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

Bicarbonate-dominated alkaline soils represent a major class of salt-affected soils that can severely limit plant growth due to the combined effect of ionic toxicity, osmotic stress, and high pH. These soils exist in many parts of the world and often limit crop productivity due to the so-called alkaline stress in the rhizosphere. Alkaline stress reduces bioavailability of essential nutrients required for plant growth, such as iron, phosphates, and manganese, disrupts proton homeostasis, and perturbs cellular redox balance. Elucidating the mechanisms that enable plants to adapt to and grow in alkaline soils is essential if saline-alkali agriculture, where crops are grown on soils with high salt concentrations and high pH, is to contribute to global food security. Here, we summarize recent advances in the physiological, biochemical, and molecular processes that enable plants to tolerate and grow in alkaline soils. Key physiological mechanisms reviewed include plasma membrane H⁺-ATPase-driven proton extrusion, regulation of ion transporters, secretion of organic acids to acidify the rhizosphere, vacuolar sequestration of sodium and associated anions, and accumulation of compatible solutes for osmotic adjustment. At the molecular level, we also review and evaluate the genes, signaling pathways, and transcription factor networks underlying alkaline stress tolerance, with an emphasis on modules governing ion/redox homeostasis and root development. We also summarize emerging evidence on root-microbe interactions and their contributions to enhanced alkaline tolerance in plants. Finally, we discuss key requirements and future research directions for translating alkaline-tolerance mechanisms into crop improvement for alkaline soils.