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).