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Cytoplasmic acidification is a specific response to freezing; it contributes to activating freezing-tolerance responses including a lipid remodeling enzyme necessary for freezing tolerance.
Low temperature is a seasonal abiotic stress that restricts native plant ranges and crop distributions. Two types of low-temperature stress can be distinguished: chilling and freezing. Much work has been done on the mechanisms by which chilling is sensed, but relatively little is known about how plants sense freezing. Recently, Arabidopsis (
Arabidopsis thaliana
) SENSITIVE TO FREEZING2 (SFR2) was identified as a protein that responds in a nontranscriptional manner to freezing. Here, we investigate the cellular conditions that allow SFR2 activation. Using a combination of isolated organelle, whole-tissue, and whole-plant assays, we provide evidence that SFR2 is activated by changes in cytosolic pH and Mg
2+
. Manipulation of pH and Mg
2+
in cold-acclimated plants is shown to cause changes similar to those of freezing. We conclude that pH and Mg
2+
are perceived as intracellular cues as part of the sensing mechanism for freezing conditions. This evidence provides a specific molecular mechanism to combat freezing.