Identification of CBF-target genes that may contribute to photosynthetic upregulation
The ecotype-dependent effect on photosynthetic upregulation in thecbf123 mutants presented an opportunity to identify CBF regulon genes potentially contributing to photosynthetic upregulation. CBF-regulated genes linked to photosynthetic upregulation under HLC should be strongly induced in the parental ecotypes, and based on the above discussion, exhibit reduced expression only in the it:cbf123 mutant and not the sw:cbf123 mutant. Two examples of genes that follow this expression pattern wereSUS1 (AT5G20830) and EGR2 (AT5G27930). SUS1 is a sucrose synthase not required for sucrose accumulation under conditions favorable for growth, but strongly induced under several abiotic-stress conditions (Barratt et al., 2009; Bieniawska et al., 2007; Branco-Price, Kawaguchi, Ferreira, & Bailey-Serres, 2005; Kilian et al., 2007). Recent evidence linked high foliar sucrose levels to increased cell height in leaves grown under high light (Hoshino et al., 2019; Katagiri et al., 2016), which may be mediated by greater rates of endocycle reduplication of DNA (amplification of genome copy number in the absence of cell division) in palisade cells contributing to cell size expansion (Katagiri et al., 2016). In this context, it should be noted thatCBF over-expressing lines exhibited increased leaf thickness and accumulation of soluble sugars, including sucrose, under LLW (Gilmour et al., 2004; Savitch et al., 2005).
The fact that it:cbf123 plants had larger rosettes relative to IT in HLC may also be associated with the regulation of cell elongation and growth. Decreased rates of cell elongation during leaf development is likely a key component of how A. thaliana reduces rosette expansion under winter conditions to reduce foliar freezing damage (Hoshino et al., 2019; Yano & Terashima, 2004). EGR2, another confirmed IT-specific CBF target gene under HLC, is a negative regulator of growth that controls cytoskeletal-mediated vesicle trafficking to the plasma membrane (Bhaskara, Wen, Nguyen, & Verslues, 2017). Over-expression of EGR2 was sufficient to reduce cell expansion and generate smaller rosettes, whereas egr2 null mutants had enhanced cell elongation and larger rosettes (Bhaskara et al., 2017). Furthermore, post-translational modification of EGR2 under chilling stress induced CBF1–3 expression (Ding et al., 2019), suggesting that EGR2 may be a regulatory link between CBF transcriptional activity and whole-plant changes in rosette growth under HLC. Overall, the resource of IT-specific CBF-regulated genes under HLC, includingEGR2, SUS1, and the four additional genes shown here to be IT-specific, may help define regulatory controls on photosynthetic upregulation to overwintering conditions. We hope that future work will seek further mechanistic insight into the ecotype-specific transcriptional control of photosynthetic pathways in response to growth environment, perhaps through phenotyping and transcriptionally profiling of Recombinant Inbred Line populations developed for these two populations under LLW and HLC conditions (Ågren et al., 2013).
v. Acknowledgements:
The lines sw:cbf2, sw:cbf123, and it:cbf123 were generously provided by Professor Michael Thomashow at Michigan State University. This work was supported by the Gordon and Betty Moore Foundation through Grant GBMF 2550.03 to the Life Sciences Research Foundation [to C.R.B]. K.K.N. is an investigator of the Howard Hughes Medical Institute. This work was also supported by the National Science Foundation [DEB-1022236 to B.D.-A. and W.W.A., IOS-1907338 to J.J.S.]; and the University of Colorado.