Common responses of the two ecotypes to high light and/or low temperature growth
Both A. thaliana ecotypes exhibited strong and significant upregulation of photosynthetic capacity in response to growth and development under HLW and to an even greater extent under HLC, a common response to low temperature or winter conditions in herbaceous winter annuals and biennials (Adams et al., 2013; Cohu et al., 2013b, 2014; Muller et al., 2014). This upregulation of photosynthetic capacity is part of a suite of acclimatory responses that support the ability to persist and thrive during winter. These responses included greater leaf mass per area, associated with more mesophyll cells and more chlorophyll per leaf area. The upregulation of photosynthesis coupled with the observed upregulation of genes involved in starch catabolism likely work synergistically to increase freezing tolerance through an elevated level of sugars that serve as cryoprotectants (Castonguay, Bertrand, Michaud, & Laberge, 2011; Strimbeck, Kjellsen, Schaberg, & Murakami, 2007).
The upregulation of photosynthetic capacity observed in both ecotypes in response to growth under high light and/or low temperature was previously shown to be accompanied by upregulation of foliar minor vein features of the phloem associated with an increased capacity for sugar export from the leaves (Adams et al. 2013, 2014, 2016, 2018; Cohu et al., 2013b; Stewart et al. 2016, 2017). On the other hand, acclimation to low versus high temperature also resulted in lower rates of transpiration and foliar vascular features (lower vein density and fewer xylem cells per minor vein) associated with a diminished capacity to distribute water to the leaves (Adams et al., 2016, 2018; Stewart et al., 2016). This latter result is consistent with the downregulation of genes associated with water transport and polar transport of auxin observed in response to growth under HLC in the present study. Vascular tissue formation in A. thaliana leaves (and thus vein density) as well as xylem differentiation is influenced by auxin synthesis and transport (Baylis, Cierlik, Sundberg, & Mattsson, 2013; Biedroñ & Banasiak, 2018; Fàbregas et al., 2015; Marcos & Berleth, 2014). There is, furthermore, a general consensus that vascular patterning arises from not only auxins but also their interaction with cytokinins (Etchells & Turner, 2017), the signaling pathways for which were found to be downregulated under HLC in the present study. Moreover, the development of xylem is a specific target of cytokinins (Kondo, Tamaki, & Fukuda, 2014).