ABA-Dependent and -Independent Regulation of Tocopherol (Vitamin E) Biosynthesis in Response to Abiotic Stress in Arabidopsis

Abiotic stresses in plants lead to elevated levels of reactive oxygen species that can have detrimental effects on photosystems and thylakoid lipids. To limit damage, plants increase the production of antioxidants such as tocopherol (vitamin E). While tocopherol biosynthesis has been investigated in detail, only little is known about the regulation of this pathway. Previously, it has been suggested that the phytohormone abscisic acid (ABA), which is involved in abiotic stress responses, may be a key regulator of tocopherol biosynthesis. To decipher the regulatory network between ABA and tocopherol biosynthesis in Arabidopsis, several ABA-deficient (aba) and ABA-insensitive (abi) mutants were exposed to (i) osmotic stress simulated by PEG8000 containing medium, (ii) drought stress on soil, and (iii) nitrogen deprived medium. Under all stress conditions the tocopherol content in the mutants increased similarly as in the wild type, except for abi4-1, abi4-102 and abi2-1 under osmotic stress. However, tocopherol synthesis was still upregulated in several aba or abi mutants. Therefore, Arabidopsis wild type plants were exposed to two additional abiotic stresses, i.e. (iv) ABA treatment and (v) high light exposure. Under all five stresses, tocopherol was increased. Transcript analyses via qPCR supported these findings showing an up-regulation of tocopherol biosynthesis genes under all conditions. LC-MS measurements revealed that ABA content increased in response to ABA treatment, drought stress and osmotic stress, but not during high light and nitrogen deprivation. This indicates that an ABA-independent pathway for regulating tocopherol biosynthesis must exist. Treatment of abi mutants with exogenous ABA showed a less pronounced increase in tocopherol content for the abi4-102 and pyl1pyr2pyr4pyr5pyr8 mutants. The expression of tocopherol biosynthesis genes is also not altered in pyl1pyr2pyr4pyr5pyr8 and especially in abi4-102. These results support the scenario that tocopherol synthesis is regulated via an ABA-dependent and an ABA-independent pathway, and that the ABA receptor proteins PYL1PYR2PYR4PYR5PYR8 and the AP2 domain transcription factor ABI4 play important roles during ABA dependent tocopherol synthesis

Suberin biosynthesis in barley roots in response to osmotic stress

In future climate change will intensify extreme weather conditions, such as drought, which will lead to decreased yields of crops. A decrease in soil water potential is the first signal of potential drought stress for plants. In fact plant roots are the first organs sensing drought and water deficit in soil. At the same time plant roots are the main organ to take up water to supply shoots and leaves. Water uptake in roots is described by the composite transport model. The main components of the model are the apoplastic pathway (cell walls), which can be blocked by Casparian bands and the biopolymer suberin, and the cell-to-cell pathway which can be regulated by aquaporins. The model plant Arabidopsis and crop plants such as rice and barley have very different root anatomical structures. This explains why a simple transfer of knowledge on root water transport from the model to the crop is not always valid. Especially the correlations between suberin amounts and water uptake require caution. The response of barley seminal roots to different levels of low water potentials (-0.4, -0.8 and -1.2 MPa) induced by PEG8000 has been studied. In this approach, various experimental methods (histochemistry, analytical chemistry, transcriptomics and transport physiology) were used to test the hypothesis whether an increased suberization of barley roots could represent an efficient response to osmotic stress thus limiting uncontrolled, passive water loss from roots to the dry soil/medium. In response to osmotic stress, genes in the suberin biosynthesis pathway were up-regulated which correlated well with increased suberin amounts in the cell walls of the endodermis and overall reduction of hydraulic conductivity (Lpr). In parallel, transcriptomic data indicated no or only weak effects of water stress on gene expression of aquaporins which are relevant for the cell-to-cell pathway. Finally, the effect of osmotic stress on seminal roots of wild and cultivated barley was compared. Wild barley has a wider diversity than cultivated barley, which is also represented in the root response to water stress. In contrast to cultivated barley, the suberization of wild barley was delayed and not much affected in response to osmotic stress. Furthermore, Lpr of wild barley was not reduced in response to osmotic stress. Most remarkably, one wild barley accession from Jordan exhibited the formation of a suberized exodermis in about 20% of the seminal roots when exposed to osmotic stress. This was never observed in cultivated barley in all water stress conditions

Experimental validation of RNA interference efficiency and off-target prediction in barley

This Bachelor thesis provides an experimental validation of the “si-Fi” software, which was designed for RNAi off-target searches and silencing efficiency predictions. The experimental approach is based on using synthetic DNA as RNAi-target as well as RNAi-trigger sequence. The data was generated by two different types of experiments using a transient gene silencing system in bombarded barley epidermal cells. The efficiency of RNAi was estimated by scoring the effect of silencing of the susceptibility-related gene Mlo on resistance of transformed cells to the powdery mildew fungus Blumeria graminis f. sp. hordei by observing reduction of fluorescent signals coming from an RNAi target fused to the green fluorescent protein. The aim of this work was a comparison between in silicio prediction of RNAi efficiency and off-target effects in barley and experimental data

Transcriptomic changes in barley leaves induced by alcohol ethoxylates indicate potential pathways of surfactant detoxification

Abstract Hardly anything is known regarding the detoxification of surfactants in crop plants, although they are frequently treated with agrochemical formulations. Therefore, we studied transcriptomic changes in barley leaves induced in response to spraying leaf surfaces with two alcohol ethoxylates (AEs). As model surfactants, we selected the monodisperse tetraethylene glycol monododecyl (C12E4) ether and the polydisperse BrijL4. Barley plants were harvested 8 h after spraying with a 0.1% surfactant solution and changes in gene expression were analysed by RNA-sequencing (RNA-Seq). Gene expression was significantly altered in response to both surfactants. With BrijL4 more genes (9724) were differentially expressed compared to C12E4 (6197). Gene families showing pronounced up-regulation were cytochrome P450 enzymes, monooxygenases, ABC-transporters, acetyl- and methyl- transferases, glutathione-S-transferases and glycosyltransferases. These specific changes in gene expression and the postulated function of the corresponding enzymes allowed hypothesizing three potential metabolic pathways of AE detoxification in barley leaves. (i) Up-regulation of P450 cytochrome oxidoreductases suggested a degradation of the lipophilic alkyl residue (dodecyl chain) of the AEs by ω- and β- oxidation. (ii) Alternatively, the polar PEG-chain of AEs could be degraded. (iii) Instead of surfactant degradation, a further pathway of detoxification could be the sequestration of AEs into the vacuole or the apoplast (cell wall). Thus, our results show that AEs lead to pronounced changes in the expression of genes coding for proteins potentially being involved in the detoxification of surfactants

Potassium deficiency reduces sugar yield in sugar beet through decreased growth of young plants

Abstract Background Optimum potassium (K) nutrition is essential for high sugar yield and quality of beet crops. Aims We quantified the effect of potassium deficiency on sugar beet dry matter and sugar yield formation during the growing season. Methods Sugar beets were grown on low, medium, and high soil K concentrations in a long‐term K fertilizer field experiment on a silt loam alluvial soil. Plants were harvested at four time points during the growing season, including the final harvest. At each harvest, the dry matter yield and K concentration of leaves and beets (when applicable) were recorded, allowing us to study their growth rates in different growth phases. At the final harvest, sugar concentration and internal beet quality were measured. Results Low soil K concentrations resulted in an approximately 10% lower sugar yield compared to high soil K supply. Interestingly, retarded growth of low K plants already occurred during the germination phase and in the first weeks thereafter, whereas later on, growth rates were similar. Higher soil K levels led to an increased K uptake rate and thus a higher final total K uptake. High K plants showed rapid youth growth and reached their maximum growth rates earlier than plants grown under low K conditions. The initial differentiation between the low and high K treatments persisted because the plants subsequently grew at similar growth rates at both low and high soil K supplies. Conclusion The early growth reduction in the low K treatment was probably caused by the low amount of K stored in sugar beet seeds in combination with low K availability in the soil. It is concluded that under overall low soil K conditions, adequate amounts of K fertilizer should be applied to meet the demand of sugar beet seedlings for maximum growth and final sugar yield. Banded K fertilizer application next to the plant rows is supposed to be efficient for this purpose