SciCombinator

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Concept: Arabidopsis

185

Alteration of leaf surface phenotypes due to virus infection has the potential to affect the likelihood of colonisation by insect vectors, or to affect their feeding activities. The aim of this study was to investigate whether viruses that rely on insects for their transmission, and which can be sensitive to the polarization of light, affect the percentage polarization of light reflected from leaves. We also set out to discover whether a correlation exists between the expression of ECERIFERUM (CER) genes involved in cuticular wax synthesis and the polarization of the light reflected from the leaf surfaces. It was found that the aphid-vectored viruses Potato virus Y and Cucumber mosaic virus (CMV) caused significant reductions in the percentage polarization of light reflected from the abaxial surfaces of leaves of Nicotiana tabacum, whereas the non-insect-vectored viruses Tobacco mosaic virus and Pepino mosaic virus did not induce this effect. In Arabidopsis thaliana, there was little difference in the impacts of CMV and the non-insect-vectored Turnip vein clearing virus on polarization reflection, with both viruses increasing the percentage polarization of light reflected from the abaxial surfaces of leaves. There was a trend towards increased accumulation of CER6 transcripts in N. tabacum and A. thaliana when infected with aphid-vectored viruses. No significant effect of infection on trichome densities was found in A. thaliana, suggesting that alterations to the formation of cuticular waxes may be the more likely phenotypic change on the leaf surface contributing to the changes in polarization reflection. The possible impacts and adaptive significance of these effects with regard to viral transmission by insects are discussed.

Concepts: Gene, Evolution, Virus, Genome, Light, Arabidopsis thaliana, Arabidopsis, Viruses

168

The effects of exposure to increasing manganese concentrations (50-1500 µM) from the start of the experiment on the functional performance of photosystem II (PSII) and photosystem I (PSI) and photosynthetic apparatus composition of Arabidopsis thaliana were compared. In agreement with earlier studies, excess Mn caused minimal changes in the PSII photochemical efficiency measured as F(v)/F(m), although the characteristic peak temperature of the S(2/3)Q(B)(-) charge recombinations was shifted to lower temperatures at the highest Mn concentration. SDS-PAGE and immunoblot analyses also did not exhibit any significant change in the relative abundance of PSII-associated polypeptides: PSII reaction centre protein D1, Lhcb1 (major light-harvesting protein of LHCII complex), and PsbO (OEC33, a 33kDa protein of the oxygen-evolving complex). In addition, the abundance of Rubisco also did not change with Mn treatments. However, plants grown under excess Mn exhibited increased susceptibility to PSII photoinhibition. In contrast, in vivo measurements of the redox transients of PSI reaction centre (P700) showed a considerable gradual decrease in the extent of P700 photooxidation (P700(+)) under increased Mn concentrations compared to control. This was accompanied by a slower rate of P700(+) re-reduction indicating a downregulation of the PSI-dependent cyclic electron flow. The abundance of PSI reaction centre polypeptides (PsaA and PsaB) in plants under the highest Mn concentration was also significantly lower compared to the control. The results demonstrate for the first time that PSI is the major target of Mn toxicity within the photosynthetic apparatus of Arabidopsis plants. The possible involvement mechanisms of Mn toxicity targeting specifically PSI are discussed.

Concepts: Photosynthesis, Arabidopsis, Photosystem, Light reactions, Photosystem I, Photosystem II, Oxygen evolution, Light-dependent reactions

160

Many plants respond to competition signals generated by neighbors by evoking the shade avoidance syndrome, including increased main stem elongation and reduced branching. Vegetation-induced reduction in the Red light:Far Red light (R:FR) provides a competition signal sensed by phytochromes. Plants deficient in phytochrome B (phyB) exhibit a constitutive shade avoidance syndrome including reduced branching. As auxin in the polar auxin transport stream (PATS) inhibits axillary bud outgrowth, its role in regulating the phyB branching phenotype was tested. Removing the main shoot PATS auxin source by decapitation or chemically inhibiting the PATS strongly stimulated branching in Arabidopsis thaliana deficient in phyB, but had a modest effect in WT. While indole-3-acetic acid (IAA) levels were elevated in young phyB seedlings, there was less IAA in mature stems compared to WT. A split plate assay of bud outgrowth kinetics indicated that low auxin levels inhibited phyB buds more than WT. Since the auxin response could be due either to the bud’s ability to export auxin into the main shoot PATS, or to auxin signaling status, both parameters were assessed. Main shoots of phyB had less absolute auxin transport capacity compared to WT, but equal or greater capacity when based on the relative amounts of native IAA in the stems. Thus, auxin transport capacity was unlikely to restrict branching. Both shoots of young phyB seedlings and mature stem segments showed elevated expression of auxin responsive genes and expression was further increased by auxin treatment, suggesting that phyB suppresses auxin signaling to promote branching.

Concepts: Bud, Arabidopsis thaliana, Arabidopsis, Plant physiology, Plant stem, Shoot, Phytochrome, The Stems

152

Flattened leaf architecture is not a default state but depends on positional information to precisely coordinate patterns of cell division in the growing primordium. This information is provided, in part, by the boundary between the adaxial (top) and abaxial (bottom) domains of the leaf, which are specified via an intricate gene regulatory network whose precise circuitry remains poorly defined. Here, we examined the contribution of the ASYMMETRIC LEAVES (AS) pathway to adaxial-abaxial patterning in Arabidopsis thaliana and demonstrate that AS1-AS2 affects this process via multiple, distinct regulatory mechanisms. AS1-AS2 uses Polycomb-dependent and -independent mechanisms to directly repress the abaxial determinants MIR166A, YABBY5, and AUXIN RESPONSE FACTOR3 (ARF3), as well as a nonrepressive mechanism in the regulation of the adaxial determinant TAS3A. These regulatory interactions, together with data from prior studies, lead to a model in which the sequential polarization of determinants, including AS1-AS2, explains the establishment and maintenance of adaxial-abaxial leaf polarity. Moreover, our analyses show that the shared repression of ARF3 by the AS and trans-acting small interfering RNA (ta-siRNA) pathways intersects with additional AS1-AS2 targets to affect multiple nodes in leaf development, impacting polarity as well as leaf complexity. These data illustrate the surprisingly multifaceted contribution of AS1-AS2 to leaf development showing that, in conjunction with the ta-siRNA pathway, AS1-AS2 keeps the Arabidopsis leaf both flat and simple.

Concepts: Arabidopsis thaliana, Arabidopsis, Fern, Leaf

110

Plants respond to diurnal and seasonal changes in temperature by reprogramming vital developmental pathways. Understanding the molecular mechanisms that define environmental modulation of plant growth and reproduction is critical in the context of climate change that threatens crop yield worldwide. Here, we report that elevated temperature accelerates fruit dehiscence in members of the Brassicaceae family including the model plant Arabidopsis thaliana and important crop species. Arabidopsis fruit development is controlled by a network of interacting regulatory genes. Among them, the INDEHISCENT (IND) gene is a key regulator of the valve-margin tissue that mediates fruit opening, hence facilitating fruit dehiscence. We demonstrated that the valve-margin development is accelerated at higher temperature and that IND is targeted for thermosensory control. Our results reveal that IND upregulation is facilitated via temperature-induced chromatin dynamics leading to accelerated valve-margin specification and dispersal of the seed. Specifically, we show that temperature-induced changes in IND expression are associated with thermosensory H2A.Z nucleosome dynamics. These findings establish a molecular framework connecting tissue identity with thermal sensing and set out directions for the production of temperature-resilient crops.

Concepts: DNA, Gene expression, Developmental biology, Model organism, Arabidopsis thaliana, Arabidopsis, Legume, Brassicaceae

32

Drought stress has often caused significant decreases in crop production which could be associated with global warming. Enhancing drought tolerance without a grain yield penalty has been a great challenge in crop improvement. Here, we report the Arabidopsis thaliana galactinol synthase 2 gene (AtGolS2) was able to confer drought tolerance and increase grain yield in two different rice (Oryza sativa) genotypes under dry field conditions. The developed Ubi:AtGolS2 transgenic lines also had higher levels of galactinol than the non-transgenic control. The increased grain yield of the transgenic rice under drought conditions was related to a higher number of panicles, grain fertility and biomass. Extensive confined field trials using transgenic lines expressing AtGolS2 under the control of the constitutive maize ubiquitin promoter (Ubi:AtGolS2) in Curinga, tropical japonica and NERICA4, interspecific hybrid across two different seasons and environments revealed the verified lines have the proven field drought tolerance of the Ubi:AtGolS2 transgenic rice. The amended drought tolerance was associated with higher relative water content of leaves, higher photosynthesis activity, lesser reduction in plant growth and faster recovering ability. Collectively, our results provide strong evidence that AtGolS2 is a useful biotechnological tool to reduce grain yield losses in rice beyond genetic differences under field drought stress. This article is protected by copyright. All rights reserved.

Concepts: Genetics, Gene expression, Model organism, Arabidopsis thaliana, Arabidopsis, Maize, Rice, Copyright

30

Over the past 20 y, many studies have examined the history of the plant ecological and molecular model, Arabidopsis thaliana, in Europe and North America. Although these studies informed us about the recent history of the species, the early history has remained elusive. In a large-scale genomic analysis of African A. thaliana, we sequenced the genomes of 78 modern and herbarium samples from Africa and analyzed these together with over 1,000 previously sequenced Eurasian samples. In striking contrast to expectations, we find that all African individuals sampled are native to this continent, including those from sub-Saharan Africa. Moreover, we show that Africa harbors the greatest variation and represents the deepest history in the A. thaliana lineage. Our results also reveal evidence that selfing, a major defining characteristic of the species, evolved in a single geographic region, best represented today within Africa. Demographic inference supports a model in which the ancestral A. thaliana population began to split by 120-90 kya, during the last interglacial and Abbassia pluvial, and Eurasian populations subsequently separated from one another at around 40 kya. This bears striking similarities to the patterns observed for diverse species, including humans, implying a key role for climatic events during interglacial and pluvial periods in shaping the histories and current distributions of a wide range of species.

Concepts: Genetics, Evolution, Genome, Africa, Sub-Saharan Africa, Model organism, Arabidopsis thaliana, Arabidopsis

28

Arsenic is a toxic metalloid and recognized carcinogen. Arsenate and arsenite are the most common arsenic species available for uptake by plants. As an inorganic phosphate (Pi) analog, arsenate is acquired by plant roots through endogenous Pi transport systems. Inside the cell, arsenate is reduced to the thiol-reactive form arsenite. Glutathione (GSH)-conjugates of arsenite may be extruded from the cell or sequestered in vacuoles by members of the ATP-binding cassette (ABC) family of transporters. In the present study we sought to enhance both plant arsenic uptake through Pi transporter overexpression, and plant arsenic tolerance through ABC transporter overexpression. We demonstrate that Arabidopsis thaliana plants overexpressing the high-affinity Pi transporter family members, AtPht1;1 or AtPht1;7, are hypersensitive to arsenate due to increased arsenate uptake. These plants do not exhibit increased sensitivity to arsenite. Co-overexpression of the yeast ABC transporter YCF1 in combination with AtPht1;1 or AtPht1;7 suppresses the arsenate-sensitive phenotype while further enhancing arsenic uptake. Taken together, our results support an arsenic transport mechanism in which arsenate uptake is increased through Pi transporter overexpression, and arsenic tolerance is enhanced through YCF1-mediated vacuolar sequestration. This work substantiates the viability of coupling enhanced uptake and vacuolar sequestration as a means for developing a prototypical engineered arsenic hyperaccumulator.

Concepts: Photosynthesis, Plant, Botany, Arabidopsis thaliana, Arabidopsis, Transport, Root, Arsenic

28

Numerous aromatic plant species produce high levels of monoterpenols, using geranyl diphosphate (GPP) as a precursor. Sweet basil (Ocimum basilicum) geraniol synthase (GES) was used to evaluate the monoterpenol profiles arising from heterologous expressions in various plant models. Grapevine (Vitis vinifera) calli were transformed using Agrobacterium tumefasciens and the plants were regenerated. Thale cress (Arabidopsis thaliana) was transformed using the floral dip method. Tobacco (Nicotiana benthamiana) leaves were agro-infiltrated for transient expression. Although, as expected, geraniol was the main product detected in the leaves, different minor products were observed in these plants (V. vinifera: citronellol and nerol; N. benthamiana: linalool and nerol; A. thaliana: none). O. basilicum GES expression was also carried out with microbial system yeasts (Saccharomyces cerevisiae) and Escherichia coli. These results suggest that the functional properties of a monoterpenol synthase depend not only on the enzyme’s amino-acidic sequence, but also on the cellular background. They also suggest that some plant species or microbial expression systems could induce the simultaneous formation of several carbocations, and could thus have a natural tendency to produce a wider spectrum of monoterpenols.

Concepts: Genome, Fungus, Model organism, Arabidopsis thaliana, Arabidopsis, Annual plant, Basil, Ocimum basilicum

28

The effect of arsenic (25 and 50μM As for 1 and 5d) was analysed in wild type (WT) and Arabidopsis thaliana (L.) Heynh plants deficient in NADPH oxidase C (AtrbohC). The content of H(2)O(2) and malondialdehyde (MDA) increased with the As concentration, while the opposite effect was found for NO in WT and AtrbohC plants. The As treatment reduced catalase and increased glutathione reductase activities to the same extent in WT and AtrbohC plants, although the induction of all SOD isoforms (mainly CuZn-SODs) was observed in WT plants, the opposite effects being found in AtrbohC plants. Glycolate oxidase (H(2)O(2) producers) considerably increased with the concentration and time of treatment with As in WT and AtrbohC mutants. Arsenic induced the uptake and translocation of P, S, Cu, Zn, and Fe in WT plants, while in AtrbohC plants the opposite trend was noted and the uptake of As became considerably lower than in WT plants. These results suggest that As causes oxidative stress by inducing glycolate oxidase, while NADPH oxidase does not appear to participate in ROS overproduction but could be critical in regulating antioxidant defences as well as the transport and translocation of As and macro/micronutrients.

Concepts: Photosynthesis, Antioxidant, Oxidative stress, Reactive oxygen species, Superoxide dismutase, NADPH oxidase, Arabidopsis, Glutathione