When resources are limited, there is a tradeoff between growth/reproduction and stress defense in plants. Most temperate plant species, including Arabidopsis thaliana, can enhance freezing tolerance through cold acclimation at low but non-freezing temperatures. Induction of the cold acclimation pathway should be beneficial in environments where plants frequently encounter freezing stress, but might represent a cost in environments where freezing events are rare. In A. thaliana, induction of the cold acclimation pathway critically involves a small subfamily of genes known as the CBFs. Here, we test for a cost of cold acclimation by utilizing (1) natural accessions of A. thaliana that originate from different regions of the species native range and that have experienced different patterns of historical selection on their CBF genes, and (2) transgenic CBF over-expression and T-DNA insertion (knockdown/knockout) lines. While benefits of cold acclimation in the presence of freezing stress were confirmed, no cost of cold acclimation was detected in the absence of freezing stress. These findings suggest that cold acclimation is unlikely to be selected against in warmer environments, and that naturally occurring mutations disrupting CBF function in the southern part of the species range are likely to be selectively neutral. An unanticipated finding was that cold acclimation, in the absence of a subsequent freezing stress, resulted in increased fruit production, i.e., fitness.

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