Cold-Regulated Genes Associated with Differential Freezing Tolerance in Perennial Ryegrass.

Turfgrass can be susceptible to freezing injury during winter and early spring months due to fluctuating temperature and moisture conditions, negatively impacting freezing tolerance. Perennial ryegrass (Lolium perenne) is a cool-season turfgrass with many beneficial agronomic traits; however, compared to other cool-season grasses this species is sensitive to temperature extremes including low temperature kill. The objectives of the study were to examine changes in freezing tolerance and gene expression during cold acclimation and cold deacclimation for two perennial ryegrass genotypes differing in freezing tolerance, described herein as freezing tolerant (TOL) and freezing sensitive (SENS). Plants were moved to a controlled environmental chamber and exposed to progressive cold acclimation from 20°C to -2°C, followed by cold deacclimation (4°C or 8°C for up to 5d). Whole plants were harvested for evaluation of freezing tolerance (lethal temperature resulting in 50% mortality, LT₅₀) and gene expression of leaves and crowns measured using quantitative polymerase chain reaction. Genes were selected based on previous research demonstrating their importance in freezing tolerance in perennial grasses, including C-repeat binding factor-3 (CBF3), Ice recrystallization inhibitor-a (IRI-a), fructosyltransferase 1 (Prft1), and intrinsic membrane protein 1 (PIP1). The TOL genotype exhibited a lower LT50 in response to cold acclimation and deacclimation, which supported evidence of higher freezing tolerance in this genotype. Higher freezing tolerance in this genotype was accompanied by higher expression of CBF3, IRI-a, and Prft1 genes during cold acclimation and deacclimation, suggesting a higher capacity to maintain cryoprotectant proteins and metabolites. The SENS genotype expressed higher levels of PIP1 during both cold acclimation and deacclimation, suggesting greater tendency for tissue rehydration and freezing injury.