Einfluss von NaCl-Stress auf die Expression von Expansinen sowie den apoplastischen pH-Wert in expandierenden Blättern

Soil salinity poses a major threat to agriculture, because during the osmotic stress phase, salt-sensitive crops such as maize (Zea mays L.) exhibit a strong growth reduction. A modified capacity of cell walls to expand irreversibly has been suggested to be the major growth-limiting factor during the salinity-induced osmotic stress phase. Elucidation of the way that salinity affects shoot growth is thus of great importance for a better understanding of processes that contribute to salt resistance. For this purpose, shoot growth and shoot growth-related factors have been compared between a salt-sensitive and a salt-resistant maize hybrid under saline condition. The 100 mM NaCl treatment of maize plants that differ in their degree of salt-resistance over a period of 8–days has revealed genotype-specific differences regarding the ability of the young shoots to maintain growth. The salt-sensitive hybrid Lector exhibited a strong reduction in growth, as is known to occur in the first phase of salt stress. In contrast, the shoots of the salt-resistant hybrid SR03 were only marginally affected and maintained growth. The up-regulation of the mRNA of wall-loosening factors (ZmXTH1, ZmXET1, ZmEXPA1, ZmEXPB2, ZmEXPB6, and ZmEXPB8) within the expanding shoots of the salt-resistant hybrid might contribute to a mechanism for improving wall extensibility under stress and thus might counteract growth reduction as occurs, for example, in the salt-sensitive hybrid. In favour of this assumption, transcripts of these wall-loosening factors were down-regulated in the size-reduced leaves of the salt-sensitive Lector hybrid. These genotypic differences are indicative of a role for these wall-loosening agents in processes related to salt-resistance. Genotypic-specific effects were also confirmed to occur on the proteome level: salinity did not affect the abundance of the vegetatively expressed β-expansins in the shoots of the salt-resistant SR03. However, β-expansin proteins were down-regulated in size-reduced shoots of the salt-sensitive cultivar. 2D-Western blotting revealed that one out of four isoform was down-regulated in size-reduced leaves of the salt-sensitive Lector. Growth might be reduced because salinity possibly impairs the synthesis of this growth-mediating enzyme. These genotypic differences are indicative of a role for the β-expansins in maintaining growth and thus of their contribution to salt resistance. After salt treatment, the apoplastic pH seemed to be differentially regulated between both hybrids. The leaf apoplast of the salt-resistant SR03 was acidified in response to salinity. The findings that (i) acidification of the leaf apoplast is a major requirement for increasing wall extensibility and that (ii) expansins are activated by an acidic pH, are both indicative that the observed acidification represents a mechanism possibly related to the maintenance of growth under saline conditions. In favour of this hypothesis, the leaf apoplast of the salt-sensitive Lector does not acidify but exhibits a strong reduction in its shoot growth. A comparative study of plants that differ in their degree of salt resistance revealed contrasting physiological features in terms of cell wall-associated agents that mediate growth. Wall-loosening agents were impaired in size-reduced leaves of the salt-sensitive hybrid but not in leaves of the salt-resistant hybrid that maintained growth. This physiological difference is indicative for a role of these wall-loosening agents for salt-resistance and thus may be used for screening for salt-resistant plants

Real-Time Imaging of Leaf Apoplastic pH Dynamics in Response to NaCl Stress

Knowledge concerning apoplastic ion concentrations is important for the understanding of many processes in plant physiology. Ion-sensitive fluorescent probes in combination with quantitative imaging techniques offer opportunities to localize, visualize, and quantify apoplastic ion dynamics in situ. The application of this technique to the leaf apoplast is complicated because of problems associated with dye loading. We demonstrate a more sophisticated dye loading procedure that enables the mapping of spatial apoplastic ion gradients over a period of 3 h. The new technique has been used for the real-time monitoring of pH dynamics within the leaf apoplast in response to NaCl stress encountered by the roots

Biofortification and subcellular localization of minerals in faba bean as influenced by Mg foliar application

Foliar application of Mg is a measure for the correction of Mg deficiency in crop plants. Foliar applied nutrients need to access the symplastic side where majority of physiological processes take place. To achieve an adequate uptake of the Mg ions through the leaf surface, high concentrations of of 100-200 mM MgSO4 are usually supplied. This can cause antagonistic perturbations on the subcellular distribution of Caand K cations. To test for such unintended side effects, we used the infiltration-centrifugation method to extract ions from the apoplastic and symplastic side of Vicia faba leaves and quantified concentrations of Mg, Ca and K in dependency to the dose of the foliar fertilized Mg. Results show that a large fraction of Mg accesses the symplast whereas the apoplastic fraction shows a concomitant increase. Symplastic and apoplastic K and Ca relations were only affected under conditions of high exogenous leaf supply of Mg (200 mM) but did not change upon moderate Mg supply (50; 100 mM). Overall, results reveal the suitability of leaf fertilization to biofortify plant-based products with magnesium. With respect to human nutrition, care must be taken that K and Ca do not become impoverished based on antagonistic effects

Saving CO2 Emissions by Reusing Organic Growing Media from Hydroponic Tomato Production as a Source of Nutrients to Produce Ethiopian Kale (Brassica carinata)

Large quantities of growing media residues that are rich in nutrients are disposed of after their use in hydroponics. The objective of this study was to investigate the benefits of different organic growing media (wood fibers, hemp fibers, sphagnum moss) residues from hydroponic tomato production as a nutrient source to produce Ethiopian kale. The amount of nutrients that can be reused as fertilizer and the associated CO2 savings have been calculated. Kale was cultivated in sand-residue mixtures, either with 25 or 50 vol% of the mentioned growing media residues. Control treatments with sand with or without nutrient addition were cultivated too. The incorporation of all growing media residues to sand increased the field capacity and growth. Plants that were supplemented with hemp fiber residues showed the strongest growth and highest yields. However, the hemp fiber residues that are used are not suitable for use in the open field due to its excessive content of certain nutrients, which restrict the output quantity. Regarding the fertilization effect of growing media residues, it was calculated that 11–300 kg nutrients ha−1 (N, P, K, Mg, Ca, S), with an average primary energy demand of 90–3435 MJ and 6–317 kg CO2 eq, could potentially be saved when different crops were considered.Peer Reviewe

One-time abscisic acid priming induces long-term salinity resistance in Vicia faba: Changes in key transcripts, metabolites, and ionic relations

Abscisic acid (ABA) priming is known to enhance plant growth and survival under salinity. However, the mechanisms mediating this long-term acclimatization to salt stress are still obscure. Specifically, the long-term transcriptional changes and their effects on ion relations were never investigated. This motivated us to study the long-term (8 days) effect of one-time 24 h root priming treatment with 10 μM ABA on transcription levels of relevant regulated key genes, osmotically relevant metabolites, and ionic concentrations in Vicia faba grown under 50 mM NaCl salinity. The novelty of this study is that we could demonstrate long-term effects of a one-time ABA application. ABA-priming was found to prevent the salt-induced decline in root and shoot dry matter, improved photosynthesis, and inhibited terminal wilting of plants. It substantially increased the mRNA level of AAPK and 14-3-3 ABA inducible kinases and ion transporters (PM H+ -ATPase, VFK1, KUP7, SOS1, and CLC1). These ABA-induced transcriptional changes went along with altered tissue ion patterns. Primed plants accumulated less Na+ and Cl- but more K+ , Ca2+ , Zn2+ , Fe2+ , Mn2+ , NO3 - , and SO4 2- . Priming changed the composition pattern of organic osmolytes under salinity, with glucose and fructose being dominant in unprimed, whereas sucrose was dominant in the primed plants. We conclude that one-time ABA priming mitigates salt stress in Vicia faba by persistently changing transcription patterns of key genes, stabilizing the ionic and osmotic balance, and improving photosynthesis and growth

Phenolic compound abundance in Pak choi leaves is controlled by salinity and dependent on pH of the leaf apoplast

Onset of salinity induces the pH of the leaf apoplast of Pak choi transiently to increase over a period of 2 to 3 hr. This pH event causes protein abundances in leaves to increase. Among them are enzymes that are key for the phenylpropanoid pathway. To answer the questions whether this short-term salt stress also influences contents of the underlying phenylpropanoids and for clarifying as to whether the apoplastic pH transient plays a role for such a putative effect, Pak choi plants were treated with 37.5 mM CaCl2 against a non-stressed control. A third experimental group, where the leaf apoplast of plants treated with 37.5 mM CaCl2, was clamped in the acidic range by means of infiltration of 5 mM citric acid/sodium citrate (pH 3.6), enabled validation of pH-dependent effects. Microscopy-based live cell imaging was used to quantify leaf apoplastic pH in planta. Phenolics were quantified shortly after the formation of the leaf apoplastic pH transient by means of HPLC-DAD-ESI-MS. Results showed that different phenolic compounds were modulated at 150 and 200 min after the onset of chloride salinity. A pH-independent reduction in phenolic acid abundance as well as an accumulation of phenolic acid:malate conjugates was quantified after 200 min of salt stress. However, at 150 min after the onset of salt stress, flavonoids were significantly reduced by salinity in a pH-dependent manner. These results provided a strong indication that the pH of the apoplast is a relevant component for the short-term metabolic response to chloride salinity.BMBF research grantDeutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659Peer Reviewe

Early Changes in Nitrate Uptake and Assimilation Under Drought in Relation to Transpiration

Soil drying combined with nitrogen (N) deficiency poses a grave threat to agricultural crop production. The rate at which nitrate (NO3−) is taken up depends partly on the uptake and transpiration of water. Rapid changes in nitrate assimilation, in contrast to other N forms, may serve as a component of the plant stress response to drought because nitrate assimilation may lead to changes in xylem pH. The modulation of xylem sap pH may be relevant for stomata regulation via the delivery of abscisic acid (ABA) to guard cells. In several factorial experiments, we investigated the interactions between nitrate and water availability on nitrate fate in the plant, as well as their possible implications for the early drought-stress response. We monitored the short-term response (2–6 days) of nitrate in biomass, transport to shoot and reduction in Pisum sativum, Hordeum vulgare, Vicia faba, and Nicotiana tabacum and correlated this with sap pH and transpiration rates (TRs). Cultivation on inorganic substrate ensured control over nutrient and water supply and prevented nodulation in legume species. NO3− content in biomass decreased in most of the species under drought indicating significant decline in NO3− uptake. Hordeum vulgare had the highest NO3− concentrations in all organs even under drought and low NO3− treatment. This species can likely respond much better to the combined adverse effects of low NO3− and water scarcity. Nitrate reductase activity (NRA) was reduced in both roots and leaves of water deficient (WD) plants in all species except H. vulgare, presumably due to its high NO3− contents. Further, transient reduction in NO3− availability had no effect on sap pH. Therefore, it seems unlikely that NRA shifts from shoot root leading to the supposed alkalization of sap. We also did not observe any interactive effects of NO3− and water deficiency on transpiration. Hence, as long as leaf NO3− content remains stable, NO3− availability in soil is not linked to short-term modulation of transpiration.Peer Reviewe

Physiological Responses of Contrasting Maize (Zea mays L.) Hybrids to Repeated Drought

Maize is the most important crop worldwide in terms of production and yield, but every year a considerable amount of yield is lost due to drought. The foreseen increase in the number of drought spells due to climate change raises the question whether the ability to recover quickly after a water pulse may be a relevant trait for overall drought resistance. We here address the following hypotheses: (i) different maize hybrids exhibit distinct physiological adaptive responses to drought stress and (ii) these responses affect the ability to recover from the stress. (iii) The relative biomass production of maize hybrids, which show severe drought symptoms but are able to recover quickly after a water pulse, is comparable to those hybrids, which invest more energy into tolerance mechanisms. The physiological responses of eight maize hybrids to repeated drought were elucidated employing physiological parameters such as electrolyte leakage, osmolality, relative water content, growth rate and gas-exchange measurements. Only one hybrid was able to maintain biomass production under drought conditions. Amongst the others, two hybrids with similar growth inhibition but contrasting physiological responses were identified by a PCA analysis. Both strategies, i.e. stabilization of leaf water content via resistance mechanisms versus high recovery potential were equally effective in maintaining aboveground biomass production in the scenario of a long drought intermitted by a water-pulse. However, each strategy might be advantageous under different drought stress scenarios. Overall, the recovery potential is underestimated in drought resistance under natural conditions, which includes periodic cycles of drought and rewatering, and should be considered in screening trials.Regional Research AllianceUniversität Hohenheim (3153)Peer Reviewe

Silencing of the sulfur rich alpha-gliadin storage protein family in wheat grains (Triticum aestivum L.) causes no unintended side-effects on other metabolites

Zoerb C, Becker D, Hasler M, et al. Silencing of the sulfur rich alpha-gliadin storage protein family in wheat grains (Triticum aestivum L.) causes no unintended side-effects on other metabolites. Frontiers in Plant Science. 2013;4:369.Wheat is an important source of proteins and metabolites for human and animal nutrition. To assess the nutritional quality of wheat products, various protein and diverse metabolites have to be evaluated. The grain storage protein family of the alpha-gliadins are suggested to be the primary initiator of the inflammatory response to gluten in Celiac disease patients. With the technique of RNAi, the alpha-gliadin storage protein fraction in wheat grains was recently knocked down. From a patient's perspective, this is a desired approach, however, this study aims to evaluate whether such a down-regulation of these problematic alpha-gliadins also has unintended side-effects on other plant metabolites. Such uncontrolled and unknown arbitrary effects on any metabolite in plants designated for food production would surely represent an avoidable risk for the consumer. In general, alpha-gliadins are rich in sulfur, making their synthesis and content depended of the sulfur supply. For this reason, the influence of the application of increasing sulfur amounts on the metabolome of alpha-gliadin-deficient wheat was additionally investigated because it might be possible that e.g., considerable high/low amounts of S might increase or even induce such unintended effects that are not observable under moderate S nutrition. By silencing the alpha-gliadin genes, a recently developed wheat line that lacks the set of 75 corresponding alpha-gliadin proteins has become available. The plants were subsequently tested for RNAi-induced effects on metabolites that were not directly attributable to the specific effects of the RNAi-approach on the alpha-gliadin proteins. For this, GC-MS-based metabolite profiles were recorded. A comparison of wild type with gliadin-deficient plants cultivated in pot experiments revealed no differences in all 109 analyzed metabolites, regardless of the S-nutritional status. No unintended effects attributable to the RNAi-based specific genetic deletion of a storage protein fraction were observed

Interactive effects of genotype and N/S-supply on glucosinolates and glucosinolate breakdown products in Chinese cabbage (<i>Brassica rapa</i> L. ssp. <i>pekinensis</i>)

Chinese cabbage is rich in glucosinolates (GLS) and their breakdown products, mainly isothiocyanates (ITC), which are assumed to be human health-promoting compounds. Sulphur and nitrogen have been shown to influence concentrations and patterns of both. Little is known as to whether the effect of varying sulphur and nitrogen nutrition on glucosinolate and isothiocyanate content is influenced by the genotype. Therefore, two cultivars of Brassica rapa L. ssp. pekinensis were grown with increasing S (0.0, 0.3, and 0.6 g pot-1) and N (1 and 2 g pot-1) supply. Results show that total GLS concentration increased with higher N and S application, but ratios between individual GLS compounds remained unchanged. High N supply reduced the concentration of GLS, especially of the aliphatic ones, while the indole and aromatic GLS exhibited statistically insignificant responses to increasing N and S application. The profile of breakdown products was dominated by epithionitriles, followed by ITCs and nitriles. The ITCs were substantially reduced in response to increasing N and decreasing S supply. This was not observed for nitriles. Overall, GLS pattern were primarily influenced by the genetic background of the cultivar and less influenced by differential nutrition. Results show that selection of the cultivar is of utmost importance when glucosinolates and their breakdown products shall be increased by fertilization.   The online version of this article (doi: 10.5073/JABFQ.2016.089.036) contains supplementary files