Proteins Involved in Energy Metabolism and Oxidative Regulation Associated with Genotypic Variations in Drought Tolerance for Tall Fescue

Protein metabolism plays an important role in plant adaptation to drought stress. The objective of this study was to identify drought-responsive proteins associated with differential drought tolerance for a tolerant genotype (RU9) and a sensitive genotype (RU18) of tall fescue (Lolium arundinacea). Plants of both genotypes were grown under well-watered conditions or subjected to drought stress by withholding irrigation for 12 days in a growth chamber controlled at the optimal growth temperatures of 23/18 8C (day/night). Physiological analysis demonstrated that RU9 was relatively more drought tolerant than RU18, as shown by the higher leaf net photosynthetic rate (Pn) and photochemical efficiency at 12 days of drought treatment. Differentially expressed proteins between RU9 and RU18 exposed to drought stress were identified by two-dimensional electrophoresis and mass spectrometry (MS). Several proteins [photosystem I reaction center subunit II, Rubisco small subunit, and Glyceraldehyde-3-phosphate dehydrogenase (GADPH)] in photosynthesis, respiration, or oxidative regulation exhibited higher abundance in RU9 than RU18 under drought stress. These results suggested the critical importance of energy and oxidative metabolism in tall fescue adaptation to drought stress. Those abundant proteins in the drought-tolerant genotype could be used as biomarkers or developed to molecular markers to develop elite drought-tolerant germplasm in tall fescue and other cool-season perennial grass species. Water availability for irrigation of turfgrass species used on landscapes, parks, and sport fields has been declining due to limited rainfall or water use restriction in many areas, which is a major factor causing the decline in turf aesthetic quality and physiological functions (Fry and Huang, 2004). Use of genetic variability in drought tolerance in breeding selection or through genetic modification has made significant progresses in developing drought-tolerant cultivars of various turfgrass species, including tall fescue. Tall fescue is a widely grown cool-season species throughout the temperate regions of the world (Mefti et al., 2016). A previous study found that tall fescue exhibits a wide range of genetic variability in drought tolerance (Ebrahimiyan et al., 2013; Pirnajmedin et al., 2015; Salehi et al., 2014). However, the metabolic mechanisms underlying the genetic variations in drought tolerance of cool-season turfgrasses, including tall fescue are still not completely understood. Protein metabolism plays essential roles in regulating plant tolerance to various stresses, including drought stress. Effects of drought stress on proteins involve complicated processes, including acceleration of the denaturation, proteolysis, and unfolding of some proteins and induction of some stressprotective proteins, such as dehydrins and chaperones (Obidiegwu et al., 2015; Pinheiro and Chaves, 2011). Proteomic profiling using two-dimensional electrophoresis is a powerful and widely used approach for the identification of proteins and metabolic pathway–regulating plant stress tolerance. Many drought-responsive genes have been identified through proteomic profiling, as reported in limited number of perennial grass species and mostly in annual crops, such as rice [Oryza sativa (Ali and Komatsu, 2006)], soybean [Glycine max (Khan and Komatsu, 2016)], wheat [Triticum aestivum (Faghani et al., 2015)], and barley [Hordeum vulgare (Wang et al., 2015)], which were found to be involved in different metabolic processes, such as antioxidant metabolism, chaperones, cell wall biosynthesis, energy metabolism, and hormone metabolism. A Received for publication 12 Mar. 2018. Accepted for publication 15 May 2018. The authors wish to thank the New Jersey Agricultural Experiment Station and Center for Turfgrass Science at Rutgers University for supporting this research project. Thanks to Stephanie Rossi and Cathryn Chapman for critical review of the manuscript. Corresponding authors. E-mail: huang@aesop.rutgers. edu or lixl@iascaas.