Crop root system plasticity for improved yields in saline soils.

Crop yields must increase to meet the demands of a growing world population. Soil salinization is increasing due to the impacts of climate change, reducing the area of arable land for crop production. Plant root systems are plastic, and their architecture can be modulated to (1) acquire nutrients and water for growth, and (2) respond to hostile soil environments. Saline soils inhibit primary root growth and alter root system architecture (RSA) of crop plants. In this review, we explore how crop root systems respond and adapt to salinity, focusing predominately on the staple cereal crops wheat, maize, rice, and barley, that all play a major role in global food security. Cereal crops are classified as glycophytes (salt-sensitive) however salt-tolerance can differ both between species and within a species. In the past, due to the inherent difficulties associated with visualising and measuring root traits, crop breeding strategies have tended to focus on optimising shoot traits. High-resolution phenotyping techniques now make it possible to visualise and measure root traits in soil systems. A steep, deep and cheap root ideotype has been proposed for water and nitrogen capture. Changes in RSA can be an adaptive strategy to avoid saline soils whilst optimising nutrient and water acquisition. In this review we propose a new model for designing crops with a salt-tolerant root ideotype. The proposed root ideotype would exhibit root plasticity to adapt to saline soils, root anatomical changes to conserve energy and restrict sodium (Na+) uptake, and transport mechanisms to reduce the amount of Na+ transported to leaves. In the future, combining high-resolution root phenotyping with advances in crop genetics will allow us to uncover root traits in complex crop species such as wheat, that can be incorporated into crop breeding programs for yield stability in saline soils.Megan C. Shelden, and Rana Munn

Integrative Multi-omics Analyses of Barley Rootzones under Salinity Stress Reveal Two Distinctive Salt Tolerance Mechanisms

The mechanisms underlying rootzone-localized responses to salinity during early stages of barley devel-opment remain elusive. In this study, we performed the analyses of multi-root-omes (transcriptomes, me-tabolomes, and lipidomes) of a domesticated barley cultivar (Clipper) and a landrace (Sahara) that main-tain and restrict seedling root growth under salt stress, respectively. Novel generalized linear modelswere designed to determine differentially expressed genes (DEGs) and abundant metabolites (DAMs)specific to salt treatments, genotypes, or rootzones (meristematic Z1, elongation Z2, and maturationZ3). Based on pathway over-representation of the DEGs and DAMs, phenylpropanoid biosynthesis isthe most statistically enriched biological pathway among all salinity responses observed. Togetherwith histological evidence, an intense salt-induced lignin impregnation was found only at stelic cellwall of Clipper Z2, compared with a unique elevation of suberin deposition across Sahara Z2. This sug-gests two differential salt-induced modulations of apoplastic flow between the genotypes. Based on theglobal correlation network of the DEGs and DAMs, callose deposition that potentially adjusted symplasticflow in roots was almost independent of salinity in rootzones of Clipper, and was markedly decreased inSahara. Taken together, we propose two distinctive salt tolerance mechanisms in Clipper (growth-sus-taining) and Sahara (salt-shielding), providing important clues for improving crop plasticity to copewith deteriorating global soil salinization.William Wing Ho Ho, Camilla B. Hill, Monika S. Doblin, Megan C. Shelden, Allison van de Meene, Thusitha Rupasinghe, Antony Bacic, and Ute Roessne

A laser ablation technique maps differences in elemental composition in roots of two barley cultivars subjected to salinity stress

In saline soils, high levels of sodium (Na+ ) and chloride (Cl- ) ions, reduce root growth by inhibiting cell division and elongation, thereby impacting on crop yield. Soil salinity can lead to Na+ toxicity of plant cells, influencing the uptake and retention of other important ions (ie. potassium (K+ )) required for growth. However, measuring and quantifying soluble ions in their native, cellular environment is inherently difficult. Technologies that allow in situ profiling of plant tissues are fundamental for our understanding of abiotic stress responses and the development of tolerant crops. Here, we employ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to quantify Na, K and other elements (calcium (Ca), magnesium (Mg), sulphur (S), phosphorus (P), iron (Fe)) at high spatial resolution in the root growth zone of two genotypes of barley (Hordeum vulgare) that differ in salt-tolerance, cv. Clipper (tolerant) and Sahara (sensitive). The data show that Na+ was excluded from the meristem and cell division zone, indicating that Na+ toxicity is not directly reducing cell division in the salt-sensitive genotype, Sahara. Interestingly, in both genotypes, K+ was strongly correlated with Na+ concentration, in response to salt stress. In addition, we also show important genetic differences and salt-specific changes in elemental composition in the root growth zone. These results show that LA-ICP-MS can be used for fine mapping of soluble ions (ie. Na+ and K+ ) in plant tissues, providing insight into the link between Na+ toxicity and root growth responses to salt stress.Megan C. Shelden, Sarah E. Gilbert and Stephen D. Tyerma

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.Borjana Arsova, Kylie J. Foster, Megan C. Shelden, Helen Bramley and Michelle Wat

Integrative multi-omics analyses of barley rootzones under salinity stress reveal two distinctive salt tolerance mechanisms

Mechanisms underlying rootzone-localised responses to salinity during early stage of barley development remains elusive. Here, we detected the multi-root-omes (transcriptomes, metabolomes, lipidomes) of a domesticated barley cultivar (Clipper) and a landrace (Sahara) which maintain and restrict seedling root growth under salt stress, respectively. Novel generalized linear models were designed to determine differentially expressed genes (DEG) and abundant metabolites (DAM) specific to salt treatments, genotypes, or rootzones (meristematic Z1, elongation Z2, maturation Z3). Based on pathway over-representation of the DEG and DAM, phenylpropanoid biosynthesis is the most statistically enriched biological pathways among all salinity responses observed. Together with histological evidence, an intense salt-induced lignin impregnation was found only at stelic cell wall of Clipper Z2, comparing to a unique elevation of suberin deposition across Sahara Z2. This suggests two differential salt-induced modulations of apoplastic flow between the genotypes. Based on global correlation network of the DEG and DAM, callose deposition that potentially adjusted symplastic flow in roots was almost independent of salinity in rootzones of Clipper, and was markedly decreased in Sahara. Taken together, we propose two distinctive salt tolerance mechanisms in Clipper (growth-sustaining) and Sahara (salt-shielding), providing important clues for improving crop plasticity to cope with deteriorating global soil salinization