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).