Differential protein expression for geothermal Agrostis scabra and turf-type Agrostis stolonifera differing in heat tolerance

Heat stress is a major factor limiting the growth of cool-season grasses in warm climatic regions by affecting many physiological processes, including protein metabolism. Protein degradation often occurs with increasing temperatures, but certain specific proteins such as heat shock proteins (HSPs) may be induced or enhanced in their expression under supraoptimal temperatures. The objectives of this study were to determine the critical temperature that causes protein induction or degradation in two Agrostis grass species differing in heat tolerance and to compare protein profiles between the two species under different temperature regimes. Plants of heat-tolerant Agrostis scabra and two cultivars of heat-sensitive Agrostis stolonifera ('L-93' and 'Penncross') were exposed to constant day/night temperatures of 20, 30, 35, 40, or 45°C for 14d. Leaf photochemical efficiency (Fv/Fm), chlorophyll and carotenoid contents, and soluble protein content declined with increasing temperatures. The decreases were the least severe for A. scabra, intermediate for 'L-93', and the most severe for 'Penncross', indicating interspecific and intraspecific variations in heat tolerance in Agrostis species. Protein degradation was observed at 30-45°C in both cultivars of A. stolonifera, and at 40-45°C in A. scabra. HSPs were induced or enhanced at 35-45°C in 'L-93' and A. scabra, and at 40-45°C in 'Penncross'. Immunoblotting also revealed stronger expressions of HSP60 and HSP70 in A. scabra or 'L-93' than in 'Penncross' at 35-45°C after 3d. The results suggested the superior heat tolerance of Agrostis grass species and cultivars could be attributed to the early induction of HSPs, particularly small molecular weight (23kDa), at a lower level of heat stress and the maintenance of protein thermostability, particularly high-molecular weight proteins (83kDa and large units of Rubisco).