Improved heat tolerance in creeping bentgrass by γ-aminobutyric acid, proline, and inorganic nitrogen associated with differential regulation of amino acid metabolism

Plant defense against heat stress involves adjustments in amino acid metabolism. The objective of this study was to identify major amino acids and associated metabolic pathways differentially regulated by γ-aminobutyric acid (GABA) and proline that may contribute to augmentation of heat tolerance in cool-season grass species. Creeping bentgrass (Agrostis stolonifera L. cv. ‘Penncross’) was exposed to non-stress (22/18 °C, day/night) or heat stress (35/30 °C, day/night) conditions for 35 d in controlled-environment growth chambers. Non-stressed and heat-stressed plants were foliar-sprayed with water (untreated control), GABA, proline, or ammonium nitrate (N) as a nitrogen source control. Under heat stress, foliar application of GABA, proline, or N significantly increased turf quality and leaf chlorophyll content compared to untreated control plants. N application had nutritional effects, resulting in increases in the content of all amino acids under heat stress. Application of GABA under heat stress significantly increased endogenous content of glutamic acid, GABA, and threonine. Plants treated with proline under heat stress had significantly higher endogenous levels of proline, GABA, glutamic acid, aspartic acid, lysine, isoleucine, leucine, valine, serine, alanine, threonine, and tryptophan compared to untreated controls. The improved heat tolerance in creeping bentgrass pathway by GABA was mainly associated with regulation of amino acid metabolism in the GABA shunt and oxaloacetate pathways. Proline-enhanced heat tolerance involved the regulation of five metabolic pathways (GABA shunt, oxaloacetate, 3-phosphoglycerate, secondary metabolism, and pyruvate). GABA and proline, as well as their responsive amino acids could be used as biomarkers to improve heat tolerance in cool-season grass species.