Effects of hypergravity environment on lignin formation in Arabidopsis.

Lignin and the secondary wall formation are essential for evolution of land plants. In this study, effects of hypergravity environment on the morphology of the secondary wall and the lignin content were examined in Arabidopsis thaliana. Xylem vessels showed intense staining with phloroglucinol-HCl and autofluorescence under UV light at the basal region of the flower stalk when seedlings grown for 3 days after hypergravity treatment for 24 hours. And, the flower stalk exposed to hypergravity showed slight increase in the lignin content. These results suggest that the lignin formation is positively regulated under hypergravity.     

Effects of hypergravity on the elongation growth in radish and cucumber hypocotyls

The elongation growth of the hypocotyls of radish and cucumber seedlings was examined under hypergravity in a newly developed centrifuge (Kasaharaet al. 1995). The effects of hypergravity on elongation growth differed between the two species. The rate of elongation of radish hypocotyls was reduced under basipetal hypergravity (H+20g) but not under acropetal hypergravity (H-13g), as compared to growth under the control conditions (C+1g and C-1g). In cucumber hypocotyls, elongation growth was inhibited not only by basipetal but also by acropetal hypergravity. Under these conditions, the reduction in the elongation growth of both radish and cucumber hypocotyls was accompanied by an increase in their thickness. Although no distinct differences in relative composition of neutral sugars were found, the amounts of cell-wall components (pectic substances, hemicelluloses and cellulose) per unit length of hypocotyls were increased by exposure to hypergravity.     

Brassinosteroids, microtubules and cell elongation in Arabidopsis thaliana. II. Effects of brassinosteroids on microtubules and cell elongation in the bul1 mutant

In order to elucidate the involvement of brassinosteroids in the cell elongation process leading to normal plant morphology, indirect immunofluorescence and molecular techniques were use to study the expression of tubulin genes in the bul1-1 dwarf mutant of Arabidopsis thaliana (L.) Heynh., the characteristics of which are reported in this issue (M. Catterou et al., 2001). Microtubules were studied specifically in the regions of the mutant plant where the elongation zone is suppressed (hypocotyls and petioles), making the reduction in cell elongation evident. Indirect immunofluorescence of α-tubulin revealed that very few microtubules were present in mutant cells, resulting in the total lack of the parallel microtubule organization that is typical of elongating cells in the wild type. After brassinosteroid treatment, microtubules reorganized and became correctly oriented, suggesting the involvement of brassinosteroids in microtubule organization. Molecular analyses showed that the microtubule reorganization observed in brassinosteroid-treated bul1-1 plants did not result either from an activation of tubulin gene expression, or from an increase in tubulin content, suggesting that a brassinosteroid-responsive pathway exists which allows microtubule nucleation/organization and cell elongation without activation of tubulin gene expression. Received: 28 April 2000 / Accepted: 6 October 2000     

Control of hypocotyl elongation in Arabidopsis thaliana by photoreceptor interaction

In order to test the interaction of different phytochromes and blue-light receptors, etiolated seedlings of wild-type Arabidopsis thaliana (L.) Heynh., a phytochrome (phy) B-overexpressor line (ABO), and the photoreceptor mutants phyA-201, phyB-5, hy4-2.23n, fha-1, phyA-201/phyB-5, and phyA-201/hy4-2.23n were exposed to red and far-red light pulses after various preirradiations. The responsiveness to the inductive red pulses is primarily mediated by phyB which is rather stable in its far-red-absorbing form as demonstrated by a very slow loss of reversibility. Without preirradiation the red pulses had an impact on hypocotyl elongation only in PHYA mutants but not in the wild type. This indicates a suppression of phyB function by the presence of phyA. Preirradiation with either far-red or blue light resulted in an inhibition of hypocotyl elongation by red pulses in the wild type. Responsiveness amplification by far-red light is mediated by phyA and disappears slowly in the dark. The extent of responsiveness amplification by blue light was identical in the wild type and in the absence of phyA, or the cryptochromes cryl (hy4-2.23n) or cry2 (fha-1). Therefore, we conclude that stimulation of phyB by blue light preirradiation is either mediated by an additional still-unidentified blue-light-absorbing pigment or that phyA, cry1 and cry2 substitute for each other completely. Both blue and red preirradiation established responsiveness to red pulses in phyA-201/phyB-5 double mutants. These results demonstrate that inhibition of hypocotyl elongation by red pulses is not only mediated by phyB but also by a phytochrome(s) other than phyA and phyB. Received: 21 July 1998 / Accepted: 7 December 1998     

Effects of gene expression on molecular evolution in Arabidopsis thaliana and Arabidopsis lyrata

We analyzed the complete genome sequence of Arabidopsis thaliana and sequence data from 83 genes in the outcrossing A. lyrata, to better understand the role of gene expression on the strength of natural selection on synonymous and replacement sites in Arabidopsis. From data on tRNA gene abundance, we find a good concordance between codon preferences and the relative abundance of isoaccepting tRNAs in the complete A. thaliana genome, consistent with models of translational selection. Both EST-based and new quantitative measures of gene expression (MPSS) suggest that codon preferences derived from information on tRNA abundance are more strongly associated with gene expression than those obtained from multivariate analysis, which provides further support for the hypothesis that codon bias in Arabidopsis is under selection mediated by tRNA abundance. Consistent with previous results, analysis of protein evolution reveals a significant correlation between gene expression level and amino acid substitution rate. Analysis by MPSS estimates of gene expression suggests that this effect is primarily the result of a correlation between the number of tissues in which a gene is expressed and the rate of amino acid substitution, which indicates that the degree of tissue specialization may be an important determinant of the rate of protein evolution in Arabidopsis.     

Identification and characterization of Arabidopsis thaliana genes involved in xylem secondary cell walls

The xylem of higher plants offers support to aerial portions of the plant body and serves as conduit for the translocation of water and nutrients. Terminal differentiation of xylem cells typically involves deposition of thick secondary cell walls. This is a dynamic cellular process accompanied by enhanced rates of cellulose deposition and the induction of synthesis of specific secondary-wall matrix polysaccharides and lignin. The secondary cell wall is essential for the function of conductive and supportive xylem tissues. Recently, significant progress has been made in identifying the genes responsible for xylem secondary cell wall formation. However, our present knowledge is still insufficient to account for the molecular processes by which this complex system operates. To acquire further information about xylem secondary cell walls, we initially focused our research effort on a set of genes specifically implicated in secondary cell wall formation, as well as on loss-of-function mutants. Results from two microarray screens identified several key candidate genes responsible for secondary cell wall formation. Reverse genetic analyses led to the identification of a glycine-rich protein involved in maintaining the stable structure of protoxylem, which is essential for the transport of water and nutrients. A combination of expression analyses and reverse genetics allows us to systematically identify new genes required for the development of physical properties of the xylem secondary wall.     

Effects of hypergravity on the elongation growth in radish and cucumber hypocotyls

The elongation growth of the hypocotyls of radish and cucumber seedlings was examined under hypergravity in a newly developed centrifuge (Kasahara et al. 1995). The effects of hypergravity on elongation growth differed between the two species. The rate of elongation of radish hypocotyls was reduced under basipetal hypergravity (H+2O g) but not under acropetal hypergravity (H-13 g), as compared to growth under the control conditions (C+1 g and C-1 g). In cucumber hypocotyls, elongation growth was inhibited not only by basipetal but also by acropetal hypergravity. Under these conditions, the reduction in the elongation growth of both radish and cucumber hypocotyls was accompanied by an increase in their thickness. Although no distinct differences in relative composition of neutral sugars were found, the amounts of cell-wall components (pectic substances, hemicelluloses and cellulose) per unit length of hypocotyls were increased by exposure to hypergravity.     

Effect of mechanical perturbation on the biomechanics, primary growth and secondary tissue development of inflorescence stems of Arabidopsis thaliana

Background and Aims

Mechanical perturbation is known to inhibit elongation of the inflorescence stem of Arabidopsis thaliana. The phenomenon has been reported widely for both herbaceous and woody plants, and has implications for how plants adjust their size and form to survive in mechanically perturbed environments. While this response is an important aspect of the plant''s architecture, little is known about how mechanical properties of the inflorescence stem are modified or how its primary and secondary tissues respond to mechanical perturbation.

Methods

Plants of the Columbia-0 ecotype were exposed to controlled brushing treatments and then submitted to three-point bending tests to determine stem rigidity and stiffness. Contributions of different tissues to the inflorescence stem geometry were analysed.

Key Results

Perturbed plants showed little difference in stem diameter, were 50 % shorter, 75 % less rigid and 70 % less stiff than controls. Changes in mechanical properties were linked to significant changes in tissue geometry – size and position of the pith, lignified interfascicular tissue and cortex – as well as a reduction in density of lignified cells. Stem mechanical properties were modified by changes in primary tissues and thus differ from changes observed in most woody plants tested with indeterminate growth – even though a vascular cambium is present in the inflorescence axis.

Conclusions

The study suggests that delayed development of key primary developmental features of the stem in this ecotype of Arabidopsis results in a ‘short and flexible’ rather than a ‘short and rigid’ strategy for maintaining upright axes in conditions of severe mechanical perturbation. The mechanism is comparable with more general phenomena in plants where changes in developmental rate can significantly affect the overall growth form of the plant in both ecological and evolutionary contexts.     

Involvement of AtLAC15 in lignin synthesis in seeds and in root elongation of Arabidopsis

Laccase, EC 1.10.3.2 or p-diphenol:dioxygen oxidoreductase, has been proposed to be involved in lignin synthesis in plants based on its in vitro enzymatic activity and a close correlation with the lignification process in plants. Despite many years of research, genetic evidence for the role of laccase in lignin synthesis is still missing. By screening mutants available for the annotated laccase gene family in Arabidopsis, we identified two mutants for a single laccase gene, AtLAC15 (At5g48100) with a pale brown or yellow seed coat which resembled the transparent testa (tt) mutant phenotype. A chemical component analysis revealed that the mutant seeds had nearly a 30% decrease in extractable lignin content and a 59% increase in soluble proanthocyanidin or condensed tannin compared with wild-type seeds. In an in vitro enzyme assay, the developing mutant seeds showed a significant reduction in polymerization activity of coniferyl alcohol in the absence of H₂O₂. Among the dimers formed in the in vitro assay using developing wild-type seeds, 23% of the linkages were β-O-4 which resembles the major linkages formed in native lignin. The evidence strongly supports that AtLAC15 is involved in lignin synthesis in plants. To our knowledge, this is the first genetic evidence for the role of laccase in lignin synthesis. Changes in seed coat permeability, seed germination and root elongation were also observed in the mutant.Electronic Supplementary Material Supplementary material is available to authorised users in the online version of this article at .     

Stimulation of root hair elongation in Arabidopsis thaliana by low phosphorus availability

Low phosphorus availability stimulates root hair elongation in many plants, which may have adaptive significance in soil phosphorus acquisition. We investigated the effect of low phosphorus on the elongation of Arabidopsis thaliana root hairs. Arabidopsis thaliana plants were grown in plant culture containing high (1000 mmol m^?3) or low (1 mmol m^?3) phosphorus concentrations, and root hair elongation was analysed by image analysis. After 15d of growth, low-phosphorus plants developed root hairs averaging 0.9 mm in length while high-phosphorus plants of the same age developed root hairs averaging 0.3 mm in length. Increased root hair length in low-phosphorus plants was a result of both increased growth duration and increased growth rate. Root hair length decreased logarithmically in response to increasing phosphorus concentration. Local changes in phosphorus availability influenced root hair growth regardless of the phosphorus status of the plant. Low phosphorus stimulated root hair elongation in the hairless axr2 mutant, exogenously applied IAA stimulated root hair elongation in wild-type high-phosphorus plants and the auxin antagonist CM PA inhibited root hair elongation in low-phosphorus plants. These results indicate that auxin may be involved in the low-phosphorus response in root hairs.     

重金属镉对拟南芥DNA甲基化的影响

将拟南芥种子点种于添加有不同浓度CdCl2的培养基中处理2周,移苗时CdCl2的胁迫即解除。低浓度CdCl2促进拟南芥种子的萌发。CdCl2为0．5mg·L^-1时萌发率最高（为97．21％）。随着CdCl2浓度的继续增加,种子萌发率即逐渐下降。幼苗期和抽薹期分别提取叶DNA,采用甲基敏感扩增多态性（MSAP）技术分析其基因组DNA甲基化的结果显示,总的来说,随着CdCl2浓度的增加,甲基化程度增高。     

Effects of growth differences due to geographic location and N-fertilisation on wood chemistry of Norway spruce

We studied the effect of long-term nitrogen fertilisation on wood chemistry at two boreal sites in Finland: the northern site (Kemijärvi) and the southern site (Heinola). N-fertilisation was repeated in five-year intervals from the 1960s. Norway spruce (Picea abies L. Karst.) trees that had been planted in 1938 and 1954, in the northern and the southern site, respectively, were harvested in October 2002. Altogether 20 trees, in five different size classes, either unfertilised or fertilised, were felled. Wood sections at breast height, consisting of five consecutive annual rings, from six (Kemijärvi) or five (Heinola) points with different distances from the pith were examined. Differences in growth between the northern and southern sites were marked in favour of the southern site. In the northern site fertilisation had clearly increased the diameter growth, while in the southern site fertilisation had no effect. Nitrogen fertilisation resulted in slight changes in wood chemistry that included increased nitrogen concentrations in the northern site and extractives in the southern site. Stem wood had higher concentrations of extractives, starch, and uronic acids, and lower concentration of cellulose, in the northern than in the southern site. Changes in the stem wood chemistry along radial axis were marked. The changes in wood chemistry are discussed in relation to the physiological function and also how the changes can influence the suitability of wood for different end-use purposes.     

表油菜素内酯对拟南芥细胞分化的影响

研究了表油菜素内酯（ｅｐｉ－ＢＲ）对拟南芥细胞体外分化的影响．表明ｅｐｉ－ＢＲ不仅能促进愈伤组织的增殖,而且还能有效地诱导愈伤组织转绿,继而分化绿芽和长成小植株,其诱导频率高达７０％以上。电镜观察表明,ｅｐｉ－ＢＲ诱导的转绿细胞中的叶绿体发育正常。     

拟南芥雄性不育突变体fne的遗传与定位分析

在T-DNA插入突变体Salk_118481株系的群体中,筛选到一株雄性不育突变体,用T-DNA序列上的一对引物进行PCR鉴定表明其基因组中没有T DNA插入。通过背景纯化与遗传分析发现该雄性不育突变体是由单个隐性基因控制的,引起不育的主要原因是在花药发育的第13~14期,花丝不能伸长以完成授粉,故该突变体命名为fne （filament no elongation）。利用图位克隆的方法对FNE基因进行了定位,结果表明FNE基因位于第五条染色体上分子标记MBD2和MMG4之间的97kb区间内。目前该区间内尚未见到控制花丝伸长基因的报道,因此,FNE基因是一个控制花丝伸长的新基因。     

Genetic and Mapping Analysis of Arabidopsis thaliana Male Sterile Mutant filament no elongation

To identify new genes important for anther development, we screened for male sterile mutants among a population of Arabidopsis ecotype Columbia (Col) mutagenized by T DNA insertion (provided by ARBC). A male sterile mutant line with normal vegetative and flora development but no seed yield was isolated from Salk_118481 line. T DNA insertion site identification showed that there were no T DNA sequences in the genome of the mutants. Genetic analysis indicated that the mutant was controlled by a single recessive nuclear gene named filament no elongation because the filament of the mutant remains very short at the 13-14 stage of anther development. The fne gene was mapped to a region of 97kb between the molecular makers MBD2 and MMG4 on chromosome 5 using map based cloning technique. No genes involved filament elongation were reported in this region, so we believe that FNE gene could be a new gene controlling filament elongation in Arabidopsis.     

环境激素DBP对拟南芥体外培养叶片超微结构的影响

报道了酞酸酯类化合物DBP对拟南芥离体培养叶片超微结构的影响。在DBP(1.0mg·L-1)处理第3天即观察到拟南芥叶片叶绿体的超微结构受到破坏。不同浓度的DBP长期处理植株(40d)出现叶片白化、节间短缩等异常现象。在0.01mg·L-1DBP处理40d植株的叶细胞中,叶绿体出现解体,破碎部分呈颗粒状,散落其间,但细胞结构完整;0.1mg·L-1DBP处理后引起叶绿体的进一步解体,叶绿体中类囊体基粒和片层结构解体,细胞出现空洞现象,其它细胞器很少观察到;1.0mg·L-1DBP和2.0mg·L-1DBP处理植株叶片叶绿体中类囊体基粒和片层结构破碎,叶绿体结构也解体,细胞中其它细胞器数目极少。     

Role of growth regulators in the senescence of Arabidopsis thaliana leaves

A homozygous, dominant, C₂H₄-resistant line of Arabidopsis thaliana (L.) Heynh (cv. Columbia; er ) was selected from ethylmethylsulfonate-mutagenized seed, and used to test the role of C₂H₄ and other growth regulators in senescence of mature leaves. Chlorophyll (Chl) loss from disks excised from leaves of er was much slower than that from wild-type (WT) disks, whether they were held in the light or in the dark. C₂H₄ accelerated Che loss from WT disks but had no effect on the yellowing of mutant disks. C₂H₄ biosynthesis was higher in disks from the mutant plants, particularly in the light. In the dark, treatment with the cytokinin, 6-benzyladenine (BA), reduced Chl loss from wild-type disks, but had no effect on mutant disks. In the light, BA treatment stimulated chlorophyll breakdown in both wild type and mutant disks. Treatment with abscisic acid (ABA) stimulated chlorophyll loss in wild-type and mutant disks, whether they were held in the light or the dark. C₂H₄ production was stimulated in ABA-treated disks, but they still yellowed even when C₂H₄ production was inhibited by application of aminooxyacetic acid (AOA). These data indicate that C₂H₄ is only one of the factors involved in leaf senescence, and that the promotion of senescence by ABA is not mediated through its stimulation of C₂H₄ production.     

Hypersensitivity to cytoskeletal antagonists demonstrates microtubule-microfilament cross-talk in the control of root elongation in Arabidopsis thaliana

Elongation of diffusely expanding plant cells is thought to be mainly under the control of cortical microtubules. Drug treatments that disrupt actin microfilaments, however, can reduce elongation and induce radial swelling. To understand how microfilaments assist growth anisotropy, we explored their functional interactions with microtubules by measuring how microtubule disruption affects the sensitivity of cells to microfilament-targeted drugs. We assessed the sensitivity to actin-targeted drugs by measuring the lengths and diameters of expanding roots and by analysing microtubule and microfilament patterns in the temperature-sensitive Arabidopsis thaliana mutant microtubule organization 1 (mor1-1), along with other mutants that constitutively alter microtubule arrays. At the restrictive temperature of mor1-1, root expansion was hypersensitive to the microfilament-disrupting drugs latrunculin B and cytochalasin D, while immunofluorescence microscopy showed that low doses of latrunculin B exacerbated microtubule disruption. Root expansion studies also showed that the botero and spiral1 mutants were hypersensitive to latrunculin B. Hypersensitivity to actin-targeted drugs is a direct consequence of altered microtubule polymer status, demonstrating that cross-talk between microfilaments and microtubules is critical for regulating anisotropic cell expansion.