Inhibition of Cell Elongation in Avena Coleoptile by Hydroxyproline

A study has been made of the hydroxyproline-induced inhibition of elongation of Avena coleoptile tissues. The isomers of 4-hydroxyproline differ in their effectiveness; only the L isomers are growth inhibitors with the cis form (allohydroxyproline) being more effective than the trans form (hydroxyproline).Hydroxyproline differs from other amino acid antagonists and protein synthesis inhibitors in respect to 2 characteristics of the growth inhibition. First, a certain increment of auxin-induced elongation must take place following addition of hydroxyproline before the growth is inhibited. In contrast, pretreatment with other amino acid antagonists or protein synthesis inhibitors completely eliminates the ability of Avena coleoptile sections to respond to auxin. Secondly, sucrose markedly increases the magnitude of the hydroxyproline inhibition; i.e., sucrose acts to inhibit rather than promote growth when in the presence of hydroxyproline.It appears that hydroxyproline is a specific inhibitor for the synthesis of some factor which is utilized in elongation. Following addition of hydroxyproline, auxin-induced elongation continues until the pool of this factor is exhausted; then elongation is inhibited.     

Hydroxyproline Formation and Its Relation to Auxin-induced Cell Elongation in the Avena Coleoptile

A study has been made of the effects on hydroxyproline formation of 4 factors that influence the rate of cell elongation in the Avena coleoptile; auxin, sugars, an external osmoticum, and actinomycin D. Hydroxyproline formation is increased by a combination of auxin and sucrose, but is affected to a much lesser extent by either factor alone. Its formation is inhibited by an external osmoticum but is scarcely affected by actinomycin D. The lack of correlation between the amount of hydroxyproline synthesis and the growth rate suggests that hydroxyproline formation is not involved in the actual process of wall loosening. It is suggested, instead, that if the wall is to retain its capacity for rapid extension, those hemicelluloses which are incorporated into it by intussusception rather than by apposition must be attached to a hydroxyproline-protein.     

Plant Cell Wall Matrix Polysaccharide Biosynthesis

The wall of an expanding plant cell consists primarily of cellulose microfibrils embedded in a matrix of hemicellulosic and pectic polysaccharides along with small amounts of structural and enzymatic proteins. Matrix polysaccharides are synthesized in the Golgi and exported to the cell wall by exocytosis, where they intercalate among cellulose microfibrils, which are made at the plasma membrane and directly deposited into the cell wall. Involvement of Golgi glucan synthesis in auxin-induced cell expansion has long been recognized; however, only recently have the genes corresponding to glucan synthases been identified. Biochemical purification was unsuccessful because of the labile nature and very low abundance of these enzymes. Mutational genetics also proved fruitless. Expression of candidate genes identified through gene expression profiling or comparative genomics in heterologous systems followed by functional characterization has been relatively successful. Several genes from the cellulose synthase-like （Csl） family have been found to be involved in the synthesis of various hemicellulosic glycans. The usefulness of this approach, however, is limited to those enzymes that probably do not form complexes consisting of unrelated proteins. Nonconventional approaches will continue to incrementally unravel the mechanisms of Golgi polysaccharide biosynthesis.     

Stem Elongation and Cell Wall Proteins in Flowering Plants

Abstract: The growth of stems (hypocotyls, epicotyls) and stem-like organs (coleoptiles) in developing seedlings is largely due to the elongation of cells in the sub-apical region of the corresponding organ. According to the organismal concept of plant development, the thick outer epidermal wall, which can be traced back to the peripheral cell wall of the zygote, creates a sturdy organ sheath that determines the rate of stem elongation. The cells of the inner tissues are the products of secondary partitioning of one large protoplast; these turgid, thin-walled cells provide the driving force for organ growth. The structural differences between these types of cell walls are described (outer walls: thick, sturdy, helicoidal cellulose architecture; inner walls: thin, extensible, transversely-oriented cellulose microfibrils). On the basis of these facts, current models of cell wall loosening (and wall stiffening) are discussed with special reference to the expansin, enzymatic polymer remodelling and osmiophilic particle hypothesis. It is concluded that the exact biochemical mechanism(s) responsible for the coordinated yielding of the growth-controlling peripheral organ wall(s) have not yet been identified.     

Effect of Alfalfa Wilts on the Hydroxyproline Content in Cell Wall

The Hyp content was studied in cell wall of alfalfa susceptible and resistant strains on the 3rd, the 7th and on the 14th day after inoculation with Verticillium albo-atrum or Corynebacterium michiganense pv. insidiosum. The changes of Hyp content after inoculation with both pathogens were markedly expressed in alfalfa roots. Resistant plants of R 337 strain responded to inoculation with V. albo-atrum or C. michiganense pv. insidiosum by the decrease of Hyp content mainly on the 3rd and on the 7th day. On the 14th day after inoculation Hyp content practically did not differ from that of the control. Susceptible plants of S 354 and S 321 srains responded to inoculation with wilt pathogens by the slight decrease of Hyp content at the 3rd day after inoculation. A significant increase of Hyp content was found on the 7th and mainly on the 14th day after inoculation in comparison with control plants. The cell wall Hyp content was also determined with 7 R-strains and 7 S-strains at 120 days after inoculation with both pathogens. In each R and S strain two categories of plants were used for chemical analyses: Wilt-free plants (0 to 1 classes) and diseased, wilted plants (2 to 6 classes). In the resistant alfalfa strains no differences in Hyp content between the wilt-free and diseased plants were found. In the susceptible alfalfa strains the Hyp content was significantly higher in roots of diseased plants comparing with the wilt-free ones. Only negligible changes in Hyp content were registered in the overground parts of all inoculated alfalfa strains.     

Architecture and Biosynthesis of the Saccharomyces cerevisiae Cell Wall

The wall gives a Saccharomyces cerevisiae cell its osmotic integrity; defines cell shape during budding growth, mating, sporulation, and pseudohypha formation; and presents adhesive glycoproteins to other yeast cells. The wall consists of β1,3- and β1,6-glucans, a small amount of chitin, and many different proteins that may bear N- and O-linked glycans and a glycolipid anchor. These components become cross-linked in various ways to form higher-order complexes. Wall composition and degree of cross-linking vary during growth and development and change in response to cell wall stress. This article reviews wall biogenesis in vegetative cells, covering the structure of wall components and how they are cross-linked; the biosynthesis of N- and O-linked glycans, glycosylphosphatidylinositol membrane anchors, β1,3- and β1,6-linked glucans, and chitin; the reactions that cross-link wall components; and the possible functions of enzymatic and nonenzymatic cell wall proteins.     

Proline, Hydroxyproline, and Lily Pollen Tube Elongation

Cytoplasm of freshly-harvested, ungerminated Lilium longiflorum,cv. Ace pollen contains 0.14 per cent soluble and 0.35 per centprotein-bound proline (pro). Their metabolic fates in germinationand tube elongation are not known with certainty. Here is reportedconversion of pro to hydroxyproline (hyp)—containing constituentsas well as distribution and isolation of these constituents.Colorimetry revealed pro and hyp in wall, trichloroacetic acid(TCA)—precipitable, and TCA-soluble cytoplasmic fractions.A balance sheet summarizing quantitative changes in pro andhyp for these fractions revealed that TCA—precipitablecytoplasmic pro could be a precursor to wall-bound pro and asubstrate for hydroxylation yielding cytoplasmic and wall-boundhyp. To determine whether hyp was a component of tube and/orgrain walls, pollen was allowed to germinate 1.5 h and thentransferred to sorbitol medium which prevented further tubeelongation. Hyp was absent from walls of transferred pollen.Electron microscope autoradiography of tubes exposed to ²H-prosuggested that a pro- and/or hyp-containing constituent waslocalized in the growing tip. Light microscope autoradiographyof intact tubes labelled with ¹⁴C-pro showed that the constituentwas distributed throughout the pollen tubes. Gel filtrationof hyp-containing material enzymically released from walls supportedthe view that they contained hyp-glycopeptides.     

Al Binding in the Epidermis Cell Wall Inhibits Cell Elongation of Okra Hypocotyl

Al inhibits root elongation at micromolar concentrations, butthe mechanisms leading to this process are unknown. In thesestudies, Al-induced inhibition of cell elongation was examinedusing hypocotyl of okra (Abelmoschus esculentus Moench cv. ClemsonSpineless) as an experimental model. One-h exposure to Al (0.5mM A1Cl₃) in the presence of 10 µM auxin in 0.5 mM CaCl₂,pH 4.0 significantly inhibited auxin-induced cell elongationof okra hypocotyl segments. Elongation was further suppressedwith increasing Al concentrations up to 1 mM. Treatment of thehypocotyl with 1 mM citrate for 10 minutes after 2-h exposureto Al resulted in significant recovery of elongation. The amountof Al in the cell wall relative to the total in the tissue was96.0, 96.2, and 85.4%, respectively, following 1-, 2-, and 3-hexposure to the Al solution. The total and cell wall Al contentwas decreased by half after the citrate desorption treatment.Further-more, 95% of Al was found in the epidermis, and 95%of the Al in the epidermis was associated with the cell wall.Experiments using split hypocotyl segments showed that Al exposureincreased the outward bending of hypocotyl segments, suggestingthat the epidermis elongation was specifically inhibited byAl. Al inhibited the autolysis of epidermis by about 20%, buthad little effect on the autolysis of core tissue. Taken together,these results suggest that Al binding in the epidermal cellwall inhibits critical components in cell wall loosening mechanism,resulting in inhibition of cell elongation.     

Inhibition of Sterol Biosynthesis and Stem Elongation of Tobacco Seedlings Induced by Some Hypocholesterolemic Agents

Incorporation of DL-[2-¹⁴C]mevalonic acid ([2-¹⁴C]MVA) into4-desmethylsterols in Nicotiana tabacum cv. Turkish Samson seedlingswas inhibited by SK&F 7997-A₃,¹ SK&F 7732-A₃, AY 9944,and the plant growth retardant, Amo 1618. Reductions in 4-desmethylsterol levels resulted from treatmentwith AY 9944 and Amo 1618, but not the SK&F compounds. Amo1618 and SK&F 7997-A₃ both significantly reduced the specificactivity of each of the four major 4-desmethylsterols examined.Although SK&F 7732-A₃ reduced the specific activity of campesterol,and AY 9944 reduced the specific activity of stigmasterol, neitherhad an effect on the specific activity of ß-sitosterol. Stem elongation of tobacco seedlings was retarded by SK&F7997-A₃, AY 9944, and SK&F 7732-A₃, particularly the former,and the retarded plants thus produced were morphologically indistinguishablefrom the Amo 1618-treated plants. Application of exogenous stigmasterol,or GA₃, to the chemically-retarded plants resulted in a reversalof stem growth retardation.     

beta-d-Glucan Antibodies Inhibit Auxin-Induced Cell Elongation and Changes in the Cell Wall of Zea Coleoptile Segments

Antiserum was raised against the Avena sativa L. caryopsis β-d-glucan fraction with an average molecular weight of 1.5 × 10⁴. Polyclonal antibodies recovered from the serum after Protein A-Sepharose column chromatography precipitated when cross-reacted with high molecular weight (1→3), (1→4)-β-d-glucans. These antibodies were effective in suppression of cell wall autohydrolytic reactions and auxin-induced decreases in noncellulosic glucose content of the cell wall of maize (Zea mays L.) coleoptiles. The results indicate antibody-mediated interference with in situ β-d-glucan degradation. The antibodies at a concentration of 200 micrograms per milliliter also suppress auxin-induced elongation by about 40% and cell wall loosening (measured by the minimum stress-relaxation time of the segments) of Zea coleoptiles. The suppression of elongation by antibodies was imposed without a lag period. Auxin-induced elongation, cell wall loosening, and chemical changes in the cell walls were near the levels of control tissues when segments were subjected to antibody preparation precipitated by a pretreatment with Avena caryopsis β-d-glucans. These results support the idea that the degradation of (1→3), (1→4)-β-d-glucans by cell wall enzymes is associated with the cell wall loosening responsible for auxin-induced elongation.     

植物次生细胞壁生物合成的转录调控网络

植物次生细胞壁包含纤维素、半纤维素和木质素, 赋予细胞壁机械强度及疏水性, 这种特性对植物直立生长、水分和营养物质运输以及抵御生物和非生物胁迫十分重要。该文总结了调控次生细胞壁生物合成的转录因子及其调控机制, 包括NAC转录因子调控次生壁合成的一级开关作用, AtMYB46/AtMYB83及其下游调控因子的二级开关作用, 以及其它转录因子对次生壁生物合成的调控作用, 并对未来研究内容和方法进行了展望, 以期为深入系统理解次生细胞壁生物合成的转录调控网络提供参考。     

The Hydroxyproline Content in Cell Wall of Resistant and Susceptible Alfalfa Plants Inoculated with Wilt Pathogens

The changes of hydroxyproline (Hyp) content in cell wall after inoculation with Corynebacterium michiganense pv. insidiosum or Verticillium albo-atrum in 8 cultivars of alfalfa (Medicago sativa) were studied. In our experiments cultivars ‘Trek’ and ‘Vernal’– resistant, ‘Du Puits’ and ‘Vertus’– susceptible to C. michiganense;‘Vela’ and ‘Maris Kabul’– resistant to V. albo-atrum, ‘Sabilt’ and ‘Lahontan’– susceptible to V. albo-atrum were used. In resistant plants inoculated with C. michiganense none or a small increase of Hyp content in comparison with control plants was registered. On the other hand, inoculated susceptible plants showed markedly increase of Hyp content in comparison with the control plants. The differences in Hyp content between control and inoculated resistant and susceptible plants with V. albo-atrum were not markedly expressed in comparison with the alfalfa –C. michiganense pair. The changes had similar tendency both in C. michiganense and V. albo-atrum.–alfalfa pairs.