4.3.3 ⎪ SW as a high-light adapted ecotype
It was previously shown that SW responds with stronger upregulation of
photosynthetic capacity and associated leaf features than IT to growth
in high light under warm temperature (Stewart et al. 2017a).
Based on the latter response, SW was classified as having a high-light
phenotype, presumably as an adaptation to the extremely long summer days
in its site of origin at high latitude (Adams et al. 2016).
Photosynthetic upregulation is a developmental process involving changes
at the organelle, cell, tissue, and whole plant levels (Yano &
Terashima 2004; Hoshino et al. 2019), and involves integration of
multiple regulatory pathways, including photoreceptors,
photosynthesis-related sugar and redox signals, and phytohormone
signals. For example, mutants in blue-light photoreceptor signaling and
foliar sucrose (López-Juez, Bowyer & Sakai 2007; Kozuka, Kong, Doi,
Shimazaki & Nagatani 2011; Katagiri et al. 2016; Hoshinoet al. 2019) have an effect of similar magnitude in increasing
leaf thickness in HL-grown plants to those observed for
CBF1–3-dependent leaf thickening in the it:cbf123 mutant under
HLC. The sucrose synthase SUS1 may contribute to the differential
leaf thickening phenotype of SW and IT in HLC conditions via
sucrose-responsive leaf thickening (Katagiri et al. 2016) given
that (a) it was induced in both ecotypes in HLC conditions, but more
strongly in SW, and (b) its induction was unchanged in the
sw:cbf123 mutant but significantly attenuated in it:cbf123relative to each respective parental ecotype. Thus, its induction
pattern in HLC closely mirrors the trends for leaf thickness reported
here. In summary, the present findings suggest that light-responsive
signaling pathways with overlapping functions compensate fully for
CBF1–3 deficiency in sw:cbf123 with respect to upregulation of
photosynthetic capacity and associated leaf features, which were
unaffected in sw:cbf123 , but significantly (albeit modestly)
reduced in it:cbf123 . The particularly pronounced photosynthetic
upregulation in SW is presumably demanded by the continuously low
temperatures at its high-latitude site of origin, whereas the IT ecotype
encounters intermittent cold spells (requiring oxidative-stress
mitigation) and can quickly resume photosynthetic activity upon return
to milder temperatures (for temperature profiles at the respective sites
of origin, see Adams et al., 2016).