Expanding insights into the role of cell proliferation in plant development

Development in plants relies largely on the activity of meristems, which are regions at the apices of shoots and roots that are capable of prolonged organogenesis. Developmental patterning and morphogenesis in plants is principally determined by post-embryonic regulation of the shoot, root and flower meristems, which enable plants to modify their form rapidly in response to different environmental conditions. Because meristems are continually generating new organs and tissues, they provide excellent model systems in which to study the processes of cell division, differentiation and organ formation. Here, we describe recent studies and several classic experiments that are helping to uncover the mechanisms controlling meristem development and the role of cell division in morphogenesis and patterning in plants.     

Structural insights into the retroviral DNA integration apparatus

Retroviral replication depends on successful integration of the viral genetic material into a host cell chromosome. Virally encoded integrase, an enzyme from the DDE(D) nucleotidyltransferase superfamily, is responsible for the key DNA cutting and joining steps associated with this process. Insights into the structural and mechanistic aspects of integration are directly relevant for the development of antiretroviral drugs. Recent breakthroughs have led to biochemical and structural characterization of the principal integration intermediates revealing the tetramer of integrase that catalyzes insertion of both 3' viral DNA ends into a sharply bent target DNA. This review discusses the mechanism of retroviral DNA integration and the mode of action of HIV-1 integrase strand transfer inhibitors in light of the recent visualization of the prototype foamy virus intasome, target DNA capture and strand transfer complexes.     

Role of transglycosylation in the integration of xyloglucan into the growing plant cell wall in vivo

Abstract

Xyloglucan endotransglycosylase (XET) activity is widespread in plant cell walls, but its action on xyloglucan in vivo has been difficult to prove because the reaction products are not expected to differ chemically from the reactants. By feeding of cultured Rosa cells with [¹³C]glucose and [³H]arabinose followed by [¹²-C]glucose, and isopyenic centrifugation of the extracted xyloglucan in caesium trifluoroacetate, we have obtained evidence for the annealing of segments of newly-secreted xyloglucan to xyloglucan chains that were already present in the cell wall. This is the first evidence for interpolymeric transglycosylation of xyloglucan in vivo.     

The charophycean green algae provide insights into the early origins of plant cell walls

Numerous evolutionary innovations were required to enable freshwater green algae to colonize terrestrial habitats and thereby initiate the evolution of land plants (embryophytes). These adaptations probably included changes in cell-wall composition and architecture that were to become essential for embryophyte development and radiation. However, it is not known to what extent the polymers that are characteristic of embryophyte cell walls, including pectins, hemicelluloses, glycoproteins and lignin, evolved in response to the demands of the terrestrial environment or whether they pre-existed in their algal ancestors. Here we show that members of the advanced charophycean green algae (CGA), including the Charales, Coleochaetales and Zygnematales, but not basal CGA (Klebsormidiales and Chlorokybales), have cell walls that are comparable in several respects to the primary walls of embryophytes. Moreover, we provide both chemical and immunocytochemical evidence that selected Coleochaete species have cell walls that contain small amounts of lignin or lignin-like polymers derived from radical coupling of hydroxycinnamyl alcohols. Thus, the ability to synthesize many of the components that characterize extant embryophyte walls evolved during divergence within CGA. Our study provides new insight into the evolutionary window during which the structurally complex walls of embryophytes originated, and the significance of the advanced CGA during these events.     

New insights into the functional roles of CrRLKs in the control of plant cell growth and development

Receptor-like kinases (RLKs) are a family of transmembrane proteins with a variable ligand-binding extracellular domain and a cytoplasmic kinase domain. In Arabidopsis, there are ∼600 RLKs believed to have diverse functions during plant growth, development and interactions with the environment. Based on the variable extracellular domain, RLKs can be classified into different subfamilies. The CrRLK subfamily contains 17 members in Arabidopsis and characterization of some of its members suggests a role for these proteins in the regulation of growth and reproduction. This review focuses on the roles of CrRLKs in the regulation of polarized growth with emphasis on the newly identified signal transduction pathways activated downstream of CrRLKs. A picture is emerging where CrRLKs are part of a conserved signal transduction cascade important for growth maintenance in different cell types.Key words: CrRLKs, FERONIA, RAC/ROP, ROS, polar growthThe ability of plants to perceive and process environmental and internal information into coordinated responses is crucial to their adaptability and survival in constantly changing environments. Most of signal perception occurs at the plasma membrane of cells where membrane-associated receptors receive signals to activate downstream signaling cascades that regulate growth and development. In plants and animals alike, receptor-like kinases (RLKs) mediate many of the signaling events at the cell surface and in the model plant Arabidopsis they comprise a monophyletic family with more than 600 members.¹ RLKs are transmembrane proteins with a variable N-terminal extracellular domain and a Ser/Thr intracellular kinase domain. The diversity of their extracellular domains suggests involvement in the transduction of a wide range of signals and allows them to be classified into different sub-families.² The CrRLK1L subfamily (from here on referred to as CrRLK) is named after the first member characterized in Catharanthus roseus cell cultures³ and contains 17 members in Arabidopsis.⁴ Several members of this family have now been implicated in growth regulatory processes.THESEUS1 (THE1) was identified through a suppressor screen of a cellulose-deficient mutant (prc1-1) which has a short hypocotyl phenotype.⁵ Loss of THE1 function resulted in reduced growth inhibition in the prc1-1 the1 double mutant. Interestingly, the the1 mutation itself has no effect in wild type background, thus leading to the suggestion that THE1 functions as a sensor of cell wall integrity in situations where the cell wall is weakened and organ elongation would be detrimental for the plant.^4,5A second CrRLK, FERONIA (FER), was first implicated in the regulation of female control of fertility. In the female gametophyte FER is involved in sensing pollen tube arrival and promoting its rupture which is necessary for double fertilization to occur.^6,7 FER is in fact involved in several processes depending on the tissue where it is expressed. In hypocotyls, FER is involved in the integration of ethylene and brassinosteroid (BR) signals to regulate hypocotyl elongation in the dark.⁸ Moreover, FER, THE1 and the closely related HERCULES1 (HERK1), were found to regulate cell elongation by interacting with BR signaling.⁹ More recently, roles for FER in the regulation of root hair development and fungal invasion have been established.^10,11 The pollen-specific ANXUR1 (ANX1) and ANXUR2 (ANX2) are closely related to FER and act redundantly to maintain pollen tube growth integrity during its journey through the style and ensure against precocious pollen tube rupture before reaching the ovule.^12,13Apparently with different biological roles, all the CrRLK members analyzed thus far have an effect on the growth of plant cells. The present review focuses on their role during cell growth with emphasis on polar cell growth and the downstream pathways activated by CrRLKs.     

New insights into plant development in New England

This year, the biannually organized FASEB meeting 'Mechanisms in Plant Development' took place in August in Vermont, USA, organized by Martin Hulskamp (University of Koln, Koln, Germany) and John Schiefelbein (University of Michigan, Ann Arbor, MI, USA). The meeting covered numerous topics, ranging from patterning and differentiation to the evolution of developmental mechanisms. Despite apparent distinctions between the sessions, many of the talks were broad ranging and most highlighted unifying developmental concepts.     

Physiological and metabolic profiles of common reed provide insights into plant adaptation to low nitrogen conditions

One of the most distinct features of the common reed (Phragmites australis) is its ability to survive under extremely low nitrogen conditions. To explore the regulation mechanisms of reed to adapt to nitrogen deficiency, we treated reed seedlings under long-term extremely low nitrogen conditions and profiled the physiological and metabolic features of photosynthesis, metabolism, growth, nutrient balance, and enzyme activities. Unexpectedly, the photosynthesis, biomass and carbon content were still maintained at high levels in reed under N-deficient conditions regardless of the decreased content of chlorophyll and nitrogenous compounds. Using mass spectrometry, we profiled metabolism of 627 metabolites and found the concentrations of lactic acid and galactinol were accumulated under the treatment. The development of underground organs and nutrient accumulation (B, P, Zn and Na) were also enhanced under the condition. Unlike the positive correlation of nitrate reductases and N levels in other plants, we found the catalytic activities of nitrate reductases were dramatically elevated in roots under the N-deficient condition, which may increase the intracellular NO₃⁻ and NH₄⁺ levels. Our experiments characterized the unique features of reed under extreme nitrogen deficiency conditions and also provided valuable information for other corps to develop the cultivars with high yield under low nitrogen input.     

New insights into the isopenicillin N transport in Penicillium chrysogenum

In Penicillium chrysogenum the beta-lactam biosynthetic pathway is compartmentalized. This fact forces the occurrence of transport processes of penicillin-intermediate molecules across cell membranes. Many aspects around this molecular traffic remain obscure but are supposed to involve transmembrane transporter proteins. In the present work, an in-depth study has been developed on a Major Facilitator-type secondary transporter from P. chrysogenum named as PenM. The reduction of penM expression level reached by penM targeted silencing, leads to a decrease in benzylpenicillin production in silenced transformants, especially in SilM-35. On the contrary, the penM overexpression from a high efficiency promoter increases the benzylpenicillin production and the expression of the biosynthetic genes. Moreover, when the silenced strain SilM-35 is cultured under penicillin production conditions with 6-aminopenicillanic acid supplementation, an increase in the benzylpenicillin production proportional to the 6-aminopenicillanic acid availability is observed. By this phenomenon, it can be concluded that due to the penM silencing the benzylpenicillin transport remains intact but the peroxisomal isopenicillin N import results affected. As a culminating result, obtained by the expression of the fluorescent recombinant PenM-DsRed protein, it was determined that PenM is naturally located in P. chrysogenum peroxisomes. In summary, our experimental results suggest that PenM is involved in penicillin production most likely through the translocation of isopenicillin N from the cytosol to the peroxisomal lumen across P. chrysogenum peroxisomal membrane.     

Extracellular transport and integration of plant secretory proteins into pathogen-induced cell wall compartments

Many fungal parasites enter plant cells by penetrating the host cell wall and, thereafter, differentiate specialized intracellular feeding structures, called haustoria, by invagination of the plant's plasma membrane. Arabidopsis PEN gene products are known to act at the cell periphery and function in the execution of apoplastic immune responses to limit fungal entry. This response underneath fungal contact sites is tightly linked with the deposition of plant cell wall polymers, including PMR4/GSL5-dependent callose, in the paramural space, thereby producing localized wall thickenings called papillae. We show that powdery mildew fungi specifically induce the extracellular transport and entrapment of the fusion protein GFP–PEN1 syntaxin and its interacting partner monomeric yellow fluorescent protein (mYFP)–SNAP33 within the papillary matrix. Remarkably, PMR4/GSL5 callose, GFP–PEN1, mYFP–SNAP33, and the ABC transporter GFP–PEN3 are selectively incorporated into extracellular encasements surrounding haustoria of the powdery mildew Golovinomyces orontii , suggesting that the same secretory defense responses become activated during the formation of papillae and haustorial encasements. This is consistent with a time-course analysis of the encasement process, indicating that these extracellular structures are generated through the extension of papillae. We show that PMR4/GSL5 callose accumulation in papillae and haustorial encasements occurs independently of PEN1 syntaxin. We propose a model in which exosome biogenesis/release serves as a common transport mechanism by which the proteins PEN1 and PEN3, otherwise resident in the plasma membrane, together with membrane lipids, become stably incorporated into both pathogen-induced cell wall compartments.     

New insights into plant somatic embryogenesis: an epigenetic view

Somatic embryogenesis plays a significant role in plant regeneration and requires complex cellular, molecular, and biochemical processes for embryo initiation and development associated with plant epigenetics. Epigenetic regulation encompasses many sensitive events and plays a vital role in gene expression through DNA methylation, chromatin remodelling, and small RNAs. Recently, regulation of epigenetic mechanisms has been recognized as the most promising occurrences during somatic embryogenesis in plants. A few reports demonstrated that the level of DNA methylation can alter in embryogenic cells under in vitro environments. Changes or modification in DNA methylation patterns is linked with regulatory mechanisms of various candidate marker genes, involved in the initiation and development of somatic embryogenesis in plants. This review summarizes the current scenario of the role of epigenetic mechanisms as candidate markers during somatic embryogenesis. It also delivers a comprehensive and systematic analysis of more recent discoveries on expression of embryogenic-regulating genes during somatic embryogenesis, epigenetic variation. Biotechnological applications of epigenetics as well as new opportunities or future perspectives in the development of somatic embryogenesis studies are covered. Further research on such strategies may serve as exciting interaction models of epigenetic regulation in plant embryogenesis and designing novel approaches for plant productivity and crop improvement at molecular levels.     

Taurine and cell volume maintenance in the shark rectal gland: cellular fluxes and kinetics

Tissue slices of shark rectal gland are studied to examine the kinetics of the cellular fluxes of taurine, a major intracellular osmolyte in this organ. Maintenance of high steady-state cell taurine (50 mM) is achieved by a ouabain-sensitive active Na+-dependent uptake process and a relatively slow efflux. Uptake kinetics are described by two saturable taurine transport components (high-affinity, Km 60 microM; and low-affinity, Km 9 mM). [14C]Taurine uptake is enhanced by external Cl-, inhibited by beta-alanine and unaffected by inhibitors of the Na+/K+/2Cl- co-transport system. Two cellular efflux components of taurine are documented. Incubation of slices in p-chloromercuribenzene sulfonate (1 mM) reduces taurine uptake, increases efflux of taurine and induces cell swelling. Studies of efflux in isotonic media with various cation and anion substitutions demonstrate that high-K+ markedly enhances taurine efflux irrespective of cell volume changes (i.e. membrane stretching is not involved). Moreover, iso-osmotic cell swelling induced in media containing propionate is not associated with enhanced efflux of taurine from the cells. It is suggested that external K+ exerts a specific effect on the cytoplasmic membrane to increase its permeability to taurine.     

Zero order kinetics of cell wall turnover inStaphylococcus aureus

Cells exponentially grown from four strains ofS. aureus (SG 511, H, 52A5G, and248 PN-1) and uniformly labeled in their walls with³H-N-acetylglucosamine, were found to turn over their old walls at constant rates of up to 25% per generation. Wall turnover was not observed to follow first order kinetics, thus ruling out the implication that maintenance of normal wall thickness was achieved by a random distribution of new wall components in the old wall. Instead, wall turnover in all cases strictly followed zero order kinetics, indicating that newly synthesized wall material was placed layer by layer beneath the inner surface of the old cell wall. This finding correlates with evidence obtained from earlier electron microscopic investigations into the regeneration of the staphylococcal cell wall after chloramphenicol treatment. Based on the experimental data presented, a simplified model for wall turnover of the growing staphylococcal cell was proposed. The model also takes into account the finding, derived from additional experiments with strainSG 511, that the total cell wall turned over at a somewhat higher rate than the old portions of the wall. The rates of cell wall turnover found inS. aureus SG 511 are the highest reported to date for pathogenic bacteria. The medical implications of this finding were discussed.     

Targeted integration of T-DNA into the tobacco genome at double-stranded breaks: new insights on the mechanism of T-DNA integration

Agrobacterium tumefaciens T-DNA normally integrates into random sites in the plant genome. We have investigated targeting of T-DNA by nonhomologous end joining process to a specific double-stranded break created in the plant genome by I-CeuI endonuclease. Sequencing of genomic DNA/T-DNA junctions in targeted events revealed that genomic DNA at the cleavage sites was usually intact or nearly so, whereas donor T-DNA ends were often resected, sometimes extensively, as is found in random T-DNA inserts. Short filler DNAs were also present in several junctions. When an I-CeuI site was placed in the donor T-DNA, it was often cleaved by I-CeuI endonuclease, leading to precisely truncated targeted T-DNA inserts. Their structure requires that T-DNA cutting occurred before or during integration, indicating that T-DNA is at least partially double stranded before integration is complete. This method of targeting full-length T-DNA with considerable fidelity to a chosen break point in the plant genome may have experimental and practical applications. Our findings suggest that insertion at break points by nonhomologous end joining is one normal mode of entry for T-DNA into the plant genome.