Christian Zörb

Potassium in agriculture--status and perspectives.

In this review we summarize factors determining the plant availability of soil potassium (K), the role of K in crop yield formation and product quality, and the dependence of crop stress resistance on K nutrition. Average soil reserves of K are generally large, but most of it is not plant-available. Therefore, crops need to be supplied with soluble K fertilizers, the demand of which is expected to increase significantly, particularly in developing regions of the world. Recent investigations have shown that organic exudates of some bacteria and plant roots play a key role in releasing otherwise unavailable K from K-bearing minerals. Thus, breeding for genotypes that have improved mechanisms to gain access to this fixed K will contribute toward more sustainable agriculture, particularly in cropping systems that do not have access to fertilizer K. In K-deficient crops, the supply of sink organs with photosynthates is impaired, and sugars accumulate in source leaves. This not only affects yield formation, but also quality parameters, for example in wheat, potato and grape. As K has beneficial effects on human health, its concentration in the harvest product is a quality parameter in itself. Owing to its fundamental roles in turgor generation, primary metabolism, and long-distance transport, K plays a prominent role in crop resistance to drought, salinity, high light, or cold as well as resistance to pests and pathogens. Despite the abundance of vital roles of K in crop production, an improvement of K uptake and use efficiency has not been a major focus of conventional or transgenic breeding in the past. In addition, current soil analysis methods for K are insufficient for some common soils, posing the risk of imbalanced fertilization. A stronger prioritization of these areas of research is needed to counter declines in soil fertility and to improve food security.

Adaptation of H+-Pumping and Plasma Membrane H+ ATPase Activity in Proteoid Roots of White Lupin under Phosphate Deficiency

White lupin (Lupinus albus) is able to adapt to phosphorus deficiency by producing proteoid roots that release a huge amount of organic acids, resulting in mobilization of sparingly soluble soil phosphate in rhizosphere. The mechanisms responsible for the release of organic acids by proteoid root cells, especially the trans-membrane transport processes, have not been elucidated. Because of high cytosolic pH, the release of undissociated organic acids is not probable. In the present study, we focused on H+ export by plasma membrane H+ ATPase in active proteoid roots. In vivo, rhizosphere acidification of active proteoid roots was vanadate sensitive. Plasma membranes were isolated from proteoid roots and lateral roots from P-deficient and -sufficient plants. In vitro, in comparison with two types of lateral roots and proteoid roots of P-sufficient plants, the following increase of the various parameters was induced in active proteoid roots of P-deficient plants: (a) hydrolytic ATPase activity, (b) V max andK m, (c) H+ ATPase enzyme concentration of plasma membrane, (d) H+-pumping activity, (e) pH gradient across the membrane of plasmalemma vesicles, and (f) passive H+ permeability of plasma membrane. In addition, lower vanadate sensitivity and more acidic pH optimum were determined for plasma membrane ATPase of active proteoid roots. Our data support the hypothesis that in active proteoid root cells, H+ and organic anions are exported separately, and that modification of plasma membrane H+ ATPase is essential for enhanced rhizosphere acidification by active proteoid roots.

Proteomic changes in maize roots after short‐term adjustment to saline growth conditions

It is of fundamental importance to understand adaptation processes leading to salt resistance. The initial effects on maize roots in the first hour after the adjustment to saline conditions were monitored to elucidate initial responses. The subsequent proteome change was monitored using a 2‐D proteomic approach. We found several new salt‐inducible proteins, whose expression has not been previously reported to be modulated by salt. A set of phosphoproteins in maize was detected but only ten proteins were phosphorylated and six proteins were dephosphorylated after the application of 25 mM NaCl for 1 h. Some of the phosphorylated maize proteins such as fructokinase, UDP‐glucosyl transferase BX9, and 2‐Cys‐peroxyredoxine were enhanced, whereas an isocitrate‐dehydrogenase, calmodulin, maturase, and a 40‐S‐ribosomal protein were dephosphorylated after adjustment to saline conditions. The initial reaction of the proteome and phosphoproteome of maize after adjustment to saline conditions reveals members of sugar signalling and cell signalling pathways such as calmodulin, and gave hint to a transduction chain which is involved in NaCl‐induced signalling. An alteration of 14‐3‐3 proteins as detected may change plasma membrane ATPase activity and cell wall growth regulators such as xyloglucane endotransglycosylase were also found to be changed immediately after the adjustment to salt stress.

Short-term effects of salt exposure on the maize chloroplast protein pattern

It is of fundamental importance to understand the physiological differences leading to salt resistance and to get access to the molecular mechanisms underlying this physiological response. The aim of this work was to investigate the effects of short‐term salt exposure on the proteome of maize chloroplasts in the initial phase of salt stress (up to 4 h). It could be shown that sodium ions accumulate quickly and excessively in chloroplasts in the initial phase of moderate salt stress. A change in the chloroplast protein pattern was observed without a change in water potential of the leaves. 2‐DE revealed that 12 salt‐responsive chloroplast proteins increased while eight chloroplast proteins decreased. Some of the maize chloroplast proteins such as CF1e and a Ca2+‐sensing receptor show a rather transient response for the first 4 h of salt exposure. The enhanced abundance of the ferredoxin NADPH reductase, the 23 kDa polypeptide of the photosystem II, and the FtsH‐like protein might reflect mechanism to attenuate the detrimental effects of Na+ on the photosynthetic machinery. The observed transient increase and subsequent decrease of selected proteins may exhibit a counterbalancing effect of target proteins in this context. Intriguingly, several subunits of the CF1–CF0 complex are unequally affected, whereas others do not respond at all.

Silicon-mediated improvement in the salt resistance of wheat (Triticum aestivum) results from increased sodium exclusion and resistance to oxidative stress

Silicon (Si) is reported to reduce the effect of salinity on wheat (Triticum aestivum L.) and other crops. In the present study, Si decreased plant Na+ uptake and shoot : root Na+ distribution of a salt-resistant as well as a salt-sensitive wheat genotype. Reduced shoot Na+ concentration and increased shoot K+ : Na+ ratio led to improved plant growth. Silicon increased cell-wall Na+ binding from 49% in SARC-1 and 37% in 7-Cerros under salinity to 87% in SARC-1 and 79% in 7-Cerros under salinity + silicon. It may also have resulted in decreased potentially toxic leaf sap Na+ concentration. The concentration of glutathione, an important antioxidant in plants, was increased due to the addition of Si under saline conditions. The salt-resistant wheat genotype SARC-1 was less Si-responsive in terms of shoot fresh weight, having a 39% increase compared with a 49% increase in 7-Cerros, as well as root fresh weight, having a 12% increase compared with a 22% in 7-Cerros. It is concluded that Si may have improved shoot growth of the salt-resistant as well as the salt-sensitive wheat genotype by decreasing plant Na+ uptake and shoot : root Na+ distribution as well as by increasing glutathione concentration. Silicon may have also improved in-plant Na+ detoxification by increasing cell-wall Na+ binding.

Salt resistance is determined by osmotic adjustment and abscisic acid in newly developed maize hybrids in the first phase of salt stress

This study investigated the mechanisms of salt resistance of four maize (Zea mays L.) hybrids [cultivar (cv.) Pioneer 3906 and newly developed hybrids SR03, SR12 and SR13] during the first phase of salt stress. Plants were grown in aerated nutrient solutions at 1 mM Na+ (control) and 100 mM Na+ (salt stress). Stress was imposed in 25 mM steps and plants were harvested after 2 days at 100 mM Na+. At 100 mM Na+ the area of the fourth leaf, which developed under salt stress, did not change significantly in SR03 and SR12 whereas significant reductions were observed in cv. Pioneer 3906 and SR13. Concentrations of assimilates (i.e. glucose, fructose and sucrose) in the shoot sap were significantly greater under salt stress in SR03 and SR12. However, the greater assimilate supply was not responsible for their salt resistance as there were no significant reductions in assimilate concentrations even in the other two genotypes. Shoot turgor and growth were maintained in SR03 and SR12 at 100 mM Na+ through significant increases in osmolality of the shoot sap. Concentrations of free ABA and ABA-glucose esters (ABA-GE) in the growing region of the fourth leaf increased significantly under salt stress in all genotypes. Leaf area at 100 mM Na(+), expressed as a percentage of that at 1 mM, showed significant positive relationships with free ABA (R(2) = 0.62) and the sum of free ABA and ABA-GE (R(2) = 0.65). Results of this study indicate clearly that a combination of partial osmotic adjustment, a possible reduction of the sensitivity of leaf growth under salt stress to increased ABA concentrations and a growth-promoting function regulated by ABA is responsible for salt resistance in the first phase of salt stress. Genotypic variation in these mechanisms can be utilized to breed salt-resistant genotypes in maize.

The influence of salt stress on ABA and auxin concentrations in two maize cultivars differing in salt resistance

The plant hormones abscisic acid (ABA) and auxin (IAA, IBA) play important roles in plant responses to environmental stresses such as salinity. Recent breeding improvements in terms of salt resistance of maize have lead to a genotype with improved growth under saline conditions. By comparing this salt-resistant hybrid with a sensitive hybrid, it was possible to show differences in hormone concentrations in expanding leaves and roots. In response to salinity, the salt-resistant maize significantly increased IBA concentrations in growing leaves and maintained IAA concentration in roots. These hormonal adaptations may help to establish favorable conditions for growth-promoting agents such as β-expansins and maintain growth of resistant maize hybrids under salt stress. Moreover, ABA concentrations significantly increased in resistant maize leaves under salt stress, which may contribute to acidifying the apoplast, which in turn is a prerequisite for growth.

Salt stress differentially affects growth-mediating β-expansins in resistant and sensitive maize (Zea mays L.).

Salinity mainly reduces shoot growth by the inhibition of cell division and elongation. Expansins loosen plant cell walls. Moreover, the expression of some isoforms is clearly correlated with growth. Effects of salinity on β-expansin transcripts protein abundance were recently reported for different crop species. This study provides a broad analysis of the impact of an 8-day 100mM NaCl stress treatment on the mRNA expression of different maize (Zea mays L.) β-Expansin isoforms using real-time quantitative RT-PCR. The composite β-expansin protein expression was analyzed by western blotting using an anti-peptide antibody raised against a conserved 15-amino-acid region shared by vegetatively expressed β-expansin isoforms. For the first time, changes in β-expansin transcript and protein abundance have been analyzed together with the salinity-induced inhibition of shoot growth. A salt-resistant and a salt-sensitive cultivar were compared in order to elucidate physiological changes. Genotypic differences in the relative concentration of six β-expansin transcripts together with differences in the abundance β-expansin protein are shown in response NaCl stress. In salt-sensitive Lector, reduced β-expansin protein expression was found to correlate positively with reduced shoot growth under stress. A down-regulation of ZmExpB2, ZmExpB6, and ZmExpB8 transcripts possibly contribute to this decrease in protein abundance. In contrast, the maintenance of shoot growth in salt-resistant SR03 might be related to an unaffected abundance of growth-mediating β-expansin proteins in the shoot. Our data suggest that the up-regulation of ZmExpB2, ZmExpB6, and ZmExpB8 may sustain the stable expression of β-expansin protein under conditions of salt stress.

Levels of compounds and metabolites in wheat ears and grains in organic and conventional agriculture.

In this work, wheat from two farming systems, organic and conventional, was analyzed. Organic agriculture is one of the fastest growing sectors in the food industry of Europe and the United States. It is an open question, whether organic or conventional agricultural management influences variables such as metabolism, nutrient supply, seed loading and metabolite composition of wheat. Our aim was to detect if organic or conventional farming systems would affect concentrations of metabolites and substances in developing ears and in corresponding matured grain. Therefore, broadband metabolite profiles together with lipids, cations, starch and protein concentrations of wheat ears in the last phase of grain development and of matured grain from organic and conventional agriculture of a rigorously controlled field trial with two organic and two conventional systems were examined. It appears that seed metabolism and supply of developing ears differ in organic and conventional agriculture. However, the differences in 62 metabolite concentrations become marginal or disappear in the matured grains, indicating an adjustment of nutrients in the matured grain from organic agriculture. This result suggests a high degree of homeostasis in the final seed set independent of the growing regime.

Metabolic contribution to salt stress in two maize hybrids with contrasting resistance

Salt stress reduces the growth of salt-sensitive plants such as maize. The cultivation of salt-resistant maize varieties might therefore help to reduce yield losses. For the elucidation of the underlying physiological and biochemical processes of a resistant hybrid, we used a gas chromatography mass spectrometry approach and analyzed five different salt stress levels. By comparing a salt-sensitive and a salt-resistant maize hybrid, we were able to identify an accumulation of sugars such as glucose, fructose, and sucrose in leaves as a salt-resistance adaption of the salt-sensitive hybrid. Although, both hybrids showed a strong decrease of the metabolite concentration in the tricarboxylic acid cycle. These decreases resulted in the same reduced catabolism for the salt-sensitive and even the salt-resistant maize hybrid. Surprisingly, the change of root metabolism was negligible under salt stress. Moreover, the salt-resistance mechanisms were the most effective at low salt-stress levels in the leaves of the salt-sensitive maize.