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.

     

Down-Regulation of ZmEXPB6 (Zea mays β-Expansin 6) Protein Is Correlated with Salt-mediated Growth Reduction in the Leaves of Z. mays L.

The salt-sensitive crop Zea mays L. shows a rapid leaf growth reduction upon NaCl stress. There is increasing evidence that salinity impairs the ability of the cell walls to expand, ultimately inhibiting growth. Wall-loosening is a prerequisite for cell wall expansion, a process that is under the control of cell wall-located expansin proteins. In this study the abundance of those proteins was analyzed against salt stress using gel-based two-dimensional proteomics and two-dimensional Western blotting. Results show that ZmEXPB6 (Z. mays β-expansin 6) protein is lacking in growth-inhibited leaves of salt-stressed maize. Of note, the exogenous application of heterologously expressed and metal-chelate-affinity chromatography-purified ZmEXPB6 on growth-reduced leaves that lack native ZmEXPB6 under NaCl stress partially restored leaf growth. In vitro assays on frozen-thawed leaf sections revealed that recombinant ZmEXPB6 acts on the capacity of the walls to extend. Our results identify expansins as a factor that partially restores leaf growth of maize in saline environments.     

Salinity and crop yield

Thirty crop species provide 90% of our food, most of which display severe yield losses under moderate salinity. Securing and augmenting agricultural yield in times of global warming and population increase is urgent and should, aside from ameliorating saline soils, include attempts to increase crop plant salt tolerance. This short review provides an overview of the processes that limit growth and yield in saline conditions. Yield is reduced if soil salinity surpasses crop‐specific thresholds, with cotton, barley and sugar beet being highly tolerant, while sweet potato, wheat and maize display high sensitivity. Apart from Na⁺, also Cl^?, Mg²⁺, SO₄^2‐ or HCO₃^‐ contribute to salt toxicity. The inhibition of biochemical or physiological processes cause imbalance in metabolism and cell signalling and enhance the production of reactive oxygen species interfering with cell redox and energy state. Plant development and root patterning is disturbed, and this response depends on redox and reactive oxygen species signalling, calcium and plant hormones. The interlink of the physiological understanding of tolerance processes from molecular processes as well as the agronomical techniques for stabilizing growth and yield and their interlinks might help improving our crops for future demand and will provide improvement for cultivating crops in saline environment.     

The relative contributions of biological and abiotic processes to carbon dynamics in subarctic sea ice

Knowledge on the relative effects of biological activity and precipitation/dissolution of calcium carbonate (CaCO₃) in influencing the air-ice CO₂ exchange in sea-ice-covered season is currently lacking. Furthermore, the spatial and temporal occurrence of CaCO₃ and other biogeochemical parameters in sea ice are still not well described. Here we investigated autotrophic and heterotrophic activity as well as the precipitation/dissolution of CaCO₃ in subarctic sea ice in South West Greenland. Integrated over the entire ice season (71 days), the sea ice was net autotrophic with a net carbon fixation of 56 mg C m^?2, derived from a sea-ice-related gross primary production of 153 mg C m^?2 and a bacterial carbon demand of 97 mg C m^?2. Primary production contributed only marginally to the TCO₂ depletion of the sea ice (7–25 %), which was mainly controlled by physical export by brine drainage and CaCO₃ precipitation. The net biological production could only explain 4 % of this sea-ice-driven CO₂ uptake. Abiotic processes contributed to an air-sea CO₂ uptake of 1.5 mmol m^?2 sea ice day^?1, and dissolution of CaCO₃ increased the air-sea CO₂ uptake by 36 % compared to a theoretical estimate of melting CaCO₃-free sea ice. There was a considerable spatial and temporal variability of CaCO₃ and the other biogeochemical parameters measured (dissolved organic and inorganic nutrients).     

Salinity Stiffens the Epidermal Cell Walls of Salt-Stressed Maize Leaves: Is the Epidermis Growth-Restricting?

As a result of salt (NaCl)-stress, sensitive varieties of maize (Zea mays L.) respond with a strong inhibition of organ growth. The reduction of leaf elongation investigated here has several causes, including a modification of the mechanical properties of the cell wall. Among the various tissues that form the leaf, the epidermis plays a special role in controlling organ growth, because it is thought to form a rigid outer leaf coat that can restrict elongation by interacting with the inner cell layers. This study was designed to determine whether growth-related changes in the leaf epidermis and its cell wall correspond to the overall reduction in cell expansion of maize leaves during an osmotic stress-phase induced by salt treatment. Two different maize varieties contrasting in their degree of salt resistance (i.e., the hybrids Lector vs. SR03) were compared in order to identify physiological features contributing to resistance towards salinity. Wall loosening-related parameters, such as the capacity of the epidermal cell wall to expand, β-expansin abundance and apoplastic pH values, were analysed. Our data demonstrate that, in the salt-tolerant maize hybrid which maintained leaf growth under salinity, the epidermal cell wall was more extensible under salt stress. This was associated with a shift of the epidermal apoplastic pH into a range more favourable for acid growth. The more sensitive hybrid that displayed a pronounced leaf growth-reduction was shown to have stiffer epidermal cell walls under stress. This may be attributable to the reduced abundance of cell wall-loosening β-expansin proteins following a high salinity-treatment in the nutrient solution (100 mM NaCl, 8 days). This study clearly documents that salt stress impairs epidermal wall-loosening in growth-reduced maize leaves.     

Early changes in apoplast composition associated with defence and disease in interactions between Phaseolus vulgaris and the halo blight pathogen Pseudomonas syringae Pv. phaseolicola

The apoplast is the arena in which endophytic pathogens such as Pseudomonas syringae grow and interact with plant cells. Using metabolomic and ion analysis techniques, this study shows how the composition of Phaseolus vulgaris leaf apoplastic fluid changes during the first six hours of compatible and incompatible interactions with two strains of P. syringae pv. phaseolicola (Pph) that differ in the presence of the genomic island PPHGI‐1. Leaf inoculation with the avirulent island‐carrying strain Pph 1302A elicited effector‐triggered immunity (ETI) and resulted in specific changes in apoplast composition, including increases in conductivity, pH, citrate, γ‐aminobutyrate (GABA) and K⁺, that are linked to the onset of plant defence responses. Other apoplastic changes, including increases in Ca²⁺, Fe^2/3+ Mg²⁺, sucrose, β‐cyanoalanine and several amino acids, occurred to a relatively similar extent in interactions with both Pph 1302A and the virulent, island‐less strain Pph RJ3. Metabolic footprinting experiments established that Pph preferentially metabolizes malate, glucose and glutamate, but excludes certain other abundant apoplastic metabolites, including citrate and GABA, until preferred metabolites are depleted. These results demonstrate that Pph is well‐adapted to the leaf apoplast metabolic environment and that loss of PPHGI‐1 enables Pph to avoid changes in apoplast composition linked to plant defences.     

Dawn regulates guard cell proteins in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> that function in ATP production from fatty acid beta-oxidation

Key message

Transgenomics for gene discovery in Populus euphratica.

Abstract

Transgenomics, a member of the omics family of methodologies, is characterized as the introduction of DNA from one organism into another on a genome-wide scale followed by the identification of recipients with altered phenotypes. This strategy allows investigators to identify the gene(s) involved in these phenotypic changes. It is particularly promising for woody plants that have a long life cycle and for which molecular tools are limited. In this study, we constructed a large-insert binary bacterial artificial chromosome library of Populus euphratica, a stress-tolerant poplar species, which included 55,296 clones with average insert sizes of about 127 kb. To date, 1077 of the clones have been transformed into Arabidopsis thaliana via Agrobacterium by the floral dip method. Of these, 69 transgenic lines showed phenotypic changes represented by diverse aspects of plant form and development, 22 of which were reproducibly associated with the same phenotypic change. One of the clones conferring transgenic plants with increased salt tolerance, 002A1F06, was further analyzed and the 127,284 bp insert in this clone harbored eight genes that have been previously reported to be involved in stress resistance. This study demonstrates that transgenomics is useful in the study of functional genomics of woody plants and in the identification of novel gene(s) responsible for economically important traits. Thus, transgenomics can also be used for validation of quantitative trait loci mapped by molecular markers.

     

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.