Protein Synthesis and Thermal Stability Associated with Root Thermotolerance in Agrostis Species.

Protein metabolism plays critical role in plant responses to heat stress. The objective of this study was to examine how protein synthesis and degradation is associated with root thermotolerance in perennial grass species. It was addressed by examining heat-adapted, Agrostis scabra (C3) that is tolerant to extremely high soil temperatures in geothermal areas in Yellowstone National Park, in comparison to a congeneric heat-sensitive species, A. stolonifera, a widely cultivated turfgrass. In order to assess whether lower rates of protein turnover in roots of A. scabra was associated with superior thermotolerance, the content of newly-synthesized proteins and the rate of protein degradation (labeled with 14C-leucine) were determined in roots exposed to heat stress (37^oC). Roots of A. stolonifera exposed to 37^oC exhibited more rapid decline in total protein content and newly-synthesized proteins than A. scabra roots. The decline in the content of total or newly-synthesized proteins suggested that protein degradation rates exceeded protein synthesis rates during heat stress, but thermal A. scabra roots may be better able to maintain protein synthesis and/or to possess relatively more thermostable proteins during heat stress. The degradation rate of the ¹⁴C-labeled protein was lower (30%) while the protein half-life was significantly longer (2 days) in A. scabra than in A. stolonifera, suggesting that newly-synthesized proteins of A. scabra roots degraded slower, which could be due to either the presence of more thermostable proteins or lower proteolytic activities. A. scabra roots had significantly lower protease activity and expression level of a gene encoding cysteine protease than A. stolonifera when exposed to 35^oC, suggesting lower proteolysis in A. scabra roots. Two-dimensional electrophoresis analysis identified up-regulation of sucrose synthase, glutathione S-transferase, superoxide dismutase, and heat shock protein Sti (stress inducible protein) may also contribute to the superior root thermotolerance of A. scabra.