4.3.2 ⎪ Role of paralog compensation
Paralog compensation is unlikely to explain the observed significant induction of genes defined as CBF1–3-target genes in CBF1–3-deficient mutants in HLC. CBF1–3 belong to the ERF/AP2 A-1 subfamily that includes three additional members located outside the CBF1–3gene locus in A. thaliana (Mizoi, Shinozaki & Yamaguchi-Shinozaki 2012); these three other ERF/AP2 A-1 subfamily members (DDF1, AT1G12610; DDF2 , AT1G63030; CBF4 , AT5G51990) were not expressed at detectable levels in leaf tissue of IT or SW under any of the four growth regimes in either the present study or a previous study (Park et al. 2018). On the other hand, while not a core topic for this work, the findings of the present study are fully consistent with the conclusion that paralog compensation significantly attenuates the effect of the sw:cbf2 mutation alone on gene expression and freezing tolerance, as assessed by electrolyte leakage and Fv/Fm post-freezing (Gilmouret al. 2004; Zhao et al. 2016; Jia et al. 2016; Park et al. 2018). Several independent A. thalianalineages have evolved loss-of-function mutations in individualCBF1–3 genes without adverse effects on survival in regions with mild winters, suggesting a potential fitness advantage to attenuated CBF1–3 activity in southern ecotypes (Kang et al. 2013; Monroeet al. 2016). Likewise, our group, furthermore, reported that IT grow faster than SW in either low light or hot temperature under controlled conditions (Stewart et al. , 2016). The complete suite of CBF1–3 transcription factors may be required for tolerance to temperatures well below those used in this work (daytime air temperature of 8°C and maximum leaf temperature up to 16°C) as consistent with the results of the freezing survivorship assays performed by Sandersonet al . (2020).