4.1 ⎪ Response of plant function and gene expression to growth
under HLC conditions
Neither cool temperature alone (maximum daytime leaf temperature of
16°C) nor high light alone strongly induced expression ofCBF1–3, but combined high light and cool temperature (HLC) acted
synergistically to induce CBF1–3 . Increased excitation pressure
in the chloroplast serves as one of the signals that induce elevatedCBF1–3 expression under HLC conditions (Hüner et al.2012, 2016) and is integrated with additional photosynthetic retrograde
signals (Lee & Thomashow 2012; Noren et al. 2016). Overwintering
herbaceous plants, experiencing cold temperatures and the associated
high excitation pressure in the chloroplast, enact the suite of
acclimatory responses demonstrated here, including upregulation of
photosynthetic capacity and leaf thickness (with more mass and
chlorophyll per area), reduced leaf expansion, and enhanced freezing
tolerance (see also Cohu et al. 2013b, 2014; Muller et al.2014; Sanderson et al. 2020). These changes can allow
overwintering species to achieve full acclimation, defined as a new
homeostasis where internal stress (with signs of oxidative stress) is
minimized or absent.
The pronounced acclimation of plant form and function in SW and IT
plants grown in HLC conditions was associated with sweeping changes in
gene expression, with approximately 5.2% of total leaf transcriptome
upregulated and 4.9% downregulated in HLC relative to growth in low
light and warm temperature (LLW). The most strongly enriched gene
ontology categories in HLC were related to starch metabolism, sucrose
biosynthesis, and cold acclimation, which is consistent with the
upregulation of photosynthetic capacity and of freezing tolerance.
Continued photosynthetic productivity under cool temperatures in the
absence of significant growth generates carbohydrate that can be stored
in sink tissues (Demmig-Adams et al. 2017; Adams et al.2018a) and also contribute to the accumulation of compatible solutes and
freezing point depression (Wanner & Junttila 1999; Reyes-Diaz et
al. 2006; Cao, Song & Su 2007).
Pathways repressed in HLC in both SW and IT included those associated
with growth hormones. Reduction of rosette expansion under winter
conditions, involving decreased rates of cell elongation during leaf
development (Yano & Terashima 2004; Hoshino et al. 2019) helps
to minimize foliar freezing damage. Pathways repressed in HLC in both SW
and IT included not only those associated with growth hormones (e.g.,
brassinosteroids and auxin polar transport) but also with water
transport. In fact, vascular tissue is one of the targets of growth
hormones (Fàbregas et al. 2015; Etchells, Peter Etchells &
Turner 2017) and acclimation to cool temperature is associated with
adjustments of vascular anatomy (Cohu et al. 2013a; Adamset al. 2016; Stewart et al. 2016). Thus, effects of
freezing on the plant vasculature may be involved in the pronounced
vulnerability of it:cbf123 plants. Further research is warranted
into the possibility that the much greater impairment of freezing
tolerance of whole plants by CBF1–3- deficiency in the IT
compared to the SW background may be due to the well-characterized
phenomenon of freeze-thaw-induced water conduit embolism and its
dependence on xylem anatomy (Lens et al. 2013). Further research
should also address plant ontology and age given that the electrolyte
leakage assays in this study were performed on excised leaves of plants
acclimated for multiple weeks, whereas, to achieve the necessary sample
size, whole plants of a younger age/smaller size were used for survival
assays.