Transport and Use of Bicarbonate in Plants: Current Knowledge and Challenges Ahead

Bicarbonate plays a fundamental role in the cell pH status in all organisms. In autotrophs, HCO₃⁻ may further contribute to carbon concentration mechanisms (CCM). This is especially relevant in the CO₂-poor habitats of cyanobacteria, aquatic microalgae, and macrophytes. Photosynthesis of terrestrial plants can also benefit from CCM as evidenced by the evolution of C₄ and Crassulacean Acid Metabolism (CAM). The presence of HCO₃⁻ in all organisms leads to more questions regarding the mechanisms of uptake and membrane transport in these different biological systems. This review aims to provide an overview of the transport and metabolic processes related to HCO₃⁻ in microalgae, macroalgae, seagrasses, and terrestrial plants. HCO₃⁻ transport in cyanobacteria and human cells is much better documented and is included for comparison. We further comment on the metabolic roles of HCO₃⁻ in plants by focusing on the diversity and functions of carbonic anhydrases and PEP carboxylases as well as on the signaling role of CO₂/HCO₃⁻ in stomatal guard cells. Plant responses to excess soil HCO₃⁻ is briefly addressed. In conclusion, there are still considerable gaps in our knowledge of HCO₃⁻ uptake and transport in plants that hamper the development of breeding strategies for both more efficient CCM and better HCO₃⁻ tolerance in crop plants.