Empowering roots-Some current aspects of root bioenergetics

Roots of higher plants provide the shoot with nutrients and water. In exchange, they receive photosynthates, which serve both as energy source and building blocks for maintenance and growth. While studies in plant bioenergetics used to focus on photosynthesis, several more recent findings also aroused or renewed interest in energy conversion and allocation in roots. Root building costs were identified as a long-undervalued trait, which turned out to be highly relevant for stress tolerance and nutrient use efficiency. Reduced building costs per root length (e.g., by aerenchyma formation or by increasing the cell size) are beneficial for exploring the soil for nutrient-rich patches, especially in low-input agrosystems. Also, an apparent mismatch was frequently found between the root energy budget in the form of the ATP pool on the one side and the apparent costs on the other side, particularly the costs of membrane transport under stress conditions, e.g., the Na+ detoxification costs resulting from Na+ sequestration at the plasma membrane. Ion transport across the plasma membrane (and also endomembranes) is coupled to the proton motive force usually believed to be exclusively generated by H+ ATPases. Recently, an alternative mechanism, the biochemical pH clamp, was identified which relies on H+ formation and binding in the apoplast and the cytosol, respectively, driven by metabolism (so-called active buffering). On this background, several aspects of root bioenergetics are discussed. These are (1) root respiration in soil, with a critical view on calorimetric vs. gas exchange measurements; (2) processes of energy conversion in mitochondria with a special focus on the role of the alternative oxidases, which allow adjusting carbon flow through metabolic pathways to membrane transport processes; and (3) energy allocation, in particular to transport across the plasma membrane forming the interface to soil solution. A concluding remark is dedicated to modeling root bioenergetics for optimizing further breeding strategies. Apparent energy spoilers may bestow the plant with a yet unidentified advantage only unfolding their beneficial effect under certain environmental conditions.

Automated summary

The roots of higher plants play a crucial role in providing nutrients and water to the shoot, while receiving photosynthates in return. Recent findings have brought attention to the energy conversion and allocation in roots, highlighting the importance of root building costs for stress tolerance and nutrient use efficiency. Additionally, a mismatch between root energy budget and apparent costs, such as membrane transport under stress conditions, has been observed. The study also discusses various aspects of root bioenergetics, including root respiration, energy conversion in mitochondria, and energy allocation across the plasma membrane.