When grown under cool temperature, winter annuals upregulate photosynthetic capacity as well as freezing tolerance. Here, the role of three cold-induced C-repeat-Binding Factor (CBF1–3) transcription factors in photosynthetic upregulation and freezing tolerance was examined in two Arabidopsis thaliana ecotypes originating from Italy (IT) or Sweden (SW), and their corresponding CBF1–3-deficient mutant lines it:cbf123 and sw:cbf123. Photosynthetic, morphological, and freezing-tolerance phenotypes as well as gene expression profiles were characterized in plants grown from seedling stage under different combinations of light level and temperature. Under high light and cool growth temperature (HLC), a greater role of CBF1–3 in IT versus SW was evident from both phenotypic and transcriptomic data, especially with respect to photosynthetic upregulation and freezing tolerance of whole plants. Overall, features of SW were consistent with a different approach to HLC acclimation than seen in IT, and an ability of SW to reach the new homeostasis through involvement of transcriptional controls other than CBF1–3. These results provide tools and direction for further mechanistic analysis of the transcriptional control of approaches to cold acclimation suitable for either persistence through brief cold spells or for maximization of productivity in environments with continuous low temperatures.

Plants in dryland ecosystems experience extreme daily and seasonal fluctuations in light, temperature, and water availability. We used an in-situ field experiment to uncover the effects of natural and reduced levels of UV on maximum PSII quantum efficiency (Fv/Fm), relative abundance of photosynthetic pigments and antioxidants, and the transcriptome in the desiccation-tolerant desert moss Syntrichia caninervis. We tested the hypotheses that: (1) S. caninervis plants undergo sustained thermal quenching of light (non-photochemical quenching; NPQ) while desiccated and after rehydration, (2) a reduction of UV will result in improved recovery of Fv/Fm, but (3) one year of UV removal will de-harden plants and increase vulnerability UV damage to photosynthetic efficiency. All field-collected plants had extremely low Fv/Fm after initial rehydration but recovered over eight days in lab-simulated winter conditions. UV-filtered plants had lower Fv/Fm during recovery, higher concentrations of photoprotective pigments and antioxidants such as zeaxanthin and tocopherols, and lower concentrations neoxanthin and chlorophyll b than plants exposed to near natural UV levels. Field-grown S. caninervis underwent sustained NPQ that took days to relax and for efficient photosynthesis to resume. Reduction of solar UV radiation adversely affected recovery of Fv/Fm following rehydration.

When grown under cool temperature, winter annuals respond with not only enhanced freezing tolerance but also photosynthetic upregulation. The role of the cold-induced C-repeat-Binding Factor (CBFs) in long-term maintenance of freezing tolerance and photosynthetic upregulation was examined in two Arabidopsis thaliana ecotypes adapted to differing climates (Italy = IT and Sweden = SW) as well as corresponding CBF-disabled mutant lines. Data on photosynthetic, morphological, and freezing-tolerance phenotypes as well as transcriptomic data were collected from plants grown for several weeks under controlled conditions with several combinations of temperature and light levels. Freezing tolerance in these acclimated plants depended strongly on CBFs in both SW and IT. In contrast, photosynthetic upregulation was the same, or modestly reduced, in cbf mutant versus parental lines of SW and IT, respectively. Physiological and transcriptomic data showed a consistent trend for a greater role of CBFs in cool-temperature-grown plants of IT versus SW. These features suggest that IT remained in a state of continuing CBF-related cold-acclimation even after weeks of acclimation, while SW entered a state of completed acclimation in which maintenance of photosynthetic upregulation no longer required CBF activation and maintenance of freezing tolerance was less dependent on CBF than in IT.