Arabidopsis VILLIN2 and VILLIN3 act redundantly in sclerenchyma development via bundling of actin filaments

The organization of the actin cytoskeleton has been implicated in sclerenchyma development. However, the molecular mechanisms linking the actin cytoskeleton to this process remain poorly understood. In particular, there have been no studies showing that direct genetic manipulation of the actin cytoskeleton affects sclerenchyma development. Villins belong to the villin/gelsolin/fragmin superfamily and are versatile actin-modifying proteins. Several recent studies have implicated villins in tip growth of single cells, but how villins act in multicellular plant development remains largely unknown. Here, we found that two closely related villin isovariants from Arabidopsis, VLN2 and VLN3, act redundantly in sclerenchyma development. Detailed analysis of cross-sections from inflorescence stems of vln2 vln3 double mutant plants revealed a reduction in stem size and in the number of vascular bundles; however, no defects in synthesis of the secondary cell wall were detected. Surprisingly, the vln2 vln3 double mutation did not affect cell elongation of inter-fascicular fibers. Biochemical analyses showed that recombinant VLN2 was able to cap, sever and bundle actin filaments, similar to VLN3. Consistent with these biochemical activities, loss of function of VLN2 and VLN3 resulted in a decrease in the amount of F-actin and actin bundles in plant cells. Collectively, our findings demonstrate that VLN2 and VLN3 act redundantly in sclerenchyma development via bundling of actin filaments.     

Hypergravity stimulus enhances primary xylem development and decreases mechanical properties of secondary cell walls in inflorescence stems of Arabidopsis thaliana

BACKGROUND AND AIMS: The xylem plays an important role in strengthening plant bodies. Past studies on xylem formation in tension woods in poplar and also in clinorotated Prunus tree stems lead to the suggestion that changes in the gravitational conditions affect morphology and mechanical properties of xylem vessels. The aim of this study was to examine effects of hypergravity stimulus on morphology and development of primary xylem vessels and on mechanical properties of isolated secondary wall preparations in inflorescence stems of arabidopsis. METHODS: Morphology of primary xylem was examined under a light microscope on cross-sections of inflorescence stems of arabidopsis plants, which had been grown for 3-5 d after exposure to hypergravity at 300 g for 24 h. Extensibility of secondary cell wall preparation, isolated from inflorescence stems by enzyme digestion of primary cell wall components (mainly composed of metaxylem elements), was examined. Plants were treated with gadolinium chloride, a blocker of mechanoreceptors, to test the involvement of mechanoreceptors in the responses to hypergravity. KEY RESULTS: Number of metaxylem elements per xylem, apparent thickness of the secondary thickenings, and cross-section area of metaxylem elements in inflorescence stems increased in response to hypergravity. Gadolinium chloride suppressed the effect of hypergravity on the increase both in the thickness of secondary thickenings and in the cross-section area of metaxylem elements, while it did not suppress the effect of hypergravity on the increase in the number of metaxylem elements. Extensibility of secondary cell wall preparation decreased in response to hypergravity. Gadolinium chloride suppressed the effect of hypergravity on cell wall extensibility. CONCLUSIONS: Hypergravity stimulus promotes metaxylem development and decreases extensibility of secondary cell walls, and mechanoreceptors were suggested to be involved in these processes.     

Improvement of cell wall digestibility in tall fescue by <Emphasis Type="Italic">Oryza sativa</Emphasis> SECONDARY WALL NAC DOMAIN PROTEIN2 chimeric repressor

Forage digestibility is one of the most important factors in livestock performance. As grasses grow and mature, dry matter increases but they become fibrous with secondary cell wall deposition and lignification of sclerenchyma cells, and forage quality drops. In rice (Oryza sativa), the SECONDARY WALL NAC DOMAIN PROTEIN2 fused with the modified EAR-like motif repression domain (OsSWN2-SRDX) reduces secondary cell wall thickening in sclerenchyma cells. We introduced OsSWN2-SRDX under the control of the OsSWN1 promoter into tall fescue (Festuca arundinacea Schreb.) to increase cell wall digestibility. Of 23 transgenic plants expressing OsSWN2-SRDX, nine had brittle internodes that were easily broken by bending. Their secondary cell walls were significantly thinner than those of the wild type in interfascicular fibers of internodes and in cortical fiber cells between leaf epidermal cells and vascular bundles. The dry matter digestibility increased by 11.8% in stems and by 6.8% in leaves compared with the wild type, and therefore forage quality was improved. In stem interfascicular fibers, acid detergent fiber and acid insoluble lignin were greatly reduced. Thus, the reduction of indigestible fiber composed of cellulose and lignin increased the degradability of sclerenchyma cell walls. OsSWN2-SRDX plants offer great potential in the genetic improvement of forage digestibility.     

Effects of hypergravity conditions on elongation growth and lignin formation in the inflorescence stem of Arabidopsis thaliana

The effects of hypergravity on elongation growth and lignin deposition in secondary cell walls of the Arabidopsis thaliana (L.) Heynh. inflorescence stem were examined in plants grown for 3 days after exposure to hypergravity in the direction from shoot to root at 300 g for 24 h. The content of acetylbromide-extractable lignins in a secondary cell wall fraction prepared by enzyme digestion of inflorescence stem segments removing primary cell wall components was significantly increased by the hypergravity stimulus. Xylem vessels, particularly in a region closer to the base of the inflorescence stem, increased in number. Gadolinium chloride at 0.1 mM, a blocker of mechanoreceptors, partially suppressed the effect of hypergravity on lignin deposition in the secondary cell wall fraction. These results suggest that mechanoreceptors are responsible for hypergravity-induced lignin deposition in secondary cell walls in A. thaliana inflorescence stems.     

Alteration of auxin polar transport in the Arabidopsis ifl1 mutants

The INTERFASCICULAR FIBERLESS/REVOLUTA (IFL1/REV) gene is essential for the normal differentiation of interfascicular fibers and secondary xylem in the inflorescence stems of Arabidopsis. It has been proposed that IFL1/REV influences auxin polar flow or the transduction of auxin signal, which is required for fiber and vascular differentiation. Assay of auxin polar transport showed that the ifl1 mutations dramatically reduced auxin polar flow along the inflorescence stems and in the hypocotyls. The null mutant allele ifl1-2 was accompanied by a significant decrease in the expression level of two putative auxin efflux carriers. The ifl1 mutants remained sensitive to auxin and an auxin transport inhibitor. The ifl1-2 mutant exhibited visible phenotypes associated with defects in auxin polar transport such as pin-like inflorescence, reduced numbers of cauline branches, reduced numbers of secondary rosette inflorescence, and dark green leaves with delayed senescence. The visible phenotypes displayed by the ifl1 mutants could be mimicked by treatment of wild-type plants with an auxin polar transport inhibitor. In addition, the auxin polar transport inhibitor altered the normal differentiation of interfascicular fibers in the inflorescence stems of wild-type Arabidopsis. Taken together, these results suggest a correlation between the reduced auxin polar transport and the alteration of cell differentiation and morphology in the ifl1 mutants.     

Gibberellin signaling

A study of stem anatomy and the sclerenchyma fibre cells associated with the phloem tissues of hemp (Cannabis sativa L.) plants is of interest for both understanding the formation of secondary cell walls and for the enhancement of fibre utility as industrial fibres and textiles. Using a range of molecular probes for cell wall polysaccharides we have surveyed the presence of cell wall components in stems of hemp in conjunction with an anatomical survey of stem and phloem fibre development. The only polysaccharide detected to occur abundantly throughout the secondary cell walls of phloem fibres was cellulose. Pectic homogalacturonan epitopes were detected in the primary cell walls/intercellular matrices between the phloem fibres although these epitopes were present at a lower level than in the surrounding parenchyma cell walls. Arabinogalactan-protein glycan epitopes displayed a diversity of occurrence in relation to fibre development and the JIM14 epitope was specific to fibre cells, binding to the inner surface of secondary cell walls, throughout development. Xylan epitopes were found to be present in the fibre cells (and xylem secondary cell walls) and absent from adjacent parenchyma cell walls. Analysis of xylan occurrence in the phloem fibre cells of hemp and flax indicated that xylan epitopes were restricted to the primary cell walls of fibre cells and were not present in the secondary cell walls of these cells.     

菰适应湿地环境的解剖和屏障结构特征研究

菰(Zizania latifolia)是一种多年生挺水植物,为了探讨该植物根、茎和叶的解剖结构、组织化学及其质外体屏障的通透性生理。该文利用光学显微镜和荧光显微镜,对菰的根、茎、叶进行了解剖学和组织化学研究。结果表明:(1)菰不定根解剖结构由外而内分别为表皮、外皮层、单层细胞的厚壁机械组织层、皮层、内皮层和维管柱;茎结构由外而内分别为角质层、表皮、周缘厚壁机械组织层、皮层、具维管束的厚壁组织层和髓腔。叶鞘具有表皮和具维管束皮层,叶片具有表皮,叶肉和维管束。(2)不定根具有位于内侧的内皮层及其邻近栓质化细胞和外侧的外皮层组成的屏障结构;茎具内侧厚壁机械组织层,外侧的角质层和周缘厚壁机械组织层组成的屏障结构,屏障结构的细胞壁具凯氏带、木栓质和木质素沉积的组织化学特点,叶表面具有角质层。(3)菰通气组织包括根中通气组织,茎、叶皮层的通气组织和髓腔。(4)菰的屏障结构和解剖结构是其适应湿地环境的重要特征,但其茎周缘厚壁层和厚壁组织层较薄。由此推测,菰适应湿地环境,但在旱生环境中分布有一定的局限性。     

Secretory low molecular weight phospholipase A2 plays important roles in cell elongation and shoot gravitropism in Arabidopsis

To elucidate the cellular functions of phospholipase A(2) in plants, an Arabidopsis cDNA encoding a secretory low molecular weight phospholipase A(2) (AtsPLA(2)beta) was isolated. Phenotype analyses of transgenic plants showed that overexpression of AtsPLA(2)beta promotes cell elongation, resulting in prolonged leaf petioles and inflorescence stems, whereas RNA interference-mediated silencing of AtsPLA(2)beta expression retards cell elongation, resulting in shortened leaf petioles and stems. AtsPLA(2)beta is expressed in the cortical, vascular, and endodermal cells of the actively growing tissues of inflorescence stems and hypocotyls. AtsPLA(2)beta then is secreted into the extracellular spaces, where signaling for cell wall acidification is thought to occur. AtsPLA(2)beta-overexpressing or -silenced transgenic plants showed altered gravitropism in inflorescence stems and hypocotyls. AtsPLA(2)beta expression is induced rapidly by auxin treatment and in the curving regions of inflorescence stems undergoing the gravitropic response. These results suggest that AtsPLA(2)beta regulates the process of cell elongation and plays important roles in shoot gravitropism by mediating auxin-induced cell elongation.     

Chlorophyll distribution in the stems and trunk of beech trees

The distribution of chlorophyll was examined in cross-sections of 2- and 6-year-old stems as well as in the bark of the stump trunk of beech trees, utilising chlorophyll autofluorescence. The investigations were conducted using a confocal microscope. The tests carried out on 2 – 6-year old stems showed a large presence of chlorophyll in the bark, in primary and secondary rays as well as in the pith, but smaller amounts in wood parenchyma cells. There was no chlorophyll in the cork, sclerenchyma: in wood in vessels, tracheids and fibers. A reduction in the chlorophyll content in 6-year-old stems was not observed. In the bark of the trunk, chlorophyll was found in large amounts directly under the cork and in vestigial amounts in the primary phloem.     

Changes in leaf,stem, and root anatomy of Chrysanthemum cv. Lillian Hoek following paclobutrazol application

Plants ofChrysanthemum cv. Lillian Hoek were treated with a paclobutrazol (PBZ) soil drench and histologically examined after 3 months. PBZ application resulted in thicker leaves, reduced stem diameter, and roots with an increased diameter and an unusual segmented appearance. Increased leaf thickness was partly due to an additional layer of palisade mesophyll, although individual palisade cells were shorter, of smaller diameter, and more tightly packed. Spongy mesophyll depth was also greater and the individual cells were more rounded and the volume of intercellular space was reduced. The narrower stems had an increased development of secondary xylem, but had a marked reduction in the number of sclerenchyma bundle caps. Increased root diameter was due to an increase in the number of rows and diameter of cortical cells. In PBZ-treated plants, root cortical cell length was 50–70% less than in untreated plants, and this reduction appeared to be associated with the segmentation of the roots. PBZ inhibited secondary vascular development in the roots. This study is similar to other relevant studies in recording thicker leaves and roots with PBZ application; however, many of the underlying anatomical changes described above have not been previously reported.     

Reduced gravitropism in inflorescence stems and hypocotyls, but not roots, of Arabidopsis mutants with large plastids

The sites of gravity perception are columella cells in roots and endodermal cells in hypocotyls and inflorescence stems. Since plastids are likely to play a role in graviperception, we investigated gravitropism in plastid mutants of Arabidopsis . Previous studies have shown that the arc 6 and arc 12 ( a ccumulation and r eplication of c hloroplasts) mutants have an average of two large plastids per leaf mesophyll cell. In this study, we found that these arc mutants have altered plastid morphology throughout the entire plant body, including the cells involved in gravity perception. There were no major differences in total starch content per cell in endodermal and columella cells of the wild-type (WT) compared to arc 6 and arc 12 as assayed by iodine staining. Thus, the total mass of plastids per cell in arc 6 and arc 12 is similar to their respective WT strains. Results from time course of curvature studies demonstrated that the plastid mutation affected gravitropism only of inflorescence stems and hypocotyls, but not roots. Thus, roots appear to have different mechanisms of gravitropism compared to stems and hypocotyls. Time course of curvature studies with light-grown seedlings were performed in the presence of latrunculin B (Lat-B), an actin-depolymerizing drug. Lat-B promoted gravitropic curvature in hypocotyls of both the WT and arc 6 but had little or no effect on gravitropism in roots of both strains. These results suggest that F-actin is not required for hypocotyl gravitropism.     

Genome-wide identification, classification and expression analysis of genes encoding putative fasciclin-like arabinogalactan proteins in Chinese cabbage (Brassica rapa L.)

Fasciclin-like arabinogalactan proteins (FLAs), a subclass of arabinogalactan proteins (AGPs), have both predicted AGP-like glycosylated regions and putative fasciclin (FAS) domains, which may function in cell adhesion and communication. Previous studies have identified 21, 27, and 34 FLAs in Arabidopsis (Arabidopsis thaliana), rice (Oryza sativa), and wheat (Triticum aestivum), respectively. In this study, we identified 33 FLAs in the annotated genome of Chinese cabbage (Brassica rapa ssp. pekinensis line Chiifu-401-42). Sequence analysis indicated that FAS domains each contain two highly conserved regions, named H1 and H2, and that 17 FLAs from B. rapa (BrFLAs) possess both of these regions. Prediction of glycosylphosphatidylinositol (GPI) modification sites suggested that 15 BrFLAs were GPI-anchored to the plasma membrane. Additionally, 25 BrFLAs may have been duplicated during the processes that shaped the triplicated genome of the mesopolyploid B. rapa. Expression analyses indicated that BrFLA1, BrFLA11, BrFLA13, BrFLA28 and BrFLA32 were specifically expressed in inflorescence. Meanwhile, BrFLA9 (homologous to AtFLA12) is specifically expressed in stem, and BrFLA6/22 (homologous to AtFLA11) is also highly expressed in stem, suggesting BrFLA6/9/22 may have the same functions as AtFLA11/12 in A. thaliana. Taken together, the identification and bioinformatic analysis of FLAs in B. rapa will open the way for studying their biological functions in plant growth and development as well as evolutionary history of this gene family from A. thaliana to B. rapa.     

Characterization and genetic analysis of a lettuce (Lactuca sativa L.) mutant,weary, that exhibits reduced gravitropic response in hypocotyls and inflorescence stems

A lettuce (Lactuca sativa L.) mutant that exhibits a procumbent growth habit was identified and characterized. In two wild type (WT) genetic backgrounds, segregation patterns revealed that the mutant phenotype was controlled by a recessive allele at a single locus, which was designated weary. Hypocotyls and inflorescence stems of plants homozygous for the weary allele exhibited reduced gravitropic responses compared with WT plants, but roots exhibited normal gravitropism. Microscopic analysis revealed differences in the radial distribution of amyloplasts in hypocotyl and inflorescence stem cells of weary and WT plants. Amyloplasts occurred in a single layer of endodermal cells in WT hypocotyls and inflorescence stems. By contrast, amyloplasts were observed in several layers of cortical cells in weary hypocotyls, and weary inflorescence stem cells lacked amyloplasts entirely. These results are consistent with the proposed role of sedimenting amyloplasts in shoot gravitropism of higher plants. The phenotype associated with the weary mutant is similar to that described for the Arabidopsis mutant sgr1/scr, which is defective in radial patterning and gravitropism.     

Comparative vegetative anatomy of Stanhopeinae (Orchidaceae)

Stanhopeinae are a group of tropical American orchids characterized by euglossine bee pollination and lateral inflorescences stemming from the bases of pseudobulbs. Leaves are hypostomatal, and all stomatal configurations are tetracytic. Chlorenchyma is homogeneous and characterized by fibre bundles in adaxial/abaxial or adaxial/median/abaxial positions. Collateral vascular bundles occur in a single row and feature phloic and xylic sclerenchymatous caps and thin-walled bundle sheath cells. Fibre bundles and vascular sclerenchyma are accompanied by stegmata containing conical silica bodies. Pseudobulbs have thick-walled turbinate epidermal cells and ground tissue of smaller, living assimilatory cells and larger, dead water-storage cells. Fibre bundles are usually absent but occur in several genera. Collateral vascular bundles show phloic sclerenchyma, but xylic sclerenchyma occurs only in die larger vascular bundles. Phloic and xylic sclerenchyma are associated with stegmata containing conical silica bodies. Roots are velamentous. Velamen cell walls have fine, spiral thickenings. Exodermal cells are thin-walled. The cortex features scattered thick-walled cells and in some cases branched bars of secondary cell wall material. Endodermis is either u-or O-thickened, but pericycle is always O-thickened opposite the phloem. Vascular tissue consists of alternating strands of xylem and phloem surrounded by a matrix of thick-walled cells. Pith cells may be parenchymatous or sclerenchymatous.     

Anatomical and histological factors affecting the ruminal degradation of stem tissues in Bothriochloa species

Anatomical and histological features affecting the degradation of stem tissues by ruminal microorganisms were examined in ‘Caucasian’, ‘Ganada,’ and ‘WW-Spar’ Old World bluestem grasses (Bothriochloa spp.). Tissue degradation and/or staining results revealed variation among cultivars in the degree of lignification of parenchyma, sclerenchyma, and epidermal cells. In both immature and mature stems, parenchyma and sclerenchyma cell degradation by microorganisms was rapid and extensive for Ganada as compared with Caucasian and WW-Spar. In general, Caucasian tissues were more resistant to degradation than WW-Spar. There also were differences in tissue composition and degradation as a result of stem maturity. Increased lignification in parenchyma cells was primarily responsible for a decreased rate and extent of degradation in older stem portions. The major difference in tissue composition among cultivars was found in older stems; WW-Spar had more vascular and less parenchyma tissues than Caucasian and Ganada. However, Caucasian had a greater percentage of xylem-metaxylem complex tissues than the other two cultivars. The amount of lignified tissues did not seem to be related strongly with rate and extent of tissue degradation. Instead, the data suggest that our previously reported differences in digestibility among the three Old World bluestem grasses could be attributed to differences in the degradation of their individual tissue types. Fungal attachment was greatest on mature tissues, hence, the rate and extent of degradation of lignified tissues may be associated with the colonization of fungi in mature stems. Also, there was minimal colonization of fungi on immature tissues suggesting that their degradation was primarily affected by enzymatic or direct involvement of bacteria.