived and host eukaryote-derived proteins residing not only in the plastid stroma but also in the cytoplasm. Based on the available data to date, a working model is presented showing the protein components involved in plastid division, their subcellular localization, and their protein interaction properties.     

The ATP-dependent Clp protease in chloroplasts of higher plants

The best-known proteases in plastids are those that belong to families common to eubacteria. One of the first identified was the ATP-dependent caseinolytic protease (Clp), whose structure and function have been well characterized in Escherichia coli . Plastid Clp proteins in higher plants are surprisingly numerous and diverse, with at least 16 distinct Clp proteins in the model plant Arabidopsis thaliana . Multiple paralogues exist for several of the different types of plastid Clp protein, with the most extreme being five for the proteolytic subunit ClpP. Both biochemical and genetic studies have recently begun to reveal the intricate structural interactions between the various Clp proteins, and their importance for chloroplast function and plant development. Much of the recent data suggests that the function of many of the Clp proteins probably affects more specific processes within chloroplasts, in addition to the more general 'housekeeping' role previously assumed.     

The genetic control of plastid division in higher plants

The division of plastids is an important part of plastid differentiation and development and in distinct cell types, such as leaf mesophyll cells, results in large populations of chloroplasts. The morphology and population dynamics of plastid division have been well documented, but the molecular controls underlying plastid division are largely unknown. With the isolation of Arabidopsis mutants in which specific aspects of plastid and proplastid division have been disrupted, the potential exists for a detailed knowledge of how plastids divide and what factors control the rate of division in different cell types. It is likely that knowledge of plant homologues of bacterial cell division genes will be essential for understanding this process in full. The processes of plastid division and expansion appear to be mutually independent processes, which are compensatory when either division or expansion are disrupted genetically. The rate of cell expansion appears to be an important factor in initiating plastid division and several systems involving rapid cell expansion show high levels of plastid division activity. In addition, observation of plastids in different cell types in higher plants shows that cell-specific signals are also important in the overall process in determining not only the differentiation pathway of plastids but also the extent of plastid division. It appears likely that with the exploitation of molecular techniques and mutants, a detailed understanding of the molecular basis of plastid division may soon be a reality.