4.3.4 ⎪ CBF1–3 and the nature of acclimation
The more pronounced photosynthetic upregulation in SW plants grown from seedling stage under HLC suggest an acclimation directed at enhanced productivity in addition to mitigation of oxidative stress. Furthermore, the lesser excitation pressure (lower QA reduction state) in HLC-grown plants of SW compared to IT represents a lesser trigger for further acclimatory adjustment and evidence of more complete acclimation to HLC conditions in SW compared to IT plants (see also Cohuet al. 2013b a, 2014; Adams et al. 2013; Stewart et al. 2017b). Additionally, as described above, genes involved in plant response to oxidative stress were consistently more strongly induced in IT relative to SW under HLC growth conditions. Rather than maximizing excitation energy utilization for photosynthetic energy production and thereby minimizing oxidant production, IT thus apparently employs multiple mechanisms that mitigate oxidative stress. CBF1–3 may play a prominent role in the mitigation of oxidative stress in IT, and presumably also during the initial stages of cold acclimation (Fowler & Thomashow 2002) compared to completed acclimation in SW (see Parket al. 2018). This difference in transcriptional control in SW and IT may stem from evolution under the different environmental conditions at the sites of origin, where IT can presumably “wait out” infrequent, short-duration cold spells, while it is advantageous for SW to maintain productivity throughout long stretches of cool conditions. These contrasting strategies would be of interest for agriculture in locations with either short cold spells or continuously low temperatures.
In conclusion, several lines of evidence at the transcriptomic and physiological levels are consistent with the CBF1–3-dependent pathway playing a disproportionately greater role under HLC in IT but not in SW. It should be noted that this trend was already evident in young plants and not only in more mature plants. The system of IT and SW, and their CBF1–3-deficient mutants, can serve as a resource to further study CBF1–3-regulated genes that mitigate oxidative stress before, or in the absence of, fully regained productivity as well as genes that remain active after productivity has been fully restored. In addition, CBF1–3-independent pathways that contribute to full HLC acclimation can also be studied in the SW background. Tools for phenotyping and transcriptional profiling of Recombinant Inbred Line populations are available for these two populations (Ågren et al. 2013; Oakleyet al. 2018).