Enzymes and other agents that enhance cell wall extensibility

Evolution of plant pathogenicity in Streptomyces

Among the multitude of soil-inhabiting, saprophytic Streptomyces species are a growing number of plant pathogens that cause economically important diseases, including potato scab. Streptomyces scabies is the dominant pathogenic species worldwide, but is only one of many that cause very similar disease symptoms on plants. Molecular genetic analysis is beginning to identify the mechanisms used by plant pathogenic species to manipulate their hosts. The nitrated dipeptide phytotoxin, thaxtomin, inhibits cellulose biosynthesis in expanding plant tissues, stimulates Ca2+ spiking, and causes cell death. A secreted necrogenic protein, Nec1, contributes to virulence on diverse plant species. The thaxtomin biosynthetic genes and nec1 lie on a large mobilizable PAI, along with other putative virulence genes including a cytokinin biosynthetic pathway and a saponinase homolog. The PAI is mobilized during conjugation and site-specifically inserts in the linear chromosome of recipient species, accounting for the emergence of new pathogens in agricultural systems. The recently available genome sequence of S. scabies will accelerate research on host-pathogen interactions.

How plants cope with complete submergence

Flooding is a widespread phenomenon that drastically reduces the growth and survival of terrestrial plants. The dramatic decrease of gas diffusion in water compared with in air is a major problem for terrestrial plants and limits the entry of CO(2) for photosynthesis and of O(2) for respiration. Responses to avoid the adverse effects of submergence are the central theme in this review. These include underwater photosynthesis, aerenchyma formation and enhanced shoot elongation. Aerenchyma facilitates gas diffusion inside plants so that shoot-derived O(2) can diffuse to O(2)-deprived plant parts, such as the roots. The underwater gas-exchange capacity of leaves can be greatly enhanced by a thinner cuticle, reorientation of the chloroplasts towards the epidermis and increased specific leaf area (i.e. thinner leaves). At the same time, plants can outgrow the water through increased shoot elongation, which in some species is preceded by an adjustment of leaf angle to a more vertical position. The molecular regulatory networks involved in these responses, including the putative signals to sense submergence, are discussed and suggestions made on how to unravel the mechanistic basis of the induced expression of various adaptations that alleviate O(2) shortage underwater.

Increased extensin levels in Arabidopsis affect inflorescence stem thickening and height

Extensins are hydroxyproline-rich glycoproteins which are abundant in the cell walls of higher plants and whose precise function has been the subject of much speculation. In order to investigate this, transgenic A. thaliana plants were generated containing the Arabidopsis Atext1 extensin coding sequence under the transcriptional control of the strong constitutive CaMV 35S promoter. Northern analysis and RT-PCR identified transgenics with high constitutive levels of Atext1 mRNA. Hydroxyproline assays confirmed that transgenic plants which over-expressed Atext1 contained high levels of hydroxyproline-rich protein. Phenotypic analysis of the transgenics showed that there were no significant phenotypic effects on the timing of different developmental stages or on the general form of the plant. However, transgenics with high extensin levels showed an enhanced increase in stem thickness, with an indirect effect on stem height. This effect, is, however, rather small, with a reduction in height of between 5-8% depending on the transgenic line being analysed.

Regulation of cell length in the Arabidopsis thaliana root by the ethylene precursor 1-aminocyclopropane- 1-carboxylic acid: a matter of apoplastic reactions

Treatment of the Arabidopsis thaliana root with the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) immediately imposes a reduced maximal cell length beyond which further elongation is blocked. Here, we investigated possible apoplastic reactions involved in the inhibition of cell elongation. Five-day-old Arabidopsis seedlings were transferred to a growth medium supplemented with ACC and the effect on root cell length was recorded after 3 h of treatment. Altered characteristics in the apoplast of the nonelongating cells in the ACC-treated root, such as 'reactive oxygen species' (ROS) production and callose deposition, were detected using specific fluorochromes. The presence of functional hydroxyproline-rich glycoproteins (HRGPs) and the crosslinking of these cell-wall proteins are essential in limiting cell elongation. The ROS that drive the oxidative crosslinking of HRGPs, accumulate in the apoplast of cells in the zone where cell elongation stops. In the same cells, callose is deposited in the cell wall. The final cell length in the Arabidopsis root treated for a short period with ACC is determined in the zone of fast elongation. Both HRGPs crosslinking by ROS and callose deposition in the cell wall of this zone are suggested as causes for the reduced cell elongation.

A root- and hypocotyl-specific gene coding for copper-containing amine oxidase is related to cell expansion in soybean seedlings

Polyamines are considered to participate in various processes of plant development. In this study, the possible implication of putrescine catabolism by the copper-containing amine oxidases (CuAOs, EC 1.4.3.6) in the development of roots and hypocotyls was examined. For this purpose, two cDNA clones of Glycine max (L.) Merr. cv. Williams, designated as GmCuAO1 and GmCuAO2, exhibiting extensive similarity with previously characterized CuAO clones from other plants, have been isolated and characterized. The expression of the GmCuAO1 gene is root- and hypocotyl-specific, while GmCuAO2 seems not to be expressed in a tissue-specific manner. Moreover, the GmCuAO1 gene is predominantly expressed in tissues which are characterized by rapid extension growth, such as the apical segments of etiolated hypocotyls. Using convex and concave segments of the etiolated hypocotyl apical hook it has been demonstrated that GmCuAO1 is strongly expressed in expanding cells of the concave part when exposed to light, while the same pattern is also followed by the activity of enzymes involved in putrescine catabolism. In dark and photoperiodically grown hypocotyls, activity measurements of the enzymes involved in putrescine catabolism have shown that the activity of these enzymes is several-fold higher in rapidly growing tissues. Furthermore, the cellular and tissue distribution of GmCuAO1 gene transcripts in the root axis and in hypocotyls confirmed their abundance in developing tissues and expanding cells. The results provide evidence suggesting that a tissue-specific gene coding for CuAO is correlated with cell expansion in fast-growing tissues of root and hypocotyls.

Growth of the plant cell wall

Plant cells encase themselves within a complex polysaccharide wall, which constitutes the raw material that is used to manufacture textiles, paper, lumber, films, thickeners and other products. The plant cell wall is also the primary source of cellulose, the most abundant and useful biopolymer on the Earth. The cell wall not only strengthens the plant body, but also has key roles in plant growth, cell differentiation, intercellular communication, water movement and defence. Recent discoveries have uncovered how plant cells synthesize wall polysaccharides, assemble them into a strong fibrous network and regulate wall expansion during cell growth.

Salinity and the growth of non-halophytic grass leaves: the role of mineral nutrient distribution

Salinity is increasingly limiting the production of graminaceous crops constituting the main sources of staple food (rice, wheat, barley, maize and sorghum), primarily through reductions in the expansion and photosynthetic yield of the leaves. In the present review, we summarise current knowledge of the characteristics of the spatial distribution patterns of the mineral elements along the growing grass leaf and of the impact of salinity on these patterns. Although mineral nutrients have a wide range of functions in plant tissues, their functions may differ between growing and non-growing parts of the grass leaf. To identify the physiological processes by which salinity affects leaf elongation in non-halophytic grasses, patterns of mineral nutrient deposition related to developmental and anatomical gradients along the growing grass leaf are discussed. The hypothesis that a causal link exists between ion deficiency and / or toxicity and the inhibition of leaf growth of grasses in a saline environment is tested.

ANISOTROPIC EXPANSION OF THE PLANT CELL WALL

Plants shape their organs with a precision demanded by optimal function; organ shaping requires control over cell wall expansion anisotropy. Focusing on multicellular organs, I survey the occurrence of expansion anisotropy and discuss its causes and proposed controls. Expansion anisotropy of a unit area of cell wall is characterized by the direction and degree of anisotropy. The direction of maximal expansion rate is usually regulated by the direction of net alignment among cellulose microfibrils, which overcomes the prevailing stress anisotropy. In some stems, the directionality of expansion of epidermal cells is controlled by that of the inner tissue. The degree of anisotropy can vary widely as a function of position and of treatment. The degree of anisotropy is probably controlled by factors in addition to the direction of microfibril alignment. I hypothesize that rates of expansion in maximal and minimal directions are regulated by distinct molecular mechanisms that regulate interactions between matrix and microfibrils.

Expression of alpha-expansin and xyloglucan endotransglucosylase/hydrolase genes associated with shoot elongation enhanced by anoxia, ethylene and carbon dioxide in arrowhead (Sagittaria pygmaea Miq.) tubers

BACKGROUND AND AIMS

Shoot elongation of arrowhead tubers (Sagittaria pygmaea Miq.) is stimulated by anoxia, ethylene and CO2. The aim of this study was to characterize anoxic elongation by comparison with elongation stimulated by ethylene and CO2.

METHODS

The effects of the inhibitors aminoethoxyvinylglycine (AVG) as an ethylene biosynthesis inhibitor, 1-methylcyclopropene (1-MCP) as a potent inhibitor of ethylene action, and pyrazol, an inhibitor of alcohol dehydrogenase, on shoot elongation were examined. Moreover, the effects of these gaseous factors on expression of genes possibly involved in modification of cell wall architecture were examined by polymerase chain reaction (PCR) methods.

KEY RESULTS AND CONCLUSIONS

In air, promotion by 5% CO2 and 5 microL L-1 ethylene of shoot elongation occurred. At 1% O2, ethylene also stimulated shoot elongation but CO2 did not. Pyrazol inhibited shoot elongation in hypoxia but not in normoxia, suggesting that alcohol fermentation contributes to elongation enhanced by hypoxia. AVG and 1-MCP partially prevented shoot elongation both in normoxia and in hypoxia, but they did not have significant effects in anoxia, suggesting that endogenous ethylene acts as a stimulator of shoot elongation in normoxia and in hypoxia but not in anoxia. Ethylene is not involved in anoxia-enhanced elongation. We cloned four cDNAs (SpEXPA1, 2, 3 and 4) encoding alpha-expansin (EXPA) and five cDNAs (SpXTH1, 2, 3, 4 and 5) encoding xyloglucan endotransglucosylase/hydrolase (XTH) from shoots of arrowhead tubers. The transcript levels of SpEXPA1 and 2 were increased by anoxia and those of SpEXPA2 were increased by 5% CO2. Ethylene slightly elevated the level of SpEXPA4 transcripts. Anoxia enhanced the transcript levels of SpXTH1 and 4; neither ethylene nor CO2 had any effect. CO2 enhanced transcript levels of SpXTH3 and depressed those of SpXTH5. Ethylene decreased transcript levels of SpXTH5. These results suggest that four SpEXPA genes and five SpXTH genes are differently responsive to anoxia, CO2 and ethylene. Enhancement of SpEXPA1 and 2, and SpXTH1 and 4 transcript levels suggests that these gene products are involved in anoxic shoot elongation through modification of cell wall architecture.

A re-examination of the root cortex in wetland flowering plants with respect to aerenchyma

AIMS

We review literature and present new observations on the differences among three general patterns of aerenchyma origin and their systematic distributions among the flowering plants, and we clarify terminology on root aerenchyma.

SCOPE

From our own previous works and some new observations, we have analysed the root cortex in 85 species of 41 families in 21 orders of flowering plants that typically grow in wetlands to determine the characteristic patterns of aerenchyma.

FINDINGS

A developmental and structural pattern that we term expansigeny, as manifested by honeycomb aerenchyma, is characteristic of all aquatic basal angiosperms (the Nymphaeales) and basal monocots (the Acorales). Expansigenous aerenchyma develops by expansion of intercellular spaces into lacunae by cell division and cell expansion. Schizogeny and lysigeny, so often characterized in recent reviews as the only patterns of root cortex lacunar formation, are present in most wetland plants, but are clearly not present in the most basal flowering plants.

CONCLUSION

We conclude that expansigeny is the basic type of aerenchyma development in roots of flowering plants and that the presence of expansigenous honeycomb aerenchyma in root cortices was fundamental to the success of the earliest flowering plants found in wetland environments.

Ethylene regulates fast apoplastic acidification and expansin A transcription during submergence-induced petiole elongation in Rumex palustris

The semi-aquatic dicot Rumex palustris responds to complete submergence by enhanced elongation of young petioles. This elongation of petiole cells brings leaf blades above the water surface, thus reinstating gas exchange with the atmosphere and increasing survival in flood-prone environments. We already know that an enhanced internal level of the gaseous hormone ethylene is the primary signal for underwater escape in R. palustris. Further downstream, concentration changes in abscisic acid (ABA), gibberellin (GA) and auxin are required to gain fast cell elongation under water. A prerequisite for cell elongation in general is cell wall loosening mediated by proteins such as expansins. Expansin genes might, therefore, be important target genes in submergence-induced and plant hormone-mediated petiole elongation. To test this hypothesis we have studied the identity, kinetics and regulation of expansin A mRNA abundance and protein activity, as well as examined pH changes in cell walls associated with this adaptive growth. We found a novel role of ethylene in triggering two processes affecting cell wall loosening during submergence-induced petiole elongation. First, ethylene was shown to promote fast net H(+) extrusion, leading to apoplastic acidification. Secondly, ethylene upregulates one expansin A gene (RpEXPA1), as measured with real-time RT-PCR, out of a group of 13 R. palustris expansin A genes tested. Furthermore, a significant accumulation of expansin proteins belonging to the same size class as RpEXPA1, as well as a strong increase in expansin activity, were apparent within 4-6 h of submergence. Regulation of RpEXPA1 transcript levels depends on ethylene action and not on GA and ABA, demonstrating that ethylene evokes at least three, parallel operating pathways that, when integrated at the whole petiole level, lead to coordinated underwater elongation. The first pathway involves ethylene-modulated changes in ABA and GA, these acting on as yet unknown downstream components, whereas the second and third routes encompass ethylene-induced apoplastic acidification and ethylene-induced RpEXPA1 upregulation.

Distinctive diel growth cycles in leaves and cladodes of CAM plants: differences from C3 plants and putative interactions with substrate availability, turgor and cytoplasmic pH

Distinct diel rhythms of leaf and cladode expansion growth were obtained in crassulacean acid metabolism (CAM) plants under water-limited conditions, with maxima at mid-day during phase III of CO₂ assimilation. This pattern coincided with the availability of CO₂ for photosynthesis and growth during the decarboxylation of malic acid, with maximum cell turgor due to the nocturnally accumulated malic acid, and with the period of low cytoplasmic pH associated with malic acid movement from vacuole to cytosol. Maximum growth rates were generally only 20% of those in C₃ plants and were reached at a different time of the day compared with C₃ plants. The results suggest that malic acid, as a source of carbohydrates, and a determinant of turgor and cytoplasmic pH, plays a major role in the control of diel growth dynamics in CAM plants under desert conditions. The observed plasticity in phasing of growth rhythms under situations of differing water availability suggests that a complex network of factors controls the diel growth patterns in CAM plants and needs to be investigated further.

Differential location of alpha-expansin proteins during the accommodation of root cells to an arbuscular mycorrhizal fungus

alpha-Expansins are extracellular proteins that increase plant cell-wall extensibility. We analysed their pattern of expression in cucumber roots in the presence and in the absence of the mycorrhizal fungus, Glomus versiforme. The distribution of alpha-expansins was investigated by use of two polyclonal antibodies (anti-EXPA1 and anti-EXPA2, prepared against two different cucumber alpha-expansins) in immunoblotting, immunofluorescence, and immunogold experiments. Immunoblot results indicate the presence of a 30-kDa band specific for mycorrhizal roots. The two antibodies identify antigens with a different distribution in mycorrhizal roots: anti-EXPA1 labels the interface zone, but the plant cell walls only weakly. By contrast, the anti-EXPA2 labels only the plant cell walls. In order to understand the potential role of alpha-expansins during the accommodation of the fungus inside root cells, we prepared semi-thin sections to measure the size of cortical cells and the thickness of cortical cell walls in mycorrhizal and non-mycorrhizal root. Mycorrhizal cortical cells were significantly larger than non-mycorrhizal cells and had thicker cell walls. In double-labelling experiments with cellobiohydrolase-gold complex, we observed that cellulose was co-localized with alpha-expansins. Taken together, the results demonstrate that alpha-expansins are more abundant in the cucumber cell walls upon mycorrhizal infection; we propose that these wall-loosening proteins are directly involved in the accommodation of the fungus by infected cortical cells.

Molecular modeling of family GH16 glycoside hydrolases: potential roles for xyloglucan transglucosylases/hydrolases in cell wall modification in the poaceae

Family GH16 glycoside hydrolases can be assigned to five subgroups according to their substrate specificities, including xyloglucan transglucosylases/hydrolases (XTHs), (1,3)-beta-galactanases, (1,4)-beta-galactanases/kappa-carrageenases, "nonspecific" (1,3/1,3;1,4)-beta-D-glucan endohydrolases, and (1,3;1,4)-beta-D-glucan endohydrolases. A structured family GH16 glycoside hydrolase database has been constructed (http://www.ghdb.uni-stuttgart.de) and provides multiple sequence alignments with functionally annotated amino acid residues and phylogenetic trees. The database has been used for homology modeling of seven glycoside hydrolases from the GH16 family with various substrate specificities, based on structural coordinates for (1,3;1,4)-beta-D-glucan endohydrolases and a kappa-carrageenase. In combination with multiple sequence alignments, the models predict the three-dimensional (3D) dispositions of amino acid residues in the substrate-binding and catalytic sites of XTHs and (1,3/1,3;1,4)-beta-d-glucan endohydrolases; there is no structural information available in the databases for the latter group of enzymes. Models of the XTHs, compared with the recently determined structure of a Populus tremulos x tremuloides XTH, reveal similarities with the active sites of family GH11 (1,4)-beta-D-xylan endohydrolases. From a biological viewpoint, the classification, molecular modeling and a new 3D structure of the P. tremulos x tremuloides XTH establish structural and evolutionary connections between XTHs, (1,3;1,4)-beta-D-glucan endohydrolases and xylan endohydrolases. These findings raise the possibility that XTHs from higher plants could be active not only on cell wall xyloglucans, but also on (1,3;1,4)-beta-D-glucans and arabinoxylans, which are major components of walls in grasses. A role for XTHs in (1,3;1,4)-beta-D-glucan and arabinoxylan modification would be consistent with the apparent overrepresentation of XTH sequences in cereal expressed sequence tags databases.

Systemic response to aphid infestation by Myzus persicae in the phloem of Apium graveolens

Little is known about the molecular processes involved in the phloem response to aphid feeding. We investigated molecular responses to aphid feeding on celery (Apium graveolenscv. Dulce) plants infested with the aphid Myzus persicae, as a means of identifying changes in phloem function. We used celery as our model species as it is easy to separate the phloem from the surrounding tissues in the petioles of mature leaves of this species. We generated a total of 1187 expressed sequence tags (ESTs), corresponding to 891 non-redundant genes. We analysed these ESTs in silico after cDNA macroarray hybridisation. Aphid feeding led to significant increase in RNA accumulation for 126 different genes. Different patterns of deregulation were observed, including transitory or stable induction 3 or 7 days after infestation. The genes affected belonged to various functional categories and were induced systemically in the phloem after infestation. In particular, genes involved in cell wall modification, water transport, vitamin biosynthesis, photosynthesis, carbon assimilation and nitrogen and carbon mobilisation were up-regulated in the phloem. Further analysis of the response in the phloem or xylem suggested that a component of the response was developed more specifically in the phloem. However, this component was different from the stress responses in the phloem driven by pathogen infection. Our results indicate that the phloem is actively involved in multiple adjustments, recruiting metabolic pathways and in structural changes far from aphid feeding sites. However, they also suggest that the phloem displays specific mechanisms that may not be induced in other tissues.

Change in XET activities, cell wall extensibility and hypocotyl elongation of soybean seedlings at low water potential

In dark-grown soybean (Glycine max [L.] Merr.) seedlings, exposing the roots to water-deficient vermiculite (psi(w)=-0.36 MPa) inhibited hypocotyl (stem) elongation. The inhibition was associated with decreased extensibility of the cell walls in the elongation zone. A detailed spatial analysis showed xyloglucan endotransglucosylase (XET; EC 2.4.1.207) activity on the basis of unit cell wall dry weight was decreased in the elongation region after seedlings were transplanted to low psi(w). The decrease in XET activity was at least partially due to an accumulation of cell wall mass. Since cell number was only slightly altered, wall mass had increased per cell and probably led to increased wall thickness and decreased cell wall extensibility. Alternatively, an increase in cell wall mass may represent a mechanism for regulating enzyme activity in cell walls, XET in this case, and therefore cell wall extensibility. Hypocotyl elongation was partially recovered after seedlings were grown in low-psi(w) vermiculate for about 80 h. The partial recovery of hypocotyl elongation was associated with a partial recovery of cell wall extensibility and an enhancement of XET activity in the hypocotyl elongation zone. Our results indicate XTH proteins may play an important role in regulating cell wall extensibility and thus cell elongation in soybean hypocotyls. Our results also showed an imperfect correlation of spatial elongation and XET activity along the hypocotyls. Other potential functions of XTH and their regulation in soybean hypocotyl growth are discussed.

Linking physiological and genetic analyses of the control of leaf growth under changing environmental conditions

Decrease in leaf growth rate under water deficit can be seen as an adaptive process. The analysis of its genetic variability is therefore important in the context of drought tolerance. Several mechanisms are widely believed to drive the reduction in leaf growth rate under water deficit, namely leaf carbon balance, incomplete turgor maintenance, and decrease in cell wall plasticity or in cell division rate, with contributions from hormones such as abscisic acid or ethylene. Each of these mechanisms is still controversial, and involves several families of genes. It is argued that gene regulatory networks are not feasible for modelling such complex systems. Leaf growth can be modelled via response curves to environmental conditions, which are considered as ‘meta-mechanisms’ at a higher degree of organisation. Response curves of leaf elongation rate to meristem temperature, atmospheric vapour pressure deficit, and soil water status were established in recombinant inbred lines (RILs) of maize in experiments carried out in the field and in the greenhouse. A quantitative trait locus (QTL) analysis was conducted on the slopes of these responses. Each parameter of the ecophysiological model could then be computed as the sum of QTL effects, allowing calculation of parameters of new RILs, either virtual or existing. Leaf elongation rates of new RILS were simulated and were similar to measurements in a growth chamber experiment. This opens the way to the simulation of virtual genotypes, known only by their alleles, in any climatic scenario. Each genotype is therefore represented by a set of response parameters, valid in a large range of conditions and deduced from the alleles at QTLs.

The Nicotiana sylvestris extensin gene, Ext 1.2A, is expressed in the root transition zone and upon wounding

The Ext 1.2A gene of Nicotiana sylvestris L. encoding an extensin, a cell wall structural protein, was characterized. Ext 1.2A encodes a polypeptide of 311 amino acids having a highly repetitive structure and showing extensin features such as Ser-(Pro)(4) repeats and a high content of Tyr and Lys. The expression profile of the gene was demonstrated using the reporter GUS (beta-glucuronidase) fused to its promoter region (-630/+124, relative to the transcription start site) and by RNA gel blots. The results show that the (-630/+124) Ext 1.2A/GUS gene fusion is expressed in the root transition zone, where cells undergo an isodiametric growth but have not yet reached the rapid elongation phase, in stem inner and outer phloems and in cortical cells at the stem/petiole junction. The Ext 1.2A gene is also induced after wounding of stems, ribs, leaves or roots. The gene fusion is expressed in stem cortical cells, in ribs and at leaf edges upon wounding. These data suggest that the (-630/+124) promoter region contains regulatory elements responsible for expression in roots and stems, as well as for response to wounding in stems and leaves.

Production of reactive oxygen intermediates (O(2)(.-), H(2)O(2), and (.)OH) by maize roots and their role in wall loosening and elongation growth

Cell extension in the growing zone of plant roots typically takes place with a maximum local growth rate of 50% length increase per hour. The biochemical mechanism of this dramatic growth process is still poorly understood. Here we test the hypothesis that the wall-loosening reaction controlling root elongation is effected by the production of reactive oxygen intermediates, initiated by a NAD(P)H oxidase-catalyzed formation of superoxide radicals (O(2)(.-)) at the plasma membrane and culminating in the generation of polysaccharide-cleaving hydroxyl radicals ((.)OH) by cell wall peroxidase. The following results were obtained using primary roots of maize (Zea mays) seedlings as experimental material. (1) Production of O(2)(.-), H(2)O(2), and (.)OH can be demonstrated in the growing zone using specific histochemical assays and electron paramagnetic resonance spectroscopy. (2) Auxin-induced inhibition of growth is accompanied by a reduction of O(2)(.-) production. (3) Experimental generation of (.)OH in the cell walls with the Fenton reaction causes wall loosening (cell wall creep), specifically in the growing zone. Alternatively, wall loosening can be induced by (.)OH produced by endogenous cell wall peroxidase in the presence of NADH and H(2)O(2). (4) Inhibition of endogenous (.)OH formation by O(2)(.-) or (.)OH scavengers, or inhibitors of NAD(P)H oxidase or peroxidase activity, suppress elongation growth. These results show that juvenile root cells transiently express the ability to generate (.)OH, and to respond to (.)OH by wall loosening, in passing through the growing zone. Moreover, inhibitor studies indicate that (.)OH formation is essential for normal root growth.

Evaluation of carbohydrates in Pukekohe Longkeeper and Grano cultivars of Allium cepa

The storage, soluble, and structural carbohydrates of two onion cultivars, the hard, pungent Pukekohe Longkeeper (PLK) and the softer, milder Houston Grano, were analyzed to determine differences that might be related to their response to sulfur nutrition received during growth as well as their postharvest attributes and end-use suitability. PLK tissue contained 1.37 times more dry matter than Grano and was composed of more fructan and sucrose and less glucose and fructose than Grano [corrected] There were also differences in neutral sugar content, especially galactose, and the amount, size, and content of pectin fractions soluble in chelator and weak alkali. These two onion cultivars differed in their capacity to take up sulfur, but there was no statistical association between sulfur supply and any measured dry matter component.

The cell wall-modifying xyloglucan endotransglycosylase/hydrolase LeXTH1 is expressed during the defence reaction of tomato against the plant parasite Cuscuta reflexa

A suppressive subtractive hybridization technique was used to identify genes, which were induced during the early phases of the interaction between dodder (Cuscuta reflexa), a phanerogamic parasite, and its incompatible host plant tomato. One of the identified genes encodes a tomato xyloglucan endotransglycosylase/hydrolase (XTH)--an enzyme involved in cell wall elongation and restructuring. The corresponding LeXTH1 mRNA accumulated 6 h after attachment of the parasite. In contrast, wounding did not influence the expression level. Subsequent to LeXTH1 mRNA accumulation, an increase in XTH activity at the infection sites as well as in adjacent tissues was observed. The effect of IAA on LeXTH1 expression was analyzed because the concentration of this phytohormone is known to increase in the tomato tissue during the interaction with the parasite. LeXTH1 mRNA accumulation was in fact induced by external application of auxin. However, in the auxin-insensitive tomato mutant diageotropica, Cuscuta induced LeXTH1-mRNA accumulated with a time course similar to wild type tomato. Thus, auxin appears not to be an essential signal for infection-induced LeXTH1 activation. Our data suggest a role for xyloglucan transglycosylation in defence reactions associated with the incompatible tomato- Cuscuta interaction.

Targeted modification of homogalacturonan by transgenic expression of a fungal polygalacturonase alters plant growth

Pectins are a highly complex family of cell wall polysaccharides comprised of homogalacturonan (HGA), rhamnogalacturonan I and rhamnogalacturonan II. We have specifically modified HGA in both tobacco (Nicotiana tabacum) and Arabidopsis by expressing the endopolygalacturonase II of Aspergillus niger (AnPGII). Cell walls of transgenic tobacco plants showed a 25% reduction in GalUA content as compared with the wild type and a reduced content of deesterified HGA as detected by antibody labeling. Neutral sugars remained unchanged apart from a slight increase of Rha, Ara, and Gal. Both transgenic tobacco and Arabidopsis were dwarfed, indicating that unesterified HGA is a critical factor for plant cell growth. The dwarf phenotypes were associated with AnPGII activity as demonstrated by the observation that the mutant phenotype of tobacco was completely reverted by crossing the dwarfed plants with plants expressing PGIP2, a strong inhibitor of AnPGII. The mutant phenotype in Arabidopsis did not appear when transformation was performed with a gene encoding AnPGII inactivated by site directed mutagenesis.

The plot thickens: New perspectives of primary cell wall modification

Recent studies have further confirmed the ubiquity of cell wall restructuring during plant growth and development, and have emphasized the fact that our understanding of the breadth of molecular processes that mediate wall modification is still rudimentary. In the past few years, both enzymatic and non-enzymatic agents that apparently contribute to wall disassembly have been identified, and it is likely that additional mechanisms will continue to be revealed. These discoveries are being propelled by the development of new biochemical and biophysical assays, by database mining in the wake of the explosion of plant sequence information from genome sequencing and expressed sequence tags, and by a variety of strategies used to catalog the cell wall proteome. The daunting question of how these mechanistically diverse and complex processes are coordinated remains unresolved.

Interaction between wall deposition and cell elongation in dark-grown hypocotyl cells in Arabidopsis

A central problem in plant biology is how cell expansion is coordinated with wall synthesis. We have studied growth and wall deposition in epidermal cells of dark-grown Arabidopsis hypocotyls. Cells elongated in a biphasic pattern, slowly first and rapidly thereafter. The growth acceleration was initiated at the hypocotyl base and propagated acropetally. Using transmission and scanning electron microscopy, we analyzed walls in slowly and rapidly growing cells in 4-d-old dark-grown seedlings. We observed thick walls in slowly growing cells and thin walls in rapidly growing cells, which indicates that the rate of cell wall synthesis was not coupled to the cell elongation rate. The thick walls showed a polylamellated architecture, whereas polysaccharides in thin walls were axially oriented. Interestingly, innermost cellulose microfibrils were transversely oriented in both slowly and rapidly growing cells. This suggested that transversely deposited microfibrils reoriented in deeper layers of the expanding wall. No growth acceleration, only slow growth, was observed in the cellulose synthase mutant cesA6(prc1-1) or in seedlings, which had been treated with the cellulose synthesis inhibitor isoxaben. In these seedlings, innermost microfibrils were transversely oriented and not randomized as has been reported for other cellulose-deficient mutants or following treatment with dichlorobenzonitrile. Interestingly, isoxaben treatment after the initiation of the growth acceleration in the hypocotyl did not affect subsequent cell elongation. Together, these results show that rapid cell elongation, which involves extensive remodeling of the cell wall polymer network, depends on normal cellulose deposition during the slow growth phase.

The expression of MaEXP1, a Melilotus alba expansin gene, is upregulated during the sweetclover-Sinorhizobium meliloti interaction

Expansins are a highly conserved group of cell wall-localized proteins that appear to mediate changes in cell wall plasticity during cell expansion or differentiation. The accumulation of expansin protein or the mRNA for specific expansin gene family members has been correlated with the growth of various plant organs. Because expansin proteins are closely associated with plant cell wall expansion, and as part of a larger study to determine the role of different gene products in the legume-Rhizobium spp. symbiosis, we investigated whether a Melilotus alba (white sweetclover) expansin gene is expressed during nodule development. A cDNA fragment encoding an expansin gene (EXP) was isolated from Sinorhizobium meliloti-inoculated sweetclover root RNA by reverse-transcriptase polymerase chain reaction using degenerate primers, and a full-length sweetclover expansin sequence (MaEXP1) was obtained using 5' and 3' rapid amplification of cDNA end cloning. The predicted amino acid of the sweetclover expansin is highly conserved with the various alpha-expansins in the GenBank database. MaEXP1 contains a series of eight cysteines and four tryptophans that are conserved in the alpha-expansin protein family. Northern analysis and whole-mount in situ hybridization analyses indicate that MaEXP1 mRNA expression is enhanced in roots within hours after inoculation with S. meliloti and in nodules. Western and immunolocalization studies using a cucumber expansin antibody demonstrated that a cross-reacting protein accumulated in the expanding cells of the nodule.

Decreased reactive oxygen species concentration in the elongation zone contributes to the reduction in maize leaf growth under salinity

Reactive oxygen species (ROS) in the apoplast of cells in the growing zone of grass leaves are required for elongation growth. This work evaluates whether salinity-induced reductions in leaf elongation are related to altered ROS production. Studies were performed in actively growing segments (SEZ) obtained from leaf three of 14-d-old maize (Zea mays L.) seedlings gradually salinized to 150 mM NaCl. Salinity reduced elongation rates and the length of the leaf growth zone. When SEZ obtained from the elongation zone of salinized plants (SEZs) were incubated in 100 mM NaCl, the concentration where growth inhibition was approximately 50%, O2*- production, measured as NBT formazan staining, was lower in these than in similar segments obtained from control plants. The NaCl effect was salt-specific, and not osmotic, as incubation in 200 mM sorbitol did not reduce formazan staining intensity. SEZs elongation rates were higher in 200 mM sorbitol than in 100 mM NaCl, but the difference could be cancelled by scavenging or inhibiting O2*- production with 10 mM MgCl2 or 200 microM diphenylene iodonium, respectively. The actual ROS believed to stimulate growth is *OH, a product of O2*- metabolism in the apoplast. SEZ(s) elongation in 100 mM NaCl was stimulated by a *OH-generating medium. Fusicoccin, an ATPase stimulant, and acetate buffer pH 4, could also enhance elongation in these segments, although both failed to increase ROS activity. These results show that decreased ROS production contributes to the salinity-associated reduction in grass leaf elongation, acting through a mechanism not associated with pH changes.

Proteinaceous inhibitors of endo-beta-glucanases

Both plants and filamentous phytopathogens secrete proteins that inhibit endo-beta-glucanases. The first endo-beta-glucanase inhibitor proteins to be discovered are XEGIP, a tomato protein that inhibits fungal xyloglucan-specific endo-beta-1,4-glucanases, and GIP1, an oomycete protein that inhibits endo-beta-1,3-glucanases produced by the plant host. These inhibitor proteins act by forming high-affinity complexes with their endoglucanase ligands. A family of XEGIP-like proteins has been identified. At least one member of this family (extracellular dermal glycoprotein, EDGP) has been shown to have endoglucanase-inhibitor activity, while other members have sequence similarity to a xylanase inhibitor from wheat (TAXI-1). The oomycete inhibitor GIP1 is a catalytically inactive serine protease homolog (SPH) whose structure is unrelated to XEGIP. Both types of inhibitor proteins are likely to affect the interactions of plants with filamentous phytopathogens, and a basic model describing their roles in pathogenesis is proposed.

Arabidopsis whole-transcriptome profiling defines the features of coordinated regulations that occur during secondary growth

Secondary growth in the inflorescence stems of Arabidopsis plants was induced by a combination of short-day and long-day treatments. The induced stems were divided into three different stem developmental stages (i.e., immature, intermediate, and mature) with regard to secondary growth. Whole transcriptome microarrays were used to examine the changes in global gene expression occurring at the different stem developmental stages. Over 70% of the Arabidopsis transcriptome was expressed in the stem tissues. In the mature stems with secondary growth, 567 genes were upregulated 5-fold or higher and 530 were downregulated, when compared to immature stems (with no secondary growth) and 10-day old seedlings (with no inflorescence stem). The transcription phenotypes obtained from the stems at different developmental stages largely confirm the existing insights into the biochemical processes involved in the sequential events that lead to wood formation. The major difference found between the stems undergoing secondary growth and only primary growth was in the expression profiles of transcriptional regulation-and signal transduction-related genes. An analysis of several shoot apical meristem (SAM) activity-related gene expression patterns in the stems indicated that the genetic control of secondary meristem activity might be governed by a different mechanism from that of SAM. The current study established the expression patterns of many unknown genes and identified candidate genes that are involved in the genetic regulation of secondary growth. The findings described in this report should improve our understanding of the molecular mechanisms that regulate the growth and development of the stem.

The promoter of the Arabidopsis thaliana Cel1 endo-1,4-beta glucanase gene is differentially expressed in plant feeding cells induced by root-knot and cyst nematodes

SUMMARY Transgenic tobacco and Arabidopsis thaliana carrying the Arabidopsis endo-1,4-beta-glucanase (EC 3.2.1.4) Cel1 promoter fused to the beta-glucuronidase (GUS) reporter gene were infected with the root-knot nematode, Meloidogyne incognita, and either the tobacco cyst nematode, Globodera tabacum (tobacco), or beet cyst nematode, Heterodera schachtii (Arabidopsis). Cel1-driven GUS expression was detected in cell elongation zones of noninfected plants and within feeding sites (giant-cells) induced in roots of both plant hosts by M. incognita. The first detectable signs of Cel1 expression within developing giant-cells occurred at the onset of giant-cell formation and continued throughout the M. incognita life cycle. UidA (Gus) transcripts were detectable within giant-cells induced in tobacco roots at 11-13 days postinoculation with M. incognita as determined by in situ mRNA hybridization. By contrast, expression of the Cel1 promoter was not detected within developing syncytia induced in tobacco or Arabidopsis roots by G. tabacum and H. schachtii, respectively, at any time point. The results demonstrate specific regulation of cell wall-degrading enzymes that may be required for cell wall modifications during feeding cell formation by sedentary endoparasitic nematodes. Differential expression of Cel1 by cyst and root-knot nematodes further supports underlying mechanistic differences in giant-cell and syncytium formation.

Mechanical effects of plant cell wall enzymes on cellulose/xyloglucan composites

Xyloglucan-acting enzymes are believed to have effects on type I primary plant cell wall mechanical properties. In order to get a better understanding of these effects, a range of enzymes with different in vitro modes of action were tested against cell wall analogues (bio-composite materials based on Acetobacter xylinus cellulose and xyloglucan). Tomato pericarp xyloglucan endo transglycosylase (tXET) and nasturtium seed xyloglucanase (nXGase) were produced heterologously in Pichia pastoris. Their action against the cell wall analogues was compared with that of a commercial preparation of Trichoderma endo-glucanase (EndoGase). Both 'hydrolytic' enzymes (nXGase and EndoGase) were able to depolymerise not only the cross-link xyloglucan fraction but also the surface-bound fraction. Consequent major changes in cellulose fibril architecture were observed. In mechanical terms, removal of xyloglucan cross-links from composites resulted in increased stiffness (at high strain) and decreased visco-elasticity with similar extensibility. On the other hand, true transglycosylase activity (tXET) did not affect the cellulose/xyloglucan ratio. No change in composite stiffness or extensibility resulted, but a significant increase in creep behaviour was observed in the presence of active tXET. These results provide direct in vitro evidence for the involvement of cell wall xyloglucan-specific enzymes in mechanical changes underlying plant cell wall re-modelling and growth processes. Mechanical consequences of tXET action are shown to be complimentary to those of cucumber expansin.

A novel endo-beta-mannanase gene in tomato LeMAN5 is associated with anther and pollen development

Endo-beta-mannanase (EC 3.2.1.78) is involved in cell wall disassembly and the weakening of plant tissues by degrading mannan polymers in the cell walls. Endo-beta-mannanase genes are expressed in tomato (Lycopersicon esculentum) seeds (LeMAN1 and LeMAN2) and fruits (LeMAN3 and LeMAN4). A novel endo-beta-mannanase gene (termed LeMAN5) was found in the tomato genome by genome-walking PCR and bacterial artificial chromosome library screening. The 5'-upstream region of this endo-beta-mannanase gene contained four copies of the pollen-specific cis-acting elements POLLEN1LELAT52 (AGAAA). A GUS-reporter gene driven with the putative LeMAN5 promoter (-543 to +38) was activated in anthers and pollen of transgenic Arabidopsis, with the highest beta-glucuronidase activity detected in pollen. beta-Glucuronidase expression was detected in mature pollen retained in sporangia, discharged pollen, and elongating pollen tubes in transgenic Arabidopsis. Consistently, expression of LeMAN5 mRNA and endo-beta-mannnanase activity was detected in tomato anthers and pollen. In anthers, the highest mRNA expression and endo-beta-mannanase activity were detected during late stages of anther development, when pollen maturation occurred. Endo-beta-mannanase activity was present in discharged pollen, which was easily eluted in a buffer, indicating that the enzyme proteins are probably secreted from, and deposited on, the surface of pollen. These data suggest that the LeMAN5 endo-beta-mannanase is associated with anther and pollen development.

Xyloglucan oligosaccharides cause cell wall loosening by enhancing xyloglucan endotransglucosylase/hydrolase activity in azuki bean epicotyls

Addition of xyloglucan-derived oligosaccharides shifted the wall-bound xyloglucans to a lower molecular mass distribution and increased the cell wall extensibility of the native epidermal tissue strips isolated from azuki bean (Vigna angularis) epicotyls. To ascertain the mechanism of oligosaccharide function, we examined the action of a xyloglucan endotransglucosylase/hydrolase (XTH) showing both endotransglucosylase and endohydrolase activities, isolated from azuki bean epicotyl cell walls, in the presence of xyloglucan oligosaccharides. The addition of xyloglucan oligosaccharides enhanced the xyloglucan-degrading activity of XTH against isolated xyloglucan substrates. When the methanol-fixed epidermal tissue strips were incubated with XTH, the molecular mass of wall-bound xyloglucans was decreased and the cell wall extensibility increased markedly in the presence of the oligosaccharides. These results suggest that xyloglucan oligosaccharides stimulate the degradation of xyloglucans by enhancing the XTH activity within the cell wall architecture, thereby increasing the cell wall extensibility in azuki bean epicotyls.

Tissue-specific and developmentally regulated expression of a cluster of tandemly arrayed cell wall-associated kinase-like kinase genes in Arabidopsis

The Arabidopsis cell wall-associated kinase (WAK) and WAK-like kinase (WAKL) family of receptor-like kinase genes encodes transmembrane proteins with a cytoplasmic serine/threonine kinase domain and an extracellular region containing epidermal growth factor-like repeats. Previous studies have suggested that some WAK members are involved in plant defense and heavy metal responses, whereas others are required for cell elongation and plant development. The WAK/WAKL gene family consists of 26 members in Arabidopsis and can be divided into four groups. Here, we describe the characterization of group 2 members that are composed of a cluster of seven tandemly arrayed WAKL genes. The predicted WAKL proteins are highly similar in their cytoplasmic region but are more divergent in their predicted extracellular ligand-binding region. WAKL7 encodes a truncated WAKL isoform that is predicted to be secreted from the cytoplasm. Ratios of nonsynonymous to synonymous substitutions suggest that the extracellular region is subject to diversifying selection. Comparison of the WAKL and WAK gene clusters suggests that they arose independently. Protein gel-blot and immunolocalization analyses suggest that WAKL6 is associated with the cell wall. Histochemical analyses of WAKL promoters fused with the beta-glucuronidase reporter gene have shown that the expressions of WAKL members are developmentally regulated and tissue specific. Unlike WAK members whose expressions were found predominately in green tissues, WAKL genes are highly expressed in roots and flowers. The expression of WAKL5 and WAKL7 can be induced by wounding stress and by the salicylic acid analog 2,6-dichloroisonicotinic acid in an nonexpressor of pathogenesis-related gene 1-dependent manner, suggesting that they, like some WAK members, are wound inducible and can be defined as pathogenesis-related genes.

Finding unexpected patterns in microarray data

We describe the performance of a protocol based on the sequential application of unsupervised and supervised methods to analyze microarray samples defined by a combination of factors. Correspondence analysis is used to visualize the emerging patterns of three set of novel or previously published data: photoreceptor mutants of Arabidopsis grown under different light/dark conditions, Arabidopsis exposed to different types of biotic and abiotic stress, and human acute leukemia. We find, for instance, that light has a dramatic effect on plants despite the absence of the four major photoreceptors, that bacterial-, fungal-, and viral-induced responses converge at later stages of attack, and that sample preparation procedures used in different hospitals have large effects on transcriptome patterns. We use canonical discriminant analysis to identify the genes associated with these patters and hierarchical clustering to find groups of coregulated genes that are easily visualized in a second round of correspondence analysis and ordered tables. The unconventional combination of standard descriptive multivariate methods offers a previously unrecognized tool to uncover unexpected information.

Genomic and physiological studies of early cryptochrome 1 action demonstrate roles for auxin and gibberellin in the control of hypocotyl growth by blue light

Blue light inhibits elongation of etiolated Arabidopsis thaliana hypocotyls during the first 30 min of irradiation by a mechanism that depends on the phototropin 1 (phot1) photoreceptor. The cryptochrome 1 (cry1) photoreceptor begins to exert control after 30 min. To identify genes responsible for the cry1 phase of growth inhibition, mRNA expression profiles of cry1 and wild-type seedlings were compared using DNA microarrays. Of the roughly 420 genes found to be differentially expressed at the point of cry1 response incipience, approximately half were expressed higher and half lower in cry1 relative to the wild type. Many of the cry1-dependent genes encoded kinases, transcription factors, cell cycle regulators, cell wall metabolism enzymes, gibberellic acid (GA) biosynthesis enzymes, and auxin response factors. High-resolution growth studies supported the hypothesis that genes in the last two categories were indeed relevant to cry1-mediated growth control. Inhibiting GA4 biosynthesis with a 3beta-hydroxylase inhibitor (Ca-prohexadione) restored wild-type response kinetics in cry1 and completely suppressed its long-hypocotyl phenotype in blue light. Co-treatment of cry1 seedlings with Ca-prohexadione plus GA4 completely reversed the effects of the inhibitor, restoring the long-hypocotyl phenotype typical of the mutant. Treatment of wild-type seedlings with GA4 was not sufficient to phenocopy cry1 seedlings, but co-treatment with IAA plus GA4 produced cry1-like growth kinetics for a period of approximately 5 h. The genomic and physiological data together indicate that blue light acting through cry1 quickly affects the expression of many genes, a subset of which suppresses stem growth by repressing GA and auxin levels and/or sensitivity.

Growth conditions, elemental accumulation and induced physiological changes in Chinese cabbage

Soils contaminated with low levels of heavy metals and other trace elements are now frequently used for vegetable growing. In this situation, heavy metals and trace elements from these polluted soils may accumulate in the agricultural plants being grown in them and thereby enter the human food chain. The objectives of this study are to elucidate the effects of growth conditions, manipulated by the crop covers, on the phytoaccumulation of elements, and to investigate the conceivable influences of these conditions on the plant biochemistry. In three consecutive years of field experiments, open air (T(0)), and floating rowcover treatments (T(1): perforated polyethylene 50 micrometers; T(2): polypropylene 17 gm(-2)) were used to produce different environmental conditions for the growth of Chinese cabbage [Brassica rapa L. (Pekinensis group) cv. 'Nagaoka 50']. Five samplings (whole tops) were carried out from transplanting to harvest and measurements of B, Al, Ag, Si and Ca concentration as well as phenolics (orto-diphenols, total phenols and anthocyanins), pectic fractions, amino acids (histidine, phenylalanine and tyrosine) and polyphenol oxidase activity, were carried out in samples. The T(1) (perforated polyethylene sheet) gave greater B, Al, Ag and Si concentration and phytoextraction (in weight units) than the open-air control. These findings can help to develop new cost-effective techniques for phytoremediation as the application of plastic covers in the field. The build-up of heavy metals in those crops would make the product less suitable for human consumption.

Comparative analysis of spore coat formation, structure, and function in Dictyostelium

Dictyostelium produces spores at the end of its developmental cycle to propagate the lineage. The spore coat is an essential feature of spore biology contributing a semipermeable chemical and physical barrier to protect the enclosed amoeba. The coat is assembled from secreted proteins and a polysaccharide, and from cellulose produced at the cell surface. They are organized into a polarized molecular sandwich with proteins forming layers surrounding the microfibrillar cellulose core. Genetic and biochemical studies are beginning to provide insight into how the deliveries of protein and cellulose to the cell surface are coordinated and how cysteine-rich domains of the proteins interact to form the layers. A multidomain inner layer protein, SP85/PsB, seems to have a central role in regulating coat assembly and contributing to a core structural module that bridges proteins to cellulose. Coat formation and structure have many parallels in walls from plant, algal, yeast, protist, and animal cells.

Regulation of cell expansion by the DISTORTED genes in Arabidopsis thaliana: actin controls the spatial organization of microtubules

The control of the directionality of cell expansion was investigated using a class of eight genes, the so-called DISTORTED (DIS) genes, that are required for proper expansion of leaf trichomes in Arabidopsis thaliana. By tracing the separation of latex beads placed on the trichome surface, we demonstrate that trichomes grow by diffuse rather than tip growth, and that in dis mutants deviations from the normal orientation of growth can occur in all possible directions. We could not detect any differences in intracellular organization between wild-type and dis-group mutants by electron microscopy. The analysis of double mutants showed that although the expression of the dis phenotype is generally independent of branching and endoreduplication, dis mutations act synthetically in combination lesions in the ZWI gene, which encodes a kinesin motor protein. Using a MAP4:GFP marker line, we show that the organization of cortical microtubules is affected in dis-group mutants. The finding that most dis-group mutants have actin defects suggested to us that actin is involved in organizing the orientation of microtubules. By analyzing the microtubule organization in plants treated with drugs that bind to actin, we verified that actin is involved in the positioning of cortical microtubules and thereby in plant cell expansion.

Hyper-gravity effects on the Arabidopsis transcriptome

Callus cultures of Arabidopsis thaliana (cv. Columbia) in Petri dishes were exposed to altered g-forces by centrifugation (1-10 g). Using semi-quantitative RT-PCR transcripts of genes coding for metabolic key enzymes (ADP-glucose pyrophosphorylase, ADPG-PP; beta-amylase, fructose-1,6-bisphosphatase, FBPase; glyceraldehyde-P dehydrogenase, GAPDH; hydroxymethylglutaryl-CoA reductase, HMG; phenylalanine-ammonium-lyase, PAL; PEP carboxylase, PEPC) were used to monitor threshold conditions for g-number (all) and time of exposure (beta-amylase) which led to altered amounts of the gene product. Exposure to approximately 5 g and higher for 1 h resulted in altered transcript levels: transcripts of beta-amylase, PAL, and PEPC were increased, those of ADPG-PP decreased, while those of FBPase, GAPDH, and HMG were not affected. This probably indicates a shift from starch synthesis to starch degradation and increased rates of anaplerosis (PEPC: supply of ketoacids for amino acid synthesis). In order to get more information about g-related effects on gene expression, we used a 1-h exposure to 7 g for a microarray analysis, using a commercial A. thaliana chip with 4105 unique annotated clusters/genes (IncyteGenomics). Transcripts of more than 200 genes were significantly increased in amount (ratio 7 g/1 g control; 2(1.6) and larger). They fall into several categories. Transcripts coding for enzymes of major pathways form the largest group (25%), followed by gene products involved in cellular organization and cell wall formation/rearrangement (17%), signalling, phosphorylation/dephosphorylation (12%), proteolysis and transport (10% each), hormone synthesis plus related events (8%), defense (4%), stress-response (2%), and gravi-sensing (2%). Many of the alterations are part of a general stress response, but some changes related to the synthesis/rearrangement of cell wall components could be more hyper-g-specific. We only found few gene products, which were decreased in relation to 1 g controls, and these were less significant (ratio < 2(1.6)). We thus assume that g-forces above a threshold of about 5 g for 1 h are sensed by plant cells in general, causing distinct metabolic responses, which obviously in part, are regulated by gene expression.

Ionic and osmotic disruptions of the lily pollen tube oscillator: testing proposed models

Two mechanisms have been proposed as the primary control of oscillating tip growth in Lilium longiflorum Thunb. pollen tubes: changes in cell wall strength (Holdaway-Clarke et al. 1997) or alternatively, changes in turgor pressure (Messerli et al. 2000). Here we have modified the ionic and osmotic concentrations of the growth medium to test predictions derived from both models. Raising the [Ca2+]o tenfold above normal reduced the amplitude of the [Ca2+]i oscillations and growth oscillations while it raised the basal [Ca2+]i and growth rate such that the average growth rate did not change. Raising the [H+] of the growth medium tenfold reversibly decreased and sometimes eliminated the [Ca2+]i and growth oscillations without changing the average growth rate. Lowering the [H+] tenfold led to irregular frequency and amplitude [Ca2+]i oscillations, reduced the average growth rate of tubes and led to cell bursting in 33% of tubes. Addition of 50 mM H+ buffer, MES, to prevent pH changes in the cell wall increased the period, amplitude and duration of both [Ca2+]i and growth oscillations. Changing the [K+]o did not markedly effect [Ca2+]i oscillations. Reducing the osmolarity of the medium led to transient large-amplitude [Ca2+]i and growth oscillations while reducing large-amplitude oscillations over long periods. In many different conditions under which growth still occurs, lily pollen tubes maintain growth oscillations, albeit with modified frequency, amplitude and duration. We conclude that modifications to both proposed models are necessary to explain oscillating growth in this system.

Elongation growth in leaf blades of Chloris gayana under saline conditions

In Chloris gayana, salinity-associated yield decreases are due mainly to leaf area reductions. To understand the physiological basis for such reduction, the effects of salinity were studied on the spatial and temporal distribution of extension in the intercalary meristem at the leaf base, and on hydraulic conductance in that zone. C. gayana plants were grown on sand irrigated with Hoagland solution with the addition of 0 or 200 mmol/L NaCl, and all measurements were performed on tiller leaf four. In salinised plants, that leaf was 20% shorter than in controls. Extension in the blade expansion zone was studied by pricking through the leaf sheaths and analysing the displacement of the pricks. In salt-treated plants, maximum growth rates were depressed by 53% and the growth zone was shorter by approximately 10 mm, nevertheless, extension proceeded for a longer period than in control plants. The analysis of specific leaf areas in the expansion zone suggests the rate of dry matter deposition was lowered by salinity and estimations of tissue displacement time within that zone suggest cell wall maturity was delayed. Hydraulic conductance was reduced by salinity and this may be the main cause for reduced growth under salinity in Chloris gayana.

Expression of an expansin gene is correlated with root elongation in soybean

Expansin is a family of proteins that catalyze long-term expansion of cell walls and has been considered a principal protein that affects cell expansion in plants. We have identified the first root-specific expansin gene in soybean (Glycine max), GmEXP1, which may be responsible for root elongation. Expression levels of GmEXP1 were very high in the roots of 1- to 5-d-old seedlings, in which rapid root elongation takes place. Furthermore, GmEXP1 mRNA was most abundant in the root tip region, where cell elongation occurs, but scarce in the region of maturation, where cell elongation ceases, implying that its expression is closely related to root development processes. In situ hybridization showed that GmEXP1 transcripts were preferentially present in the epidermal cells and underlying cell layers in the root tip of the primary and secondary roots. Ectopic expression of GmEXP1 accelerated the root growth of transgenic tobacco (Nicotiana tabacum) seedlings, and the roots showed insensitivity to obstacle-touching stress. These results imply that the GmEXP1 gene plays an important role in root development in soybean, especially in the elongation and/or initiation of the primary and secondary roots.

CsAGP1, a gibberellin-responsive gene from cucumber hypocotyls, encodes a classical arabinogalactan protein and is involved in stem elongation

Fluorescence differential display was used to isolate the gibberellin (GA)-responsive gene, CsAGP1, from cucumber (Cucumis sativus) hypocotyls. A sequence analysis of CsAGP1 indicated that the gene putatively encodes a "classical" arabinogalactan protein (AGP) in cucumber. Transgenic tobacco (Nicotiana tabacum) plants overexpressing CsAGP1 under the control of the cauliflower mosaic virus 35S promoter produced a Y(betaGlc)(3)-reactive proteoglycan in addition to AGPs present in wild-type tobacco plants. Immuno-dot blotting of the product, using anti-AGP antibodies, showed that the CsAGP1 protein had the AGP epitopes common to AGP families. The transcription level of CsAGP1 in cucumber hypocotyls increased in response not only to GA but also to indole-3-acetic acid. Although CsAGP1 is expressed in most vegetative tissues of cucumber, including the shoot apices and roots, the GA treatment resulted in an increase in the mRNA level of CsAGP1 only in the upper part of the hypocotyls. Y(betaGlc)(3), which selectively binds AGPs, inhibited the hormone-promoted elongation of cucumber seedling hypocotyls. Transgenic plants ectopically expressing CsAGP1 showed a taller stature and earlier flowering than the wild-type plants. These observations suggest that CsAGP1 is involved in stem elongation.

Spatial and temporal effects of free-air CO2 enrichment (POPFACE) on leaf growth, cell expansion, and cell production in a closed canopy of poplar

Leaf expansion in the fast-growing tree, Populus x euramericana was stimulated by elevated [CO(2)] in a closed-canopy forest plantation, exposed using a free air CO(2) enrichment technique enabling long-term experimentation in field conditions. The effects of elevated [CO(2)] over time were characterized and related to the leaf plastochron index (LPI), and showed that leaf expansion was stimulated at very early (LPI, 0-3) and late (LPI, 6-8) stages in development. Early and late effects of elevated [CO(2)] were largely the result of increased cell expansion and increased cell production, respectively. Spatial effects of elevated [CO(2)] were also marked and increased final leaf size resulted from an effect on leaf area, but not leaf length, demonstrating changed leaf shape in response to [CO(2)]. Leaves exhibited a basipetal gradient of leaf development, investigated by defining seven interveinal areas, with growth ceasing first at the leaf tip. Interestingly, and in contrast to other reports, no spatial differences in epidermal cell size were apparent across the lamina, whereas a clear basipetal gradient in cell production rate was found. These data suggest that the rate and timing of cell production was more important in determining leaf shape, given the constant cell size across the leaf lamina. The effect of elevated [CO(2)] imposed on this developmental gradient suggested that leaf cell production continued longer in elevated [CO(2)] and that basal increases in cell production rate were also more important than altered cell expansion for increased final leaf size and altered leaf shape in elevated [CO(2)].