Energy costs of mechanisms for salinity tolerance in crop plants: a cost-benefit analysis
Rationale and scope
Breeding more salt tolerant crop and pasture species is critical for maintaining the increase in food production needed globally as agriculture moves into dryer and more saline environments. Traits for salt tolerance include a low sodium accumulation in leaves (called ‘leaf sodium exclusion’) or alternatively high sodium accumulation with intrinsic tolerance (called ‘tissue tolerance’). The more salt-tolerant species have high Na+ and Cl– concentrations in their leaves, equal to or higher than in the soil solution or rooting medium. However, within crop species, genetic variation in salt tolerance is often associated with variation in Na+ concentrations in the leaf blades, with the more tolerant genotypes having the lowest Na+. This presents a paradox in terms of energy demand because osmotic adjustment using organic solutes such as sucrose is energy-expensive. The cost of using organic solutes alone for osmotic adjustment is prohibitive once the soil salinity exceeds 150 mM NaCl (Munns and Gilliham, New Phytologist Tansley Insight, 2015). However osmotic adjustment using Na+ must have a significant energy cost as leaf Na+ exclusion correlates with salt tolerance.
Another paradox is linked to the coupling of salt flow with water flow in transpiration. This occurs through convection of salt with transpiration-driven flow in the soil, the xylem and the leaf, but may also occur at the level of membrane transport proteins. This coupling has implications for energy costs for osmoregulation and salt exclusion. However, there is no strong link between rate of transpiration and the accumulation of salt in leaves. The issue is further complicated by the discovery that ion flow (Na+) can occur through aquaporins and that changes in aquaporin expression can have a profound effect on the accumulation of Na+ in plants. This has not been explained simply based on water flow and requires re-examination of our paradigm for how salt and water movement occurs in plants and the implications for energy use.
Signalling is also important to consider in terms of how energy efficiency and salt transport are regulated. Recent discoveries of the way metabolic status is signalled to membrane transport will be incorporated into the analysis.
This meeting of minds is timely as it picks up on papers in the 1980s that were highly cited but the science did not develop as it lacked the modern framework of knowledge provided by molecular biology and electrophysiology. We know a lot more about ion transporters, and whether they are coupled to proton flow and membrane potential. Also we know now that water flows across membranes via aquaporins and that these may carry ions or molecules important in energy currency (e.g. CO2, sugars, ammonia) and that they are rapidly regulated in response to salt stress.
Many of the research questions that must be addressed to solve these paradoxes have not previously been integrated. We will bring together expertise that together will explore a new way of examining salinity tolerance in terms of energy used and energy produced in the process of salt and water movement in crop plants.
The New Phytologist Workshop is an ideal format to address these problems, which require expertise in different levels of plant structure and complexity; in transport processes across organelles, cells, organs and the whole plant. Emerging science is raising new concepts such as the co-transport of water and ions, and the generation of energy from transpired water. Some of these are controversial, and can best be debated face-to-face so to quickly reach a resolution or consensus.