Overexpression of OsKTN80a,a katanin P80ortholog,caused the repressed cell elongation and stalled cell division mediated by microtubule apparatus defects in primary root in Oryza sativa

Katanin, a microtubule‐severing enzyme, consists of two subunits: the catalytic subunit P60, and the regulatory subunit P80. In several species, P80 functions in meiotic spindle organization, the flagella biogenesis, the neuronal development, and the male gamete production. However,the P80 function in higher plants remains elusive. In this study, we found that there are three katanin P80 orthologs(OsKTN80a, OsKTN80b, and OsKTN80c) in Oryza sativa L.Overexpression of OsKTN80a caused the retarded root growth of rice seedlings. Further investigation indicates that the retained root growth was caused by the repressed cell elongation in the elongation zone and the stalled cytokinesis in the division zone in the root tip. The in vivo examination suggests that OsKTN80a acts as a microtubule stabilizer. We prove that OsKTN80a, possibly associated with OsKTN60, is involved in root growth via regulating the cell elongation and division.     

水稻和小麦根尖细胞壁多糖的铝积累能力比较

采用水培法比较4种禾本科植物水稻（Oryza sativa L.）、玉米（Zea mays L.）、高粱（Sorghum bicolor（L.）Moench）和小麦（Triticum aestivum L.）8个基因型的抗铝（Al）能力,并对他们在Al积累后细胞壁的多糖组分进行分析。结果显示,在5~200 μmol/L Al处理下,水稻抗Al能力较强,而小麦抗Al能力较弱。在50 μmol/L Al处理下,小麦根尖的果胶和半纤维素1含量的增幅明显高于水稻。水稻基因型‘日本晴’与‘浙辐802’的细胞壁Al含量分别占根尖总Al含量的78.7%和91.6%;小麦基因型‘扬麦18’与‘扬麦16’Al含量分别占根尖总Al含量的64.9%和72.1%。Al吸附-解吸实验结果显示,小麦根尖细胞壁上Al的吸附量高于水稻。研究结果表明,细胞壁是Al积累的主要部位,对Al敏感的水稻和小麦基因型细胞壁中的Al主要分布在果胶中;而对Al耐性较强的水稻和小麦基因型细胞壁中的Al主要分布在半纤维素1中。     

Changes in onion root development induced by the inhibition of peptidyl-prolyl hydroxylase and influence of the ascorbate system on cell division and elongation

Post-translational hydroxylation of peptide-bound proline residues, catalyzed by peptidyl-prolyl-4 hydroxylase (EC 1.14.11.2) using ascorbate as co-substrate, is a key event in the maturation of a number of cell wall-associated hydroxyproline-rich glycoproteins (HRGPs), including extensins and arabinogalactan-proteins, which are involved in the processes of wall stiffening, signalling and cell proliferation. Allium cepa L. roots treated with 3,4-DL-dehydroproline (DP), a specific inhibitor of peptidyl-prolyl hydroxylase, showed a 56% decrease in the hydroxyproline content of HRGP. Administration of DP strongly affected the organization of specialized zones of root development, with a marked reduction of the post-mitotic isodiametric growth zone, early extension of cells leaving the meristematic zone and a huge increase in cell size. Electron-microscopy analysis showed dramatic alterations both to the organization of newly formed cell walls and to the adhesion of the plasma membranes to the cell walls. Moreover, DP administration inhibited cell cycle progression. Root tips grown in the presence of DP also showed an increase both in ascorbate content (+53%) and ascorbate-specific peroxidase activity in the cytosol (+72%), and a decrease in extracellular “secretory” peroxidase activity (−73%). The possible interaction between HRGPs and the ascorbate system in the regulation of both cell division and extension is discussed. Received: 14 October 1998 / Accepted: 31 May 1999     

Correlation of cytokinin levels in the endosperms and roots with cell number and cell division activity during endosperm development in rice

Cell number and cell division activity in rice (Oryza sativa) endosperms are possibly regulated by cytokinin levels in the endosperm and its source in the roots. This study tried to find the possible correlations among them. Six rice genotypes were grown in nutrient solution. Two patterns of endosperm cell division, synchronous and asynchronous, were observed among the genotypes based on the cell division rate of superior and inferior spikelets. Contents of zeatin (Z) + zeatin riboside (ZR) were much higher than those of N6-isopentenyladenine (iP) and N6-isopentenyladenosine (iPR) in both endosperms and roots. Changes in Z + ZR levels in endosperms were significantly correlated with those in roots, and both were very significantly correlated with the cell division rate. Changes in iP + iPR contents in the roots were not significantly correlated with those in the endosperms and the cell division rate. When roots were treated with kinetin, endosperm cell number and grain weight were increased. Such enhancement was more significantly achieved by the root kinetin treatment than by spraying kinetin on leaves and panicles. The results suggest that the cell number and cell division activity in rice endosperms are regulated by cytokinin levels in the endosperm and that root-derived Z + ZR play a pivotal role.     

Leaf expansion and cell division are affected by reducing absorbed light before but not after the decline in cell division rate in the sunflower leaf

The effect of absorbed photosynthetic photon flux density (PPFD) on leaf expansion is a key issue for analysing the phenotypic variability between plants and for modelling feedback loops. Expansion and epidermal cell division in leaf 8 of sunflower were analysed in a series of five experiments where absorbed photosynthetic photon flux density (PPFD) was reduced either by shading or by covering part of the leaf area. These treatments were imposed at different times during leaf development. Expansion and cell division were affected by a reduction in absorbed PPFD only in the first part of leaf development, while the leaf area was less than 2% of its final value and while absolute expansion rate was slow. In contrast, it was not affected if imposed later when the leaf was visible and absolute expansion rate was at maximum. A reduction in absorbed PPFD caused the same reduction in expansion and in cell division whether it was due to a reduction in incident PPFD or to a reduction in photosynthetic leaf area, suggesting that carbon metabolism was involved. Relative expansion rate recovered to control levels when relative division rate began to decline, in all experiments and in all zones of a leaf. This was probably linked to the source–sink transition, after which the leaf had such a high priority in carbon allocation that it was largely insensitive to changes in absorbed PPFD. The final leaf area was therefore closely related to the cumulated PPFD absorbed by the plant from leaf initiation to the end of exponential cell division.     

Grain growth and endosperm cell size under high night temperatures in rice (Oryza sativa L.)

BACKGROUND AND AIMS: High night temperatures are more harmful to grain weight in rice than high day temperatures. Grain growth rate and growth duration were investigated to determine which was the cause of the decrease in final grain weight under high night temperatures. Endosperm cell number and cell sizes were also examined to determine which might cause the decrease in final grain weight. METHODS: Rice plants were grown outdoors in plastic pots and moved at heading time to three temperature-controlled glasshouses under high night temperature (HNT; 22/34 degrees C), high day temperature (HDT; 34/22 degrees C) and control conditions (CONT; 22/22 degrees C). Grains were sampled periodically, and the time-course of grain growth was divided into rate and duration by logistic regression analysis. Endosperm cell numbers and cell sizes were analysed by digitalized hand-tracing images of endosperm cross-sections. KEY RESULTS: The duration of grain growth was reduced by high temperature both day and night. However, the rate of grain growth was lower in HNT than in HDT. The number of cells in endosperm cross-sections in HNT was similar to that in HDT, and higher than that in CONT. The average cell area was smaller in HNT than in either CONT or HDT. The differences in average cell areas between HNT and HDT were greater at distances 60-80 % from the central point of endosperm towards the endosperm surface. CONCLUSIONS: The results show that HNT compared with HDT reduced the final grain weight by a reduction in grain growth rate in the early or middle stages of grain filling, and also reduced cell size midway between the central point and the surface of endosperm.     

A mutation of the CRUMPLED LEAF gene that encodes a protein localized in the outer envelope membrane of plastids affects the pattern of cell division, cell differentiation, and plastid division in Arabidopsis

We identified a novel mutation of a nuclear-encoded gene, designated as CRUMPLED LEAF (CRL), of Arabidopsis thaliana that affects the morphogenesis of all plant organs and division of plastids. Histological analysis revealed that planes of cell division were distorted in shoot apical meristems (SAMs), root tips, and embryos in plants that possess the crl mutation. Furthermore, we observed that differentiation patterns of cortex and endodermis cells in inflorescence stems and root endodermis cells were disturbed in the crl mutant. These results suggest that morphological abnormalities observed in the crl mutant were because of aberrant cell division and differentiation. In addition, cells of the crl mutant contained a reduced number of enlarged plastids, indicating that the division of plastids was inhibited in the crl. The CRL gene encodes a novel protein with a molecular mass of 30 kDa that is localized in the plastid envelope. The CRL protein is conserved in various plant species, including a fern, and in cyanobacteria, but not in other organisms. These data suggest that the CRL protein is required for plastid division, and it also plays an important role in cell differentiation and the regulation of the cell division plane in plants. A possible function of the CRL protein is discussed.     

OsLOL1, a C2C2‐type zinc finger protein,interacts with OsbZIP58 to promote seed germination through the modulation of gibberellin biosynthesis in Oryza sativa

Seed germination is a key developmental process in the plant life cycle that is influenced by various environmental cues and phytohormones through gene expression and a series of metabolism pathways. In the present study, we investigated a C2C2‐type finger protein, OsLOL1, which promotes gibberellin (GA) biosynthesis and affects seed germination in Oryza sativa (rice). We used OsLOL1 antisense and sense transgenic lines to explore OsLOL1 functions. Seed germination timing in antisense plants was restored to wild type when exogenous GA₃ was applied. The reduced expression of the GA biosynthesis gene OsKO2 and the accumulation of ent‐kaurene were observed during germination in antisense plants. Based on yeast two‐hybrid and firefly luciferase complementation analyses, OsLOL1 interacted with the basic leucine zipper protein OsbZIP58. The results from electrophoretic mobility shift and dual‐luciferase reporter assays showed that OsbZIP58 binds the G‐box cis‐element of the OsKO2 promoter and activates LUC reporter gene expression, and that interaction between OsLOL1 and OsbZIP58 activates OsKO2 gene expression. In addition, OsLOL1 decreased SOD1 gene expression and accelerated programmed cell death (PCD) in the aleurone layer of rice grains. These findings demonstrate that the interaction between OsLOL1 and OsbZIP58 influences GA biosynthesis through the activation of OsKO2 via OsbZIP58, thereby stimulating aleurone PCD and seed germination.     

Factors responsible for deep-sowing tolerance in wheat seedlings: varietal differences in cell proliferation and the co-ordinated synchronization of epidermal cell expansion and cortical cell division for the gibberellin-mediated elongation of first internodes

MethodsThe lengths and numbers of epidermal and cortical cells of the first internodes in three wheat cultivars were measured. These parameters were compared in wheat seedlings treated with gibberellin A₃ (GA₃) or an inhibitor of GA biosynthesis, uniconazole.ConclusionsThe deep-sowing-tolerant cultivar ‘Hong Mang Mai’ is able to elongate the first internode to a greater degree due to enhanced cell division and a heightened response to GA. In addition, cell expansion in the epidermis and cell division in the cortex are synchronized for the elongation of the first internodes. In response to GA, this well-co-ordinated synchronization yields the rapid elongation of the first internodes in wheat seedlings.     

Silicon reduces sodium uptake in rice (Oryza sativa L.) in saline conditions and this is accounted for by a reduction in the transpirational bypass flow

Rice is relatively sensitive to salinity and is classified as a silicon accumulator. There have been reports that silicon can reduce sodium uptake in crop grasses in saline conditions, but the mechanism by which silicon might alleviate salinity damage is unclear. We report on the effects of silicon on growth, gas exchange and sodium uptake in rice genotypes differing in salt tolerance. In non-saline media there were no effects of supplementary silicate upon shoot fresh or dry weight or upon root dry weight, indicating that the standard culture solution was not formally deficient with respect to silicon. Plants grown with supplementary silicate had slightly, but significantly, shorter leaves than plants grown in a standard culture solution. Salinity reduced growth and photosynthetic gas exchange. Silicate supplementation partly overcame the reduction in growth and net photosynthesis caused by salt. This amelioration was correlated with a reduction in sodium uptake. Silicate supplementation increased the stomatal conductance of salt-treated plants, showing that silicate was not acting to reduce sodium uptake via a reduction in the transpiration rate. Silicate reduced both sodium transport and the transport of the apoplastic tracer trisodium-8-hydroxy-1,3,6-pyrenetrisulphonic acid (PTS). This implies that the mode of action of silicate was by partial blockage of the transpirational bypass flow, the pathway by which a large proportion of the uptake of sodium in rice occurs. Mechanisms by which silicate might reduce the transpirational bypass flow directly are discussed.     

Inactivation of DNA replication origins by the cell cycle regulator, trigonelline, in root meristems of Lactuca sativa

 The effects of trigonelline (TRG) on the cell cycle in root meristems of Lactuca sativa L. were examined in the knowledge that TRG is a cell cycle regulator that causes cell arrest in G2, and prevents ligation of replicons in S-phase. The hypothesis was tested that continuous exposure to TRG would perturb DNA replication which, in turn, would lengthen the cell cycle and impair root elongation. Using DNA fibre autoradiography, mean replicon size was 31 and 13 μm in the TRG (3 mM) and control treatments, respectively. Trigonelline also resulted in a lengthening of both S-phase and the cell cycle and a decrease in primary root elongation. Hence, replicon inactivation was responsible for the protracted S-phase. Trigonelline treatment also resulted in a 1.6-fold increase in fork rate (13.8 μm h⁻¹) compared with the control (8.4 m h⁻¹). The faster fork rate in the larger replicons is in accord with the highly significant positive relationship already established between fork rate and replicon size for various unrelated higher plants. Received: 11 October 1999 / Accepted: 23 December 1999     

Larval-to-adult conversion of a myogenic system in the frog, Xenopus laevis, by larval-type myoblast-specific control of cell division, cell differentiation, and programmed cell death by triiodo-L-thyronine

For the clarification of larval-to-adult muscle conversion, the authors established primary culture methods for adult- and larval-type myoblasts in the frog, Xenopus laevis, and examined the hormonal response in each case. The cell types were enzymatically dissociated from adult frog leg and tadpole tail muscles, respectively. The cells became attached to culture plates, proliferated, and fused with each other to form multinucleated myotubes within one week. Five significant differences between the two cell types were noted. (1) Adult cells showed greater proliferation activity than larval cells, the former increasing 5.5-fold over 6 days while the latter increase only 2.5-fold. (2) Differentiation (fusion) of larval type myoblasts started earlier. Cell fusion began on day 2 or 3 in larval cells and on day 4 in adult cells. (3) The metamorphic hormone, triiodo-L-thyronine (T3) decreased larval cell numbers to 56% of that of control-cultures on day 7 but had no effect on adult cell number. DNA synthetic activity (3H-thymidine incorporation) in larval cells decreased under T3 (10(-8) M) to 45% of the control level on day 7. (4) Differentiation of adult myoblasts into myotubes was promoted by T3, whereas that of larval cells diminished by half. (5) Myotube death was induced by T3 specifically in larval but not in adult cultures. In addition to the myotube death, double staining with TUNEL (in situ DNA nick end labeling) and anti-desmin antibody indicated that T3 induces myoblast (desmin+ cell) death specifically in larval but not in adult cells. It is thus evident that the conversion of a larval-type myogenic system during metamorphosis becomes possible through nearly totally specific control of cell division, cell differentiation, and programmed cell death at a precursor cell level by T3.