Impact of Drought on Soluble Sugars and Free Proline Content in Selected Arabidopsis Mutants

Simple Summary Drought has severe effects on plants, negatively impacting economic, agricultural and environmental processes. Depending on the duration and the strength of water stress conditions, plants adjust a series of physiological, cellular, and molecular mechanisms aimed at providing a correct stress response and, if possible, at establishing stress tolerance. The model plant Arabidopsis thaliana is one of the best tools for analyzing the involvement of specific genes in the response to drought. Thanks to this tool, the role of two genes encoding enzymes involved in sugars metabolism, and one gene encoding an enzyme involved in proline synthesis, have been investigated. In addition to suggesting an interaction between the metabolism of proline and that of soluble sugars, results broaden our understanding of the predominant role played by the accumulation of soluble sugars in counteracting mild osmotic stress. Water shortage is an increasing problem affecting crop yield. Accumulation of compatible osmolytes is a typical plant response to overcome water stress. Sucrose synthase 1 (SUS1), and glucan, water dikinase 2 (GWD2) and delta(1)-pyrroline-5-carboxylate synthetase 1 (P5CS1) are members of small protein families whose role in the response of Arabidopsis thaliana plants to mild osmotic stress has been studied in this work. Comparative analysis between wild-type and single loss-of-function T-DNA plants at increasing times following exposure to drought showed no differences in the content of water-insoluble carbohydrate (i.e., transitory starch and cell wall carbohydrates) and in the total amount of amino acids. On the contrary, water-soluble sugars and proline contents were significantly reduced compared to wild-type plants regardless of the metabolic pathway affected by the mutation. The present results contribute to assigning a physiological role to GWD2, the least studied member of the GWD family; strengthening the involvement of SUS1 in the response to osmotic stress; showing a greater contribution of soluble sugars than proline in osmotic adjustment of Arabidopsis in response to drought. Finally, an interaction between proline and soluble sugars emerged, albeit its nature remains speculative and further investigations will be required for complete comprehension.