Arabidopsis thaliana ICE2 gene: Phylogeny, structural evolution and functional diversification from ICE1

The ability to tolerate environmental stresses is crucial for all living organisms, and gene duplication is one of the sources for evolutionary novelties. Arabidopsis thaliana INDUCER OF CBF EXPRESSION1 and 2 (ICE1 and ICE2) encode MYC-type bHLH (basic helix-loop-helix) transcription factors. They confer cold stress tolerance by induction of the CBF/DREB1 regulon and regulate stomata formation. Although ICE2 is closely related to ICE1, its origin and role in cold response remains uncertain. Here, we used a bioinformatics/phylogenetic approach to uncover the ICE2 evolutionary history, structural evolution and functional divergence from the putative ancestral gene. Sequence diversification from ICE1 included the gain of cis-acting elements in ICE2 promoter sequence that may provide meristem-specific and defense-related gene expression. By analyzing transgenic Arabidopsis lines with ICE2 over-expression we showed that it contributes to stomata formation, flowering time regulation and cold response. Constitutive ICE2 expression led to induced meristem freezing tolerance, resulting from activation of CBF1 and CBF3 genes and ABA biosynthesis by NCED3 induction. We presume that ICE2 gene has originated from a duplication event about 17.9 MYA followed by sub- and neofunctionalization of the ancestral ICE1 gene. Moreover, we predict its role in pathogen resistance and flowering time regulation.