Organic acids: Versatile stress response roles in plants.

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Organic acids (OAs) are central to cellular metabolism. Many plant stress responses involve exudation of OAs at the root-soil interface that can improve soil mineral acquisition and toxic metal tolerance. Because of their simple structure, the Low Molecular Weight Organic Acids (LMWOAs) are widely studied. We discuss the conventional roles of OAs, along with some newly emerging roles in plant stress tolerance. OAs are more versatile in their role in plant stress tolerance and are efficient chelating agents when compared with other acids, such as amino acids. Root OA exudation is important in soil carbon sequestration. These functions are key processes combating climate change and helping with more sustainable food production. We briefly review the mechanisms behind enhanced biosynthesis, secretion and regulation of these activities under different stresses. Also, an outline of the transgenic approaches targeted towards the enhanced production and secretion of OAs is provided. A re-occurring theme of OAs in plant biology is their roles as either ‘acids’ modifying pH or ‘chelators’ binding metals or as ‘carbon sources’ for microbes. We argue that these multiple functions are key factors for understanding these molecules important roles in plant stress biology. Finally, we contemplate how the functions of OAs in plant stress responses can be made use of and what the important unanswered questions are.