Developmental regulation of water uptake in wheat

Dynamics in plant roots and shoots minimize stress, save energy and maintain water and nutrient uptake

Plants are inherently dynamic. Dynamics minimize stress while enabling plants to flexibly acquire resources. Three examples are presented for plants tolerating saline soil: transport of sodium chloride (NaCl), water and macronutrients is nonuniform along a branched root; water and NaCl redistribute between shoot and soil at night-time; and ATP for salt exclusion is much lower in thinner branch roots than main roots, quantified using a biophysical model and geometry from anatomy. Noninvasive phenotyping and precision agriculture technologies can be used together to harness plant dynamics, but analytical methods are needed. A plant advancing in time through a soil and atmosphere space is proposed as a framework for dynamic data and their relationship to crop improvement.

Effects of Climate Change at Treeline: Lessons from Space-for-Time Studies, Manipulative Experiments, and Long-Term Observational Records in the Central Austrian Alps

This review summarizes the present knowledge about effects of climate change on conifers within the treeline ecotone of the Central Austrian Alps. After examining the treeline environment and the tree growth with respect to elevation, possible effects of climate change on carbon gain and water relations derived from space-for-time studies and manipulative experiments are outlined. Finally, long-term observational records are discussed, working towards conclusions on tree growth in a future, warmer environment. Increases in CO2 levels along with climate warming interact in complex ways on trees at the treeline. Because treeline trees are not carbon limited, climate warming (rather than the rising atmospheric CO2 level) causes alterations in the ecological functioning of the treeline ecotone in the Central Austrian Alps. Although the water uptake from soils is improved by further climate warming due to an increased permeability of root membranes and aquaporin-mediated changes in root conductivity, tree survival at the treeline also depends on competitiveness for belowground resources. The currently observed seedling re-establishment at the treeline in the Central European Alps is an invasion into potential habitats due to decreasing grazing pressure rather than an upward-migration due to climate warming, suggesting that the treeline in the Central Austrian Alps behaves in a conservative way. Nevertheless, to understand the altitude of the treeline, one must also consider seedling establishment. As there is a lack of knowledge on this particular topic within the treeline ecotone in the Central Austrian Alps, we conclude further research has to focus on the importance of this life stage for evaluating treeline shifts and limits in a changing environment.

Transcriptional and physiological data reveal the dehydration memory behavior in switchgrass (Panicum virgatum L.)

BACKGROUND

Switchgrass (Panicum virgatum L.) is a model biofuel plant because of its high biomass, cellulose-richness, easy degradation to ethanol, and the availability of extensive genomic information. However, a little is currently known about the molecular responses of switchgrass plants to dehydration stress, especially multiple dehydration stresses.

RESULTS

Studies on the transcriptional profiles of 35-day-old tissue culture plants revealed 741 dehydration memory genes. Gene Ontology and pathway analysis showed that these genes were enriched in phenylpropanoid biosynthesis, starch and sucrose metabolism, and plant hormone signal transduction. Further analysis of specific pathways combined with physiological data suggested that switchgrass improved its dehydration resistance by changing various aspects of its responses to secondary dehydration stress (D2), including the regulation of abscisic acid (ABA) and jasmonic acid (JA) biosynthesis and signal transduction, the biosynthesis of osmolytes (l-proline, stachyose and trehalose), energy metabolism (i.e., metabolic process relating to photosynthetic systems, glycolysis, and the TCA cycle), and lignin biosynthesis. The transcriptional data and chemical substance assays showed that ABA was significantly accumulated during both primary (D1) and secondary (D2) dehydration stresses, whereas JA accumulated during D1 but became significantly less abundant during D2. This suggests the existence of a complicated signaling network of plant hormones in response to repeated dehydration stresses. A homology analysis focusing on switchgrass, maize, and Arabidopsis revealed the conservation and species-specific distribution of dehydration memory genes.

CONCLUSIONS

The molecular responses of switchgrass plants to successive dehydration stresses have been systematically characterized, revealing a previously unknown transcriptional memory behavior. These results provide new insights into the mechanisms of dehydration stress responses in plants. The genes and pathways identified in this study will be useful for the genetic improvement of switchgrass and other crops.

Soil warming increased whole-tree water use of Pinus cembra at the treeline in the Central Tyrolean Alps

This study quantified the effect of soil warming on sap flow density (Qs) of Pinus cembra L. at the treeline in the Central Tyrolean Alps. To enhance soil temperature we installed a transparent roof construction above the forest floor around six trees. Six other trees served as controls in the absence of any manipulation. Roofing enhanced growing season mean soil temperature by 1.6, 1.3 and 1.0 °C at 5, 10 and 20 cm soil depth, respectively, while soil water availability was not affected. Sap flow density (using Granier-type thermal dissipation probes) and environmental parameters were monitored throughout three growing seasons. During the first year of treatment, no warming effect was detected on Qs. However, soil warming caused Qs to increase significantly by 11 and 19% above levels in control trees during the second and third year, respectively. This effect appeared to result from warming-induced root production, a reduction in viscosity and perhaps an increase also in root hydraulic conductivity. Hardly affected were leaf-level net CO2 uptake rate and conductance for water vapour, so that water-use efficiency stayed unchanged as confirmed by needle δ(13)C analysis. We conclude that tree water loss will increase with soil warming, which may alter the water balance within the treeline ecotone of the Central Austrian Alps in a future warming environment.

Hydraulic conductivity and contribution of aquaporins to water uptake in roots of four sunflower genotypes

BACKGROUND

This article evaluates the potential of intraspecific variation for whole-root hydraulic properties in sunflower. We investigated genotypic differences related to root water transport in four genotypes selected because of their differing water use efficiency (JAC doi: 10.1111/jac.12079. 2014). We used a pressure-flux approach to characterize hydraulic conductance (L ₀ ) which reflects the overall water uptake capacity of the roots and hydraulic conductivity (Lp _r ) which represents the root intrinsic water permeability on an area basis. The contribution of aquaporins (AQPs) to water uptake was explored using mercuric chloride (HgCl₂), a general AQP blocker.

RESULTS

There were considerable variations in root morphology between genotypes. Mean values of L ₀ and Lp _r showed significant variation (above 60% in both cases) between recombinant inbred lines in control plants. Pressure-induced sap flow was strongly inhibited by HgCl₂ treatment in all genotypes (more than 50%) and contribution of AQPs to hydraulic conductivity varied between genotypes. Treated root systems displayed markedly different L ₀ values between genotypes whereas Lp _r values were similar.

CONCLUSIONS

Our analysis points to marked differences between genotypes in the intrinsic aquaporin-dependent path (Lp _r in control plants) but not in the intrinsic AQP-independent paths (Lp _r in HgCl₂ treated plants). Overall, root anatomy was a major determinant of water transport properties of the whole organ and can compensate for a low AQP contribution. Hydraulic properties of root tissues and organs might have to be taken into account for plant breeding since they appear to play a key role in sunflower water balance and water use efficiency.

Four distinct types of dehydration stress memory genes in Arabidopsis thaliana

BACKGROUND

How plants respond to dehydration stress has been extensively researched. However, how plants respond to multiple consecutive stresses is virtually unknown. Pre-exposure to various abiotic stresses (including dehydration) may alter plants' subsequent responses by improving resistance to future exposures. These observations have led to the concept of 'stress memory' implying that during subsequent exposures plants provide responses that are different from those during their first encounter with the stress. Genes that provide altered responses in a subsequent stress define the 'memory genes' category; genes responding similarly to each stress form the 'non-memory' category.

RESULTS

Using a genome-wide RNA-Seq approach we determine the transcriptional responses of Arabidopsis plants that have experienced multiple exposures to dehydration stress and compare them with the transcriptional behavior of plants encountering the stress for the first time. The major contribution of this study is the revealed existence of four distinct, previously unknown, transcription memory response patterns of dehydration stress genes in A.thaliana. The biological relevance for each of the four memory types is considered in the context of four overlapping strategies employed by a plant to improve its stress tolerance and/or survival: 1) increased synthesis of protective, damage-repairing, and detoxifying functions; 2) coordinating photosynthesis and growth under repetitive stress; 3) re-adjusting osmotic and ionic equilibrium to maintain homeostasis; and 4) re-adjusting interactions between dehydration and other stress/hormone regulated pathways.

CONCLUSIONS

The results reveal the unknown, hitherto, existence of four distinct transcription memory response types in a plant and provide genome-wide characterization of memory and non-memory dehydration stress response genes in A.thaliana. The transcriptional responses during repeated exposures to stress are different from known responses occurring during a single exposure. GO analyses of encoded proteins suggested implications for the cellular/organismal protective, adaptive, and survival functions encoded by the memory genes. The results add a new dimension to our understanding of plants' responses to dehydration stress and to current models for interactions between different signaling systems when adjusting to repeated spells of water deficits.