Rana Munns

Under saline conditions, plants consistently maintain cytosolic Na ⁺ concentrations between 10 and 30 mM, sequestering excess Na ⁺ to the vacuole. We demonstrate that this cytosolic Na ⁺ homeostasis is regulated by inward Na ⁺ -permeable channels and outward Na ⁺ :H ⁺ antiporters at both the plasma membrane and tonoplast. Sodium’s interplay with K ⁺ transport adds complexity and selective transport is crucial to avoid conflicting ion fluxes. Our models predict that Na ⁺ :H ⁺ antiport regulation at the plasma membrane significantly impacts cytosolic Na ⁺ levels, while channel and antiport regulation are equally important at the tonoplast. The energetic implications of these transport mechanisms are discussed. In contrast to the cytosol, chloroplast Na ⁺ concentrations vary significantly between species and increase with soil salinity, raising questions as to how C ₄ and CAM plants acquire pyruvate under saline conditions. However, modelling transport activity at the chloroplast membrane requires far more knowledge of the associated transport systems and the chloroplastic Na ⁺ content.

Summary: Climate change induces many abiotic stresses, including soil salinity, significantly challenging global agriculture. Salinity stress tolerance (SST) is a complex trait, both physiologically and genetically, and is conferred at various levels of plant functional organization. As both the sustainability and profitability of agricultural production systems are critically dependent on SST, plant breeders are trying to design and develop salinity‐smart crop plants capable of thriving under high salinity conditions. The accessibility of extreme‐quality reference genomes for cultivated crops, naturally salinity‐smart plants, and crop wild relatives has fast‐tracked the discovery of key genes and quantitative trait loci (QTLs), marker development, genotyping assays and molecular breeding products with improved SST. Employing fast‐forward breeding tools, namely genomic selection (GS), haplotype‐based breeding (HBB), artificial intelligence (AI) and high‐throughput phenotyping (HTP), has shown influence not only for fast‐tracking genetic gains but also for reducing the time and cost of developing commercial cultivars with enhanced SST and yield stability. This review discusses the advancement and prospects of various genomics‐assisted breeding (GAB) tools, including genome sequencing, QTL mapping, GWAS, GS, HBB, pan‐genomics, single‐cell/tissue genomics and phenotyping, epigenomics and transgenomics, to exploit the genetic landscape for improving SST. Additionally, we explore the integration of HTP and AI, which demonstrates how these innovative approaches can optimize breeding efficiency and guide large‐scale breeding efforts for designing salinity‐smart crops to ensure sustainable agriculture and global food security. The collective adoption of these tools suggests bridging the gap between research and field application to deliver stress‐smart varieties designed for saline‐affected regions worldwide.

Analysis of salinity tolerance processes in wheat has focused on salt exclusion from shoots while root phenotypes have received limited attention. Here we consider the varying phenotypic response of four bread wheat varieties that differ in their type and degree of salt tolerance and consider in detail their molecular responses to salinity and changes in root cell wall lignification. These varieties were Westonia introgressed with Nax1 and Nax2 root sodium transporters ( HKT1;5-A ) that reduce Na ⁺ accumulation in leaves, as well as the ‘tissue tolerant’ Portugese landrace Mocho de Espiga Branca that has a mutation in the homologous gene HKT1;5-D and has high Na ⁺ concentration in leaves. These three varieties were compared with the more salt-sensitive cultivar Gladius. Through the use of root structural analysis, ion concentrations, as well as differential proteomics and targeted metabolomics we provide an integrated view of the wheat root response to salinity. We show different metabolic re-arrangements in energy conversion, primary metabolic machinery and phenylpropanoid pathway leading to monolignol production in a genotype and genotype by treatment dependent manner that alters the extent and localisation of root lignification which correlated with an improved capacity of wheat roots to cope better under salinity stress.

Abiotic stresses such as drought and heat continue to impact crop production in a warming world. This review distinguishes seven inherent capacities that enable plants to respond to abiotic stresses and continue growing, although at a reduced rate, to achieve a productive yield. These are the capacities to selectively take up essential resources, store them and supply them to different plant parts, generate the energy required for cellular functions, conduct repairs to maintain plant tissues, communicate between plant parts, manage existing structural assets in the face of changed circumstances, and shape-shift through development to be efficient in different environments. By illustration, we show how all seven plant capacities are important for reproductive success of major crop species during drought, salinity, temperature extremes, flooding, and nutrient stress. Confusion about the term ‘oxidative stress’ is explained. This allows us to focus on the strategies that enhance plant adaptation by identifying key responses that can be targets for plant breeding.

The existence of salinity stress can be traced well before the domestication of crops, but the documentation and mitigation of this menace started only 100 years ago. Due to the unavailability of appropriate equipment and lack of sophisticated techniques, the salinity appraisal of soils and crop injury at early times was done visually. Initially, the major focus of scientists was on reclamation and management of salt-affected soils to render them fit for agriculture. Later, they strived to assess the degree of salt tolerance of different plant species using growth and morphological traits as well as some fundamental physiological criteria, most importantly ion uptake, and accumulation. In the early 20th century, the idea of developing salt tolerant crops, as an alternative to soil reclamation was realized, and the terms “biological fix” or in general “biological approach” were coined. This triggered plant breeders to initiate breeding programs aimed at developing salt tolerant crop cultivars. Although conventional selection and breeding has several limitations, mainly its slowness, it has yielded many salt tolerant lines and cultivars of different crops. To speed up the crop breeding programs, a genetic engineering approach referred to as “transgenic approach” was introduced during the late 20th century. Plant biotechnologists have produced large numbers of transgenic lines of different crops however their use in developing salt tolerant cultivars is not remarkable. Furthermore, genetically modified (GM) crops are prohibited in many countries because of putative health risks and biosafety concerns. More recently, for precise editing of genomes of organisms, new molecular tools have been developed. For example, CRISPR-Cas9 is being used to precisely edit genes involved in abiotic stress tolerance, including salt tolerance. Its success in terms of developing cultivars tolerant to multiple stresses including salt stress is expected.

Summary: The impact of salinity on wheat plants is often studied by analysis of shoot responses, even though the main mechanism of tolerance is shoot Na ⁺ exclusion. There is a need to understand the molecular responses of root tissues that directly experience rising NaCl concentrations. We have combined analysis of root growth, ion content and respiration with proteome responses in wheat root tip and mature root tissues under saline conditions. We find significant changes in translation and protein synthesis, energy metabolism and amino acid metabolism in a root tissue specific manner. Translation and protein synthesis related proteins showed significant decreases in abundance only in root tips, as did most of the glycolytic enzymes and selected TCA cycle enzymes and ATP synthase subunits. This selective root tip proteome response indicates protein synthesis capacity and energy production were impaired under salt stress, correlating with the anatomical response of roots and reduced root tip respiration rate. Wheat roots respond directly to soil salinity, therefore shoot responses such as reduction in shoot growth and photosynthetic capacity need to be considered in light of these effects.

Soil salinity affects sugarcane (Saccharum officinale L.) production in arid and semiarid climates, severely reducing productivity. This study aimed to identify differentially regulated proteins in two cultivars that differ markedly in tolerance of saline soil. Plants were grown for 30 days and then subjected to treatments of 0 and 160 mM NaCl for 15 days. The tolerant cultivar showed a 3-fold upregulation of lipid metabolising enzymes, GDSL-motif lipases, which are associated with defence to abiotic stress, and which were not upregulated in the sensitive cultivar. Lipoxygenase was 2-fold upregulated in the tolerant cultivar but not in the sensitive cultivar, as were Type III chlorophyll a/b binding proteins. Other differences were that in the sensitive cultivar, the key enzyme of C4 photosynthesis, phosphoenolpyruvate carboxylase was downregulated, along with other chloroplast enzymes. Na+ concentrations had not reached toxic concentrations in either cultivar by this time of exposure to salt, so these changes would be in response to the osmotic effect of the soil salinity, and likely be in common with plants undergoing drought stress.

Summary: 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.