Alkaline stress is far more than salt stress with a higher pH, the review’s authors from institutions including the Chinese Academy of Sciences, Northeast Forestry University, Texas Tech University, and the University of Otago emphasize. Neutral salt stress, typically caused by sodium chloride or sodium sulfate, damages plants mainly through ion toxicity and osmotic disruption without greatly altering soil pH. Alkaline stress adds an entirely different layer of injury: bicarbonate and carbonate ions actively strip protons from the root environment, collapse the proton gradients plants depend on to feed themselves, and precipitate essential nutrients such as calcium, magnesium, iron, and manganese into insoluble forms. The result is a quadruple threat combining osmotic stress, sodium toxicity, high-pH injury, and severe nutrient starvation. Bicarbonate also interferes with ion transport, weakens membrane stability, and disturbs intracellular pH regulation, making alkaline stress substantially more complex and damaging than neutral salt stress alone.

The frontline of plant defense, the review finds, is a family of molecular machinery called proton pumps. Plasma membrane H⁺-ATPases use ATP hydrolysis to pump hydrogen ions out of root cells, building an electrochemical gradient that powers nutrient uptake and drives sodium expulsion through the Salt Overly Sensitive 1 transporter. This continuous proton extrusion also acidifies the rhizosphere, partially counteracting the alkaline soil itself. Studies of the alkaline-tolerant grass Puccinellia tenuiflora, a halophyte that thrives on sodium carbonate soils, show that sodium carbonate treatment increases both the expression and the activity of plasma membrane and vacuolar proton pumps alongside vacuolar Na⁺/H⁺ exchangers, which use the proton gradient to sequester sodium inside vacuoles while stabilizing cytosolic pH. Genetic engineering of these proton pump components has already improved alkaline performance in crops: expressing a vacuolar H⁺-ATPase subunit from the salt marsh grass Spartina alterniflora in rice enhanced salt tolerance, and a vacuolar H⁺-pyrophosphatase from Zoysia matrella improved salt tolerance when introduced into Arabidopsis.

Equally remarkable is the strategy of organic acid secretion. In Puccinellia tenuiflora, alkaline treatment triggers a surge of citrate and malate released into the rhizosphere, shifting soil pH from strongly alkaline toward near-neutral. Metabolomic and proteomic analyses reveal that this is no passive byproduct accumulation: the plant actively reroutes carbon metabolism, upregulating the tricarboxylic acid cycle, glyoxylate metabolism, and glycolysis, and increasing the abundance of dehydrogenases and synthases devoted to organic acid production. Once secreted, citrate and malate partially dissociate, releasing protons that neutralize alkalinity and form carbonic acid that decomposes into carbon dioxide and water. Their carboxyl groups also chelate iron and manganese, unlocking these micronutrients from the insoluble grip of high-pH soils, while released organic anions help maintain membrane charge balance. Comparable responses have been documented in barley, alfalfa, Leymus chinensis, Yorkshire fog, and red clover, with alkaline salt treatments consistently inducing stronger organic acid accumulation than neutral salt treatments, and increased organic acid content correlating with root growth recovery and reduced sodium-to-potassium ratios.

Inside the cell, tolerant plants wage a coordinated battle over ion balance. Halophytes exploit sodium salts themselves as osmoticum, storing them harmlessly in vacuoles while keeping cytosolic sodium low and preserving potassium. This dual strategy of vacuolar sequestration and potassium retention protects metabolism while maintaining turgor pressure. Yet alkaline stress threatens potassium homeostasis directly: membrane depolarization and reactive oxygen species can activate outward-rectifying potassium channels, causing damaging potassium leakage. The review highlights that halophytes counter this by rapidly enhancing plasma membrane H⁺-ATPase activity, which repolarizes the membrane, suppresses potassium efflux, and sustains the driving force for high-affinity potassium uptake. Under both salt and alkaline stresses, halophytes maintain far higher cytosolic potassium-to-sodium ratios than sensitive glycophytes, evidence of tighter coordination among proton pumps, potassium transport, and sodium-hydrogen exchange.

The antioxidant dimension of alkaline tolerance is equally sophisticated. Osmotic stress and ion toxicity drive excess production of reactive oxygen species, which, if unchecked, oxidize lipids, proteins, and DNA. High apoplastic pH and iron deficiency compound the problem by further promoting ROS formation. Halophytes such as Atriplex, Suaeda, and the extremophile model Eutrema salsugineum maintain higher baseline activities of superoxide dismutase, catalase, and peroxidases, and induce these enzymes more rapidly than glycophytes, showing correspondingly less lipid peroxidation under alkaline conditions. Beyond the enzymatic cascade, which converts superoxide to hydrogen peroxide and then to water, non-enzymatic antioxidants including ascorbate and glutathione form the ascorbate-glutathione cycle, a major route for hydrogen peroxide detoxification in chloroplasts and the cytosol. Halophytes retain higher glutathione contents and glutathione-to-oxidized-glutathione ratios than glycophytes, while accumulating phenolics and flavonoids that scavenge ROS through hydrogen atom donation and metal chelation.

The review also brings the rhizosphere microbiome into the tolerance equation. Plant growth-promoting rhizobacteria isolated from saline-alkali soils secrete organic acids and extracellular polysaccharides that lower rhizosphere pH, dissolve calcium carbonate, and improve iron and phosphate availability. These bacteria also produce phytohormones and ACC deaminase, an enzyme that reduces stress-induced ethylene accumulation and reshapes root architecture. High-throughput sequencing of halophyte rhizospheres in Salicornia europaea, Suaeda salsa, and Atriplex species shows microbial communities distinct from adjacent bulk soil, enriched in functions for polysaccharide synthesis, osmotic adjustment, and antioxidant activity. Arbuscular mycorrhizal fungi add another layer, extending the effective absorptive surface of roots and alleviating growth inhibition under alkaline stress in Leymus chinensis seedlings and poplar, improving osmotic regulation and ion balance.

At the molecular level, the review catalogues a growing arsenal of regulatory genes. Early alkalinity perception likely involves extracellular pH sensing through peptide-receptor complexes such as RGF1-RGFR and Pep1-PEPR, although no dedicated bicarbonate receptor has yet been identified, and whether these receptors act under actual alkaline stress remains an open question. Downstream, calcium signals activate the SOS pathway, while regulators such as PKS5, the calcium sensor SCaBP3, the wheat proteins TaCCD1 and TaSAUR215, and the tomato 14-3-3 protein TFT4 fine-tune proton pump activity under high pH. Striking examples of breeding-relevant natural variation have emerged: the AT1/GS3 locus, encoding an atypical G-protein gamma subunit, modulates hydrogen peroxide distribution across root membranes and its modification improves alkaline tolerance in sorghum, rice, maize, and wheat. In maize, natural variation in the EF-hand calcium-binding protein ZmNSA1 reduces shoot sodium accumulation, while tomato lost saline-alkaline tolerance during domestication through weakened expression of SlSCaBP8, a defect an introgression line carrying the wild promoter haplotype reverses. Transcription factor networks involving NAC, HD-Zip, AP2/ERF, bZIP, TIFY/JAZ, MYB, and bHLH families coordinate hormone signaling, redox balance, iron acquisition, and root development, with genes such as GsERF71 enhancing proton pumping and grapevine VvERF1B stimulating organic acid exudation.

The authors argue that translating these mechanisms into crops will require moving beyond descriptive studies toward prioritized, testable hypotheses connecting sensing, rhizosphere regulation, molecular control, and field performance. They propose cell-type-resolved imaging of pH, calcium, and ROS signals, comparative omics under matched salt and alkaline treatments, synthetic microbial communities, and genome editing approaches including promoter editing and allele stacking. Durable tolerance, they conclude, will come from combining early pH sensing, proton extrusion, sodium compartmentation, nutrient acquisition, ROS buffering, and root remodeling, validated under realistic field conditions where salts, high pH, nutrient limitation, and drought collide. With agriculture worldwide facing expanding saline-alkali soils and rising food demand, the plants that already survive these extremes may hold the genetic blueprints for feeding the future.

Subject of Research: Physiological and molecular mechanisms of plant tolerance to bicarbonate-induced alkaline soil stress

Article Title: Physiological and molecular processes of plant tolerance to bicarbonate-induced alkaline stress

Article References: Fu, J., Li, L., Xing, M., Xie, W., Zhu, C., Yang, K., Nie, X., Yin, X., Mostofa, M. G., Burritt, D., Tran, L.-S. P., Bu, Y., & Li, W. (2026). Physiological and molecular processes of plant tolerance to bicarbonate-induced alkaline stress. Stress Biology, 6(1), Article 53. https://doi.org/10.1007/s44154-026-00325-1

Image Credits: AI Generated

DOI: 10.1007/s44154-026-00325-1

Keywords: alkaline stress, bicarbonate, halophytes, proton pumps, organic acid secretion, ion homeostasis, reactive oxygen species, rhizosphere microbiome, transcription factors, saline-alkali agriculture, crop breeding, soil pH