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).