Vernalization Requirement and the Chromosomal VRN1-Region can Affect Freezing Tolerance and Expression of Cold-Regulated Genes in Festuca pratensis

Abstract

Plants adapted to cold winters go through annual cycles of gain followed by loss of freezing tolerance (cold acclimation and deacclimation). Warm spells during winter and early spring can cause deacclimation, and if temperatures drop, freezing damage may occur. Many plants are vernalized during winter, a process making them competent to flower in the following summer. In winter cereals, a coincidence in the timing of vernalization saturation, deacclimation, downregulation of cold-induced genes, and reduced ability to reacclimate, occurs under long photoperiods and is under control of the main regulator of vernalization requirement in cereals, VRN1, and/or closely linked gene(s). Thus, the probability of freezing damage after a warm spell may depend on both vernalization saturation and photoperiod. We investigated the role of vernalization and the VRN1-region on freezing tolerance of meadow fescue (Festuca pratensis Huds.), a perennial grass species. Two F2 populations, divergently selected for high and low vernalization requirement, were studied. Each genotype was characterized for the copy number of one of the four parental haplotypes of the VRN1-region. Clonal plants were cold acclimated for 2 weeks or vernalized/cold acclimated for a total of 9 weeks, after which the F2 populations reached different levels of vernalization saturation. Vernalized and cold acclimated plants were deacclimated for 1 week and then reacclimated for 2 weeks. All treatments were given at 8 h photoperiod. Flowering response, freezing tolerance and expression of the cold-induced genes VRN1, MADS3, CBF6, COR14B, CR7 (BLT14), LOS2, and IRI1 was measured. We found that some genotypes can lose some freezing tolerance after vernalization and a deacclimation–reacclimation cycle. The relationship between vernalization and freezing tolerance was complex. We found effects of the VRN1-region on freezing tolerance in plants cold acclimated for 2 weeks, timing of heading after 9 weeks of vernalization, expression of COR14B, CBF6, and LOS2 in vernalized and/or deacclimated treatments, and restoration of freezing tolerance during reacclimation. While expression of VRN1, COR14B, CBF6, LOS2, and IRI1 was correlated, CR7 was associated with vernalization requirement by other mechanisms, and appeared to play a role in freezing tolerance in reacclimated plants.

Optimal Regulation of the Balance between Productivity and Overwintering of Perennial Grasses in a Warmer Climate

“…Oliver et al [107] therefore suggested that VRN1 and COR14B may be regulated by similar mechanisms in early cold acclimation, possibly through the action of CBF transcription factors. Several studies show that CBF6 and COR14B are down-regulated in cereals and meadow fescue by prolonged cold, but only under long photoperiods [105]. Taken together, these results suggest that VRN1 and the CBF regulon are co-regulated during cold acclimation of temperate grasses and as long as photoperiods are short, but that VRN1 down-regulates CBFs when photoperiods become long (Figure 3).…”

“…For example, carbohydrates did not accumulate to the same extent during re-acclimation as during initial cold acclimation in winter wheat [114], and several cold induced genes upregulated by cold acclimation in meadow fescue were not upregulated during re-acclimation [105]. The mechanisms behind these differences are not known, but are likely to be related to a coordinated regulation of growth and freezing tolerance as described in Section 2.3.…”

“…After short-term cold exposure, when the expression level of VRN1 is still very low, Oliver et al [120] found similar kinetics in the initial transcription of VRN1 and COR14B upon cold exposure (24 h) in barley. Under short photoperiods, high COR14B expression or positive correlation between expression of VRN1 and COR14B remained after long-term cold treatment in T. monococcum [112], barley [121], and meadow fescue [105]. COR14B is induced by CBFs, and barley VRN1 also has a putative CBF binding site in its promoter [122].…”

“…Re-acclimation at a time when spring growth has been initiated may be provided by other mechanisms than those employed during initial cold acclimation in the autumn. For example, while expression of COR14B could explain some of the variation in freezing resistance in de-acclimated meadow fescue, CR7 was, unlike COR14B, significantly upregulated by re-acclimation and could explain some of the variation in freezing resistance after re-acclimation [105].…”

“…There are several reports describing deacclimation and re-acclimation responses in perennial grass species [99][100][101][102][103][104][105], and some of them also report a negative association between freezing resistance and leaf growth during the de-acclimation period. In cereals, it has been shown that both freezing resistance and expression of cold-induced genes are down-regulated in shoot base tissue when the vernalization requirement is saturated, but before any development of the apex is visible in the microscope [106][107][108].…”