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Soil salinity reduces crop yield. The extent and severity of salt-affected agricultural land is predicted to worsen as a result of inadequate drainage of irrigated land, rising water tables and global warming. The growth and yield of most plant species are adversely affected by soil salinity, but varied adaptations can allow some crop cultivars to continue to grow and produce a harvestable yield under moderate soil salinity. Significant costs are associated with saline soils: the economic costs to the farming community and the energy costs of plant adaptations. We briefly consider mechanisms of adaptation and highlight recent research examples through a lens of their applicability to improving the energy efficiency of crops under saline field conditions.

Salinity tolerance comes from genes that limit the rate of salt uptake from the soil and the transport of salt throughout the plant, adjust the ionic and osmotic balance of cells in roots and shoots, and regulate leaf development and the onset of senescence. This review lists some candidate genes for salinity tolerance, and draws together hypotheses about the functions of these genes and the specific tissues in which they might operate. Little has been revealed by gene expression studies so far, perhaps because the studies are not tissue-specific, and because the treatments are often traumatic and unnatural. Suggestions are made to increase the value of molecular studies in identifying genes that are important for salinity tolerance.

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.

Plant roots must exclude almost all of the Na⁺ and Cl⁻ in saline soil while taking up water, otherwise these ions would build up to high concentrations in leaves. Plants evaporate c. 50 times more water than they retain, so 98% exclusion would result in shoot NaCl concentrations equal to that of the external medium. Taking up just 2% of the NaCl allows a plant to osmotically adjust the Na⁺ and Cl⁻ in vacuoles, while organic solutes provide the balancing osmotic pressure in the cytoplasm. We quantify the costs of this exclusion by roots, the regulation of Na⁺ and Cl⁻ transport through the plant, and the costs of osmotic adjustment with organic solutes in roots.

Halophytes are the flora of saline soils. They adjust osmotically to soil salinity by accumulating ions and sequestering the vast majority of these (generally Na(+) and Cl(-)) in vacuoles, while in the cytoplasm organic solutes are accumulated to prevent adverse effects on metabolism. At high salinities, however, growth is inhibited. Possible causes are: toxicity to metabolism of Na(+) and/or Cl(-) in the cytoplasm; insufficient osmotic adjustment resulting in reduced net photosynthesis because of stomatal closure; reduced turgor for expansion growth; adverse cellular water relations if ions build up in the apoplast (cell walls) of leaves; diversion of energy needed to maintain solute homeostasis; sub-optimal levels of K(+) (or other mineral nutrients) required for maintaining enzyme activities; possible damage from reactive oxygen species; or changes in hormonal concentrations. This review discusses the evidence for Na(+) and Cl(-) toxicity and the concept of tissue tolerance in relation to halophytes. The data reviewed here suggest that halophytes tolerate cytoplasmic Na(+) and Cl(-) concentrations of 100-200 mm, but whether these ions ever reach toxic concentrations that inhibit metabolism in the cytoplasm or cause death is unknown. Measurements of ion concentrations in the cytosol of various cell types for contrasting species and growth conditions are needed. Future work should also focus on the properties of the tonoplast that enable ion accumulation and prevent ion leakage, such as the special properties of ion transporters and of the lipids that determine membrane permeability.

Salt stress impacts multiple aspects of plant metabolism and physiology. For instance it inhibits photosynthesis through stomatal limitation, causes excessive accumulation of sodium and chloride in chloroplasts, and disturbs chloroplast potassium homeostasis. Most research on salt stress has focused primarily on cytosolic ion homeostasis with few studies of how salt stress affects chloroplast ion homeostasis. This review asks the question whether membrane-transport processes and ionic relations are differentially regulated between glycophyte and halophyte chloroplasts and whether this contributes to the superior salt tolerance of halophytes. The available literature indicates that halophytes can overcome stomatal limitation by switching to CO₂ concentrating mechanisms and increasing the number of chloroplasts per cell under saline conditions. Furthermore, salt entry into the chloroplast stroma may be critical for grana formation and photosystem II activity in halophytes but not in glycophytes. Salt also inhibits some stromal enzymes (e.g. fructose-1,6-bisphosphatase) to a lesser extent in halophyte species. Halophytes accumulate more chloride in chloroplasts than glycophytes and appear to use sodium in functional roles. We propose the molecular identities of candidate transporters that move sodium, chloride and potassium across chloroplast membranes and discuss how their operation may regulate photochemistry and photosystem I and II activity in chloroplasts.

Agriculture is expanding into regions that are affected by salinity. This review considers the energetic costs of salinity tolerance in crop plants and provides a framework for a quantitative assessment of costs. Different sources of energy, and modifications of root system architecture that would maximize water vs ion up take are addressed. Energy requirements for transport of salt (NaCl) to leaf vacuoles for osmotic adjustment could be small if there are no substantial leaks back across plasma membrane and tonoplast in root and leaf. The coupling ratio of the H⁺ -ATPase also is a critical component. One proposed leak, that of Na⁺ influx across the plasma membrane through certain aquaporin channels, might be coupled to water flow, thus conserving energy. For the tonoplast, control of two types of cation channels is required for energy efficiency. Transporters controlling the Na⁺ and Cl⁻ concentrations in mitochondria and chloroplasts are largely unknown and could be a major energy cost. The complexity of the system will require a sophisticated modelling approach to identify critical transporters, apoplastic barriers and root structures. This modelling approach will inform experimentation and allow a quantitative assess ment of the energy costs of Na Cl tolerance to guide breeding and engineering of molecular components.

This Perspective discusses the emerging advances in plant membrane transporters, which can be used to improve crop yields, nutritional value, and environmental stress resistance. Enhanced plant membrane transporters Transport proteins embedded in the cell membranes are key targets for improving the efficiency with which plants take up and use water and nutrients. In this Perspective article, Julian Schroeder et al . discuss recent work on the development of specialized plant membrane transporters that can enhance crop yields, increase nutritional value and boost resistance to stresses including pathogens. Promising lines of development include aluminium-tolerant cereals that thrive in acid soil, salt-tolerant varieties that grow in soils affected by salinity or sodium toxicity, and plants containing high levels of the iron and zinc micronutrients often scarce in predominantly plant-based diets in developing countries. With the global population predicted to grow by at least 25 per cent by 2050, the need for sustainable production of nutritious foods is critical for human and environmental health. Recent advances show that specialized plant membrane transporters can be used to enhance yields of staple crops, increase nutrient content and increase resistance to key stresses, including salinity, pathogens and aluminium toxicity, which in turn could expand available arable land.

This review describes physiological mechanisms and selectable indicators of gene action, with the aim of promoting new screening methods to identify genetic variation for increasing the salt tolerance of cereal crops. Physiological mechanisms that underlie traits for salt tolerance could be used to identify new genetic sources of salt tolerance. Important mechanisms of tolerance involve Na+ exclusion from the transpiration stream, sequestration of Na+ and Cl− in the vacuoles of root and leaf cells, and other processes that promote fast growth despite the osmotic stress of the salt outside the roots. Screening methods for these traits are discussed in relation to their use in breeding, particularly with respect to wheat. Precise phenotyping is the key to finding and introducing new genes for salt tolerance into crop plants.

Durum wheat (Triticum turgidum L. subsp. durum Desf.) Line 149 contains two novel major genes for excluding Na⁺ from leaf blades, named Nax1 and Nax2. The genes were separated into families containing a single gene and near-isogenic homozygous lines were selected. Lines containing either Nax1 or Nax2 had lower rates of Na⁺ transport from roots to shoots than their near-isogenic pairs due to lower rates of net loading of the xylem, not to lower rates of net uptake from the soil or higher rates of retranslocation in the phloem. Nax1 and Nax2 lines also had higher rates of K⁺ transport from root to shoot, resulting in an enhanced discrimination of K⁺ over Na⁺. Lines containing Nax1 differed from those containing Nax2 by unloading Na⁺ from the xylem as it entered the shoot so that Na⁺ was retained in the base of the leaf, leading to a high sheath to blade ratio of Na⁺ concentration. Gradients in tissue concentrations of Na⁺ along the leaf suggested that Na⁺ was continually removed from the xylem. The Nax2 line did not retain Na⁺ in the base of the leaf, suggesting that it functioned only in the root. The Nax2 gene therefore has a similar function to Kna1 in bread wheat (Triticum aestivum).