The plant hormone auxin: insight in sight.

Physiological experiments conducted over the last 60 years indicate that the plant hormone auxin regulates a diverse set of developmental processes via changes in cell division, cell elongation and cell differentiation. Recent studies using transgenic plants with altered auxin levels support these conclusions and promise to provide more detailed information on the role of auxin during plant development. Although it is possible that all auxin responses are mediated by the same primary biochemical events, the studies described in this review are more consistent with multiple modes of auxin action. The development of molecular and genetic approaches to the study of hormone action should resolve this issue. The accelerated rate of progress in this field suggests that real insight into the mechanism of auxin action may be forthcoming.     

Germination of Excised Fraxinus ornus Embryos With and Without Phleomycin

In excised embryos of Fraxinus ornus, the first macroscopic sign of germination is a curvature near the root apex. Histological studies show that this curvature is due to cell elongation on the convex side and is accompanied by cell division. Metabolic changes, manifested by the disintegration of protein bodies and tissue differentiation, also occur during this time. In embryos treated with phleomycin cell division is completely inhibited but elongation, root hair formation, disintegration of protein bodies, and differentiation are still detectable. Phleomycin also drastically inhibits the formation of chlorophylls and the fresh weight increase of embryos.     

Proteomic dissection of plant development

Plant development is controlled by complex endogenous genetic programs and responses to environmental cues. Proteome analyses have recently been introduced to plant biology to identify proteins instrumental in these developmental processes. To date most plant proteome studies have been employed to generate reference maps of the most abundant soluble proteins of plant organs at a defined developmental stage. However, proteomics is now also utilized for genetic studies comparing the proteomes of different plant genotypes, for physiological studies analyzing the influences of exogenous signals on a particular plant organ, and developmental studies investigating proteome changes during development. Technical advances are now beginning to allow a proteomic dissection of individual cell types, thus greatly increasing the information revealed by proteome analyses.     

Cell surface patterning and morphogenesis: biogenesis of a periodic surface array during Caulobacter development

Shape changes, extended processes, and other surface elaborations are associated with cellular differentiation, and the cell membranes involved with these developmental changes often are reshaped without a major alteration in biochemical composition. Caulobacter crescentus produces a hexagonally-packed periodic surface layer that covers the entire cell and further, mimics some of the membrane-mediated changes of higher organisms by forming a membranous stalk during its distinctive life cycle. Growth of the surface layer was examined during the cell cycle by treating synchronously growing cells with surface layer antibody, continuing growth, and then labeling for electron microscopy with a protein A-colloidal gold conjugate. Three regions of distinctive surface array biogenesis were resolved. The periodic surface layer on the main cell body was enlarged by insertion of new material at numerous uniformly distributed points. In contrast, the surface layer on the stalk appeared as entirely new synthesis. In examining growth of the stalk in subsequent generations, we noted that growth of stalk surface persisted at the stalk-cell body junction. The region of cell division also showed a pattern of entirely new surface layer production at late stages in division, similar to the stalk. The immunocytological method also facilitated a careful examination of stalk initiation and growth. Although initiation was under precise temporal and spatial regulation, the rate of stalk elongation was variable from cell to cell and apparently no longer under cell cycle control. The similarity of surface layer biogenesis on the stalk and the site of cell division may be a significant reflection of other events occurring at the cell pole. A model suggested by this and other studies that can account for the temporal pattern of polar morphogenesis is discussed, as is the potential relationship between the geometrically ordered surface array and the formation or maintenance of the stalk.     

哺乳动物卵母细胞及早期胚胎蛋白质组学研究进展

雌性生殖细胞发育是动物繁殖的基石,哺乳动物卵母细胞和早期胚胎在其生长发育过程中有许多独特的现象和规律,涉及一系列蛋白质合成/降解和磷酸化等状态的动态改变。对卵母细胞分裂、成熟调控机理以及植入前胚胎发育规律的研究是发育生物学领域的一项重要课题。蛋白质组学是以细胞或组织内全部的蛋白质为研究对象,系统鉴定、定量蛋白质并研究这些蛋白质功能的科学。随着蛋白质分离、鉴定技术的快速发展,蛋白质组学为卵母细胞发生、分化、成熟以及质量控制等相关研究提供了新的方法和内容,如在蛋白质定量、修饰、定位和相互作用等方面提供其他组学技术不可获得的重要信息。这些信息将有助于揭示哺乳动物卵母细胞成熟和早期胚胎发育的分子机制,对于进一步完善卵母细胞的体外成熟培养体系,提高胚胎体外生产、体细胞克隆和转基因动物生产效率具有重要意义。     

植物蛋白质组学研究进展

 蛋白质组学是后基因组时代功能基因组学研究的新兴学科和热点领域。该文简要介绍了蛋白质组学产生的科学背景、研究方法和研究内容。蛋白质组学研究方法主要有双向聚丙烯酰胺凝胶电泳（2D-PAGE）、质谱(Mass-spectrometric)技术、蛋白质芯片（Protein chips）技术、酵母双杂交系统（Yeast two-hybrid system）、植物蛋白质组数据库等。其应用的范围包括植物群体遗传学、在个体水平上植物对生物和非生物环境的适应机制、植物的发育和组织器官的分化过程,以及不同亚细胞结构在生理生态过程中的作用等诸多方面。同时对植物蛋白质组学的发展前景进行了展望。     

Regulation of the polarity of protein trafficking by phosphorylation

The asymmetry of environmental stimuli and the execution of developmental programs at the organism level require a corresponding polarity at the cellular level, in both unicellular and multicellular organisms. In plants, cell polarity is important in major developmental processes such as cell division, cell enlargement, cell morphogenesis, embryogenesis, axis formation, organ development, and defense. One of the most important factors controlling cell polarity is the asymmetric distribution of polarity determinants. In particular, phosphorylation is implicated in the polar distribution of the determinant protein factors, a mechanism conserved in both prokaryotes and eukaryotes. In plants, formation of local gradients of auxin, the morphogenic hormone, is critical for plant developmental processes exhibiting polarity. The auxin efflux carriers PIN-FORMEDs (PINs) localize asymmetrically in the plasma membrane and cause the formation of local auxin gradients throughout the plant. The asymmetry of PIN distribution in the plasma membrane is determined by phosphorylationmediated polar trafficking of PIN proteins. This review discusses recent studies on the role of phosphorylation in polar PIN trafficking.     

Actin microfilament dynamics and actin side-binding proteins in plants

Actin microfilaments are highly organized and essential intracellular components of organelle movement and cell morphogenesis in plants. The organization of these microfilaments undergoes dynamic changes during cell division, elongation, and differentiation. Recent live-cell imaging of plant actin microfilaments has revealed their native organization and remarkable dynamics. In addition, characterization of plant actin side-binding proteins has progressed rapidly by genetic, biochemical, and bioinformatic approaches. The gathering and integration of microscopy-based information from actin microfilament dynamics and the molecular identification of actin side-binding proteins have provided considerable insights into actin microfilament-dependent events and actin microfilament organization in plants.     

Expression of genes for cell-wall proteins in dividing and wounded tissues ofZea mays L.

Hydroxyproline-rich glycoproteins (HRGPs) fromZea mays have been immunolocalized in the cell wall of root tip cells using ultrathin sections and antibodies ellicited against the purified protein. The accumulation of mRNA corresponding to this protein was studied using the cDNA probe. Maximum accumulation of the mRNA was found in tissues with a high proportion of dividing cells such as those in the root tip of young maize seedlings and a close relationship with cellular division was also observed in in-vitro cultures. However, the level of the mRNA in elongating tissues was minimal, as shown by studies carried out on the elongation zones of root tips and coleoptiles. The mRNA was induced by stress conditions, particularly by wounding young leaves and coleoptiles. It is concluded that in maize this group of proline-rich cell-wall proteins accumulates during cell division and not during cell elongation or differentiation, and participates in the stress-response mechanisms of the plant.     

Latrunculin B-induced plant dwarfism: Plant cell elongation is F-actin-dependent

Marine macrolides latrunculins are highly specific toxins which effectively depolymerize actin filaments (generally F-actin) in all eukaryotic cells. We show that latrunculin B is effective on diverse cell types in higher plants and describe the use of this drug in probing F-actin-dependent growth and in plant development-related processes. In contrast to other eukaryotic organisms, cell divisions occurs in plant cells devoid of all actin filaments. However, the alignment of the division planes is often distorted. In addition to cell division, postembryonic development and morphogenesis also continue in the absence of F-actin. These experimental data suggest that F-actin is of little importance in the morphogenesis of higher plants, and that plants can develop more or less normally without F-actin. In contrast, F-actin turns out to be essential for cell elongation. When latrunculin B was added during germination, morphologically normal Arabidopsis and rye seedlings developed but, as a result of the absence of cell elongation, these were stunted, resembling either genetic dwarfs or environmental bonsai plants. In conclusion, F-actin is essential for the plant cell elongation, while this F-actin-dependent cell elongation is not an essential feature of plant-specific developmental programs.     

Dynamic proteomic analysis reveals a switch between central carbon metabolism and alcoholic fermentation in rice filling grains

Accumulation of reserve materials in filling grains involves the coordination of different metabolic and cellular processes, and understanding the molecular mechanisms underlying the interconnections remains a major challenge for proteomics. Rice (Oryza sativa) is an excellent model for studying grain filling because of its importance as a staple food and the available genome sequence database. Our observations showed that embryo differentiation and endosperm cellularization in developing rice seeds were completed approximately 6 d after flowering (DAF); thereafter, the immature seeds mainly underwent cell enlargement and reached the size of mature seeds at 12 DAF. Grain filling began at 6 DAF and lasted until 20 DAF. Dynamic proteomic analyses revealed 396 protein spots differentially expressed throughout eight sequential developmental stages from 6 to 20 DAF and determined 345 identities. These proteins were involved in different cellular and metabolic processes with a prominently functional skew toward metabolism (45%) and protein synthesis/destination (20%). Expression analyses of protein groups associated with different functional categories/subcategories showed that substantially up-regulated proteins were involved in starch synthesis and alcoholic fermentation, whereas the down-regulated proteins in the process were involved in central carbon metabolism and most of the other functional categories/subcategories such as cell growth/division, protein synthesis, proteolysis, and signal transduction. The coordinated changes were consistent with the transition from cell growth and differentiation to starch synthesis and clearly indicated that a switch from central carbon metabolism to alcoholic fermentation may be important for starch synthesis and accumulation in the developmental process.     

Emerging facets of plastid division regulation

Plastids are complex organelles that are integrated into the plant host cell where they differentiate and divide in tune with plant differentiation and development. In line with their prokaryotic origin, plastid division involves both evolutionary conserved proteins and proteins of eukaryotic origin where the host has acquired control over the process. The plastid division apparatus is spatially separated between the stromal and the cytosolic space but where clear coordination mechanisms exist between the two machineries. Our knowledge of the plastid division process has increased dramatically during the past decade and recent findings have not only shed light on plastid division enzymology and the formation of plastid division complexes but also on the integration of the division process into a multicellular context. This review summarises our current knowledge of plastid division with an emphasis on biochemical features, the functional assembly of protein complexes and regulatory features of the overall process.     

Cell elongation as an inseparable component of growth in terrestrial plants

The review is dedicated to the role of cell elongation in plant growth and morphogenesis. The ratios of cell division to elongation, cell competence for the initiation of elongation, main features of the metabolism of elongating cells, and physiological processes realizing elongation have been considered on the examples of seed germination and growth of roots, stems, and leaves. A special attention was paid to the vacuole as a specific feature of plant cells, pathways of its formation, and its role in maintenance of ion and water homeostasis in the elongating cell. The plant can modify its morphology according to changes in the environmental conditions via cell elongation.     

Cell elongation as an inseparable component of growth in terrestrial plants

The review is dedicated to the role of cell elongation in plant growth and morphogenesis. The ratios of cell division to elongation, cell competence for the initiation of elongation, main features of the metabolism of elongating cells, and physiological processes realizing elongation have been considered on the examples of seed germination and growth of roots, stems, and leaves. A special attention was paid to the vacuole as a specific feature of plant cells, pathways of its formation, and its role in maintenance of ion and water homeostasis in the elongating cell. The plant can modify its morphology according to changes in the environmental conditions via cell elongation.     

The evolution of the regulatory mechanism of chloroplast division

Chloroplasts arose from a cyanobacterial endosymbiont and multiply by division, reminiscent of their free-living ancestor. However, chloroplasts can not divide by themselves, and the division is performed and controlled by proteins that are encoded by the host nucleus. The continuity of chloroplasts was originally established by synchronization of endosymbiotic cell division with host cell division, as seen in existent algae. In contrast, land plant cells contain multiple chloroplasts, the division of which is not synchronized, even in the same cell. Land plants have evolved cell and chloroplast differentiation systems in which the size and number of chloroplasts (or other types of plastids) change along with their respective cellular function by changes in the division rate. We recently reported that PLASTID DIVISION (PDV) proteins, land-plant specific components of the chloroplast division apparatus, determined the rate of chloroplast division. The level of PDV protein is regulated by the cell differentiation program based on cytokinin, and the increase or decrease of the PDV level gives rise to an increase or decrease in the chloroplast division rate. Thus, the integration of PDV proteins into the chloroplast division machinery enabled land plant cells to change chloroplast size and number in accord with the fate of cell differentiation.Key words: chloroplast division, cell cycle, cell differentiation, cytokinin, endosymbiosis, evolution