A role for SENSITIVE TO FREEZING2 in protecting chloroplasts against freeze‐induced damage in Arabidopsis

Abstract

SummaryThe SENSITIVE TO FREEZING2 (SFR2) gene has an important role in freezing tolerance in Arabidopsis thaliana. We show that homologous genes are present, and expressed, in a wide range of terrestrial plants, including species not able to tolerate freezing. Expression constructs derived from the cDNAs of a number of different plant species, including examples not tolerant to freezing, are able to complement the freezing sensitivity of the Arabidopsis sfr2 mutant. In Arabidopsis the SFR2 protein is localized to the chloroplast outer envelope membrane, as revealed by the analysis of transgenic plants expressing SFR2 fusions to GFP, by confocal microscopy, and by the immunological analysis of isolated chloroplasts treated with thermolysin protease. Moreover, the chloroplasts of the sfr2 mutant show clear evidence of rapid damage after a freezing episode, suggesting a role for SFR2 in the protection of the chloroplast.

Structural Determinants Allowing Transferase Activity in SENSITIVE TO FREEZING 2, Classified as a Family I Glycosyl Hydrolase

“…2B), consistent with the confident model of the core fold of SFR2 (Fig. 3) and previously reported data (12). Because no further data opposed the model, and the model itself is of good quality, it was used to inform further experiments.…”

“…2B) (12). In the same work, it was predicted that SFR2 may have a second transmembrane domain between residues 448 and 470.…”

“…5, A-C) and the model itself (Fig. 3) appear to oppose the previous data showing that a portion of SFR2 is protected from thermolysin digestion by presence of the chloroplast outer envelope membrane (12). To test whether the previously observed SFR2 protein fragment was intrinsically thermolysin-resistant, isolated A. thaliana chloroplasts were digested with increasing levels of thermolysin, with or without the presence of membrane-disrupting digitonin.…”

“…Enhancing freezing tolerance of crop plants has agricultural value (11), and SFR2 is a potential tool. Protein and transcript levels of SFR2 do not appear to change upon cold acclimation; instead, SFR2 appears to be constitutively present (12). However, its products are not detectable prior to freezing conditions (3), i.e.…”

“…In the same work, it was predicted that SFR2 may have a second transmembrane domain between residues 448 and 470. This prediction was based on positive results from transmembrane domain calculators and the presence of an SFR2 fragment protected from thermolysin digestion of isolated chloroplasts (12). However, if this region constitutes a second transmembrane domain, it would disrupt the seventh backbone helix (␣7), displacing the eighth helix (␣8) and strand (␤8) to the other side of the membrane.…”