Varying Atmospheric CO Mediates the Cold-Induced CBF-Dependent Signaling Pathway and Freezing Tolerance in Arabidopsis.

Changes in the stomatal aperture in response to CO levels allow plants to manage water usage, optimize CO uptake and adjust to environmental stimuli. The current study reports that sub-ambient CO up-regulated the low temperature induction of the C-repeat Binding Factor (CBF)-dependent cold signaling pathway in Arabidopsis () and the opposite occurred in response to supra-ambient CO. Accordingly, cold induction of various downstream cold-responsive genes was modified by CO treatments and expression changes were either partially or fully CBF-dependent. Changes in electrolyte leakage during freezing tests were correlated with COs effects on CBF expression. Cold treatments were also performed on Arabidopsis mutants with altered stomatal responses to CO, i.e., high leaf temperature 1-2 (, CO hypersensitive) and β-carbonic anhydrase 1 and 4 (, CO insensitive). The cold-induced expression of CBF and downstream CBF target genes plus freezing tolerance of was consistently less than that for Col-0, suggesting that HT1 is a positive modulator of cold signaling. The mutant had diminished CBF expression during cold treatment but the downstream expression of cold-responsive genes was either similar to or greater than that of Col-0. This finding suggested that βCA1/4 modulates the expression of certain cold-responsive genes in a CBF-independent manner. Stomatal conductance measurements demonstrated that low temperatures overrode low CO-induced stomatal opening and this process was delayed in the cold tolerant mutant, , compared to the cold sensitive mutant, . The similar stomatal responses were evident from freezing tolerant line, Ox-CBF, overexpression of compared to wild-type ecotype Ws-2. Together, these results indicate that CO signaling in stomata and CBF-mediated cold signaling work coordinately in Arabidopsis to manage abiotic stress.