1 INTRODUCTION
With the recently published 6th report of the
Intergovernmental Panel on Climate Change, IPCC (IPCC 2022;
https://www.ipcc.ch/report/ar6/wg2/), we have again been warned that
ongoing global warming will expose plants and animals to conditions
exceeding the limits of their historical and eventual evolutionary
experience. Species inhabiting polar and alpine regions that are already
being influenced by a warming climate will face challenges induced by
these changes, including population decline (Descamps et al., 2017),
habitat loss, and even extinction. These challenges surpass the limits
of adaptation, even for less vulnerable species and systems (e.g.
Román-Palacios & Wiens, 2020). Studying the potential of plant species
to adapt to differing and rapidly changing conditions is now more
relevant than ever. Habitats at the coldest margins of the Earth, such
as Arctic and alpine ecosystems, are considered most affected by rapidly
proceeding global warming (Ernakovich et al., 2014); this is especially
true for arctic-alpine species at the limit of their distribution range
(Lescia et al., 2004). Even though temperatures are rising on a global
scale, winter at high latitudes and elevations is over-proportionally
affected by climate change, as average winter temperatures are
increasing more rapidly in Arctic and alpine habitats (ACIA, 2005;
Kreyling, 2010). Winter warming has led to a decline in the amount and
duration of snow cover, which is crucial to the survival of subnivean
species in several regions. During the last 30 years, Arctic snow cover
has already declined by 10%, and model projections expect an additional
decrease of 10-20% until the end of this century (ACIA, 2005; Kreyling,
2010). In Europe, a 40-80% decline in days with snow coverage is
predicted (Kreyling, 2010). The rate of snow cover reduction is among
the most dramatic signs of climate change, and its impacts are severe.
Snow cover is one of the most important factors affecting the survival
of arctic and alpine plants. Its availability, depth, and duration
determine environmental variables such as soil moisture, temperature,
and freezing depth (Wipf & Rixen, 2010). Snow has an insulating effect
because it contains a large amount of air, which protects species living
underneath from wind and extreme winter temperatures (Pomeroy & Brun,
2001). This dynamic creates a relatively mild microclimate with
temperatures only slightly below 0°C, whereas ambient temperatures may
drop as low as -45°C, thereby ensuring the survival of many plant
species (Bokhorst et al., 2009; Armstrong et al., 2015). Earlier
snowmelt and decreased snow depth caused by a reduction in snowfall and
mid-winter thawing events followed by cold periods are consequences of
global warming that can threaten the survival of arctic and alpine
plants. Plants may be exposed to considerably lower temperatures, cold
winds, and temperature fluctuations, which can cause substantial frost
damage and may lead to plant die-offs (Sonesson & Callaghan, 1991;
Bokhorst et al., 2009). Winter warming has led to a lengthening of the
vegetative growing season in the Northern Hemisphere, especially at
higher latitudes (Parmesan, 2006). As plants often react to warming with
increased productivity and less investment in protection, frost events
after a period of warming can be fatal in many cases because they occur
more frequently. Considering this dynamic, plant species inhabiting
areas experiencing changes in snow cover may paradoxically require an
increased tolerance to freezing temperatures, as these changes
counteract the overall effect of global warming (Kreyling, 2010). As
climatic conditions change drastically, plant populations may no longer
be best adapted to ambient conditions (Franks et al., 2014). Reactions
of plant species to changes in ecological conditions include strategies
such as migration and adaptation through evolutionary changes or
developing ample phenotypic plasticity (Aitken et al., 2008; Williams et
al., 2008; Franks et al., 2014). Migration allows for the exploration of
more suitable conditions. This is, however, a limited option, as
migration attempts may not keep pace with rapidly progressing changes,
and such explorations may require the movement of entire ecosystems
(Loarie et al., 2009). The expected response of northern plant species
to rising temperatures is relocation (ACIA, 2005). With improvements in
livability for warm-loving plant species in arctic regions, poleward
migrations are expected, leaving arctic species at risk of displacement;
this risk is especially pressing for species whose northward migration
is hindered by the Arctic Ocean. Similarly, a documented upward shift in
the distribution of montane species has been caused by the inability of
species to adapt to local conditions at their current elevation. In
addition, successful colonization increases with longer growing seasons
and higher temperatures (Walther et al., 2005; Parolo et al., 2008).
These shifts in distribution patterns have led to increased species
richness in both Arctic and high alpine regions, which may lead to
increased competition for species originally inhabiting these areas
(Loarie et al., 2009). If species currently adapted to the coldest
regions and highest mountain summits, where no space for further
migration is left, fail to adapt to unfavorable conditions in time, the
eventual eradication of these species is a real possibility and a
serious threat. Therefore, local adaptation may be a quicker and more
efficient response to changing conditions. Several plant populations may
need to respond to climate change through phenotypic plasticity, which
refers to the ability of a genotype to express different phenotypes or
adaptive evolution (Franks et al., 2014). The extent to which plant
populations can adapt to locally changing conditions is highly relevant
in predicting future responses and the chances of their survival in
periods of rapidly progressing global warming. Nevertheless, these
responses may be insufficient to keep pace with the current rate of
climate change (Franks et al., 2014).
An important adaptation of plants inhabiting cold-characterized regions
is their ability to protect against frost damage induced by minimum
winter temperatures through a process called cold acclimation. Minimum
winter temperatures remain a major factor in determining species
distribution ranges (Armstrong et al., 2020). Tolerance to freezing
temperatures is a trait most common in temperate as well as arctic and
alpine plants, which are frequently exposed to sub-zero temperatures,
whereas most tropical and subtropical plants suffer injuries from
temperatures below 10 °C (Xin & Browse, 2000). Freeze-induced cellular
dehydration caused by membrane destabilization is commonly accepted as
the primary cause of freezing damage (Uemura et al., 1995). The
formation of ice crystals in extracellular spaces and cell walls leads
to decreased water potential outside the cells, thereby drawing water
and electrolytes out by osmosis and eventually leading to cell death.
Because freezing damage primarily occurs in cell membranes, changes in
membrane behavior are critical for developing freezing tolerance (Xin &
Browse, 2000). In nature, temperate plants can increase their ability to
withstand freezing temperatures after being exposed to low but
non-freezing temperatures for a certain period, which makes them
tolerant against seasonal changes and protects them against freezing
damage in winter and early spring (Ritonga & Chen, 2020). The process
of cold acclimation is mostly initiated by decreasing temperatures in
late fall and can take a few days to several weeks, depending on the
plant species (Xin & Browse, 2000). For Arabidopsis thaliana , an
acclimation period of a few days is sufficient to improve cold tolerance
(Gilmour et al., 1988), whereas 2-4 weeks of cold acclimation is
necessary for wheat or rye to develop an increased frost tolerance
(Brule-Babel & Fowler, 1989). The molecular basis of cold acclimation
has long been investigated but has not yet been fully resolved. The
process involves regulating hundreds of genes and is accompanied by
complex biochemical and physiological changes, as well as structural
changes (Xin & Browse, 2000; Thomashow, 2010). Such changes include the
accumulation of soluble sugars, which act as cryoprotectants for
enzymes, prevent excessive dehydration, and promote membrane stability,
as well as compositional and structural changes in the cell membrane
(Reyez-Díaz et al., 2006; Uemura et al., 2006); cold acclimation is
therefore clearly a multifactorial process in which many genes act in
parallel to ensure maximum freezing tolerance (Renaut et al., 2005).
There is natural variation in the freezing tolerances of different
plants and their potential to acquire and increase tolerance following
cold acclimation; this variation exists among species and even
populations within species (Xin & Browse, 2000; Hannah et al., 2006);
Some of this variation may be linked to the species’ or populations’
distribution (Armstrong et al., 2020). Varying environmental conditions
in different geographic regions may expose plants to different selection
pressures, which may lead to differences in phenotypic adaptation to
cold in geographically distant populations (Zuther et al., 2012).
Therefore, an increased selection for cold tolerance is to be expected
in northern populations, whereas in southern regions, there may be less
selection for cold tolerance (Armstrong et al., 2020). This cline has
been reported for accessions of different Arabidopsis species
(Hannah et al, 2006; Zuther et al., 2012), whereas other studies have
not confirmed this phenomenon (Armstrong et al., 2015; Armstrong et al.,
2020). Freezing tolerance may also be pronounced along longitudinal and
altitudinal gradients (Zuther et al., 2012; Davey et al., 2018). Cold
acclimation is not a constitutive trait but instead must be induced by
specific environmental cues. Several researchers have hypothesized that
the process may, therefore, be biologically costly, thereby leading to
trade-offs between the degree of cold tolerance and other traits such as
productivity. Constitutive cold tolerance may be costly in environments
that rarely encounter freezing events, as extensive physiological and
biochemical changes must be maintained (Zhen et al., 2011). For species
with a distribution spanning steep temperature gradients, the same
degree of cold tolerance would not be beneficial across the full
distribution range (Meireles et al., 2017). This cost of cold tolerance
has been reported for Olea europaea (Arias et al., 2017). For
herbaceous plants such as Arabidopsis thaliana , there is no
indication of a trade-off between the level of protection and
productivity (Zhen et al., 2011; Wos & Willi, 2015; Wos & Willi,
2018). The ability to prepare for freezing events through cold
acclimation is especially important for arctic and alpine plant species
because they are exposed to a wider temperature range (Davey et al.,
2018).
Although several studies have intensively investigated the freezing
tolerance and acclimation potential of different Arabidopsisaccessions and ecotypes (e.g. Hannah et al., 2006), minimal information
is known about this trait in diverse but evolutionarily related species
assemblages, such as the tribe Cochlearieae from the Brassicaceae family
(mustards). This tribe belongs to one of the few isolated lineages of
the Brassicaceae family, comprising approximately 30 species (Koch,
2012). It represents a promising system to study cold adaptation, as it
shows distinctive traits that have evolved over a short time span. These
traits include adaptations to extreme bedrock types, heavy metal soils,
salt habitats, and high alpine and Arctic regions (Wolf et al., 2021).
The tribe includes two genera, Cochlearia and Ionopsidium ,
which were separated by a deep evolutionary split during the
mid-Miocene, approximately 13.8 million years ago (Koch, 2012). WhereasIonopsidium species adapted to western Mediterranean bioclimatic
conditions, progenitors of present-day Cochlearia expanded their
distribution range to the northern hemisphere and rapidly diversified
during the Pleistocene to ecological conditions in central and northern
Europe and the circumarctic (Koch, 2012; Wolf et al., 2021). Even though
these taxa are closely related, they have colonized substantially
different ecological niches, such as salt marshes, sand dunes, coastal
areas, cold and calcareous springs, and high alpine and arctic
environments. Separating most Cochlearia species fromIonopsidium is the coldness of their habitats (Koch, 2012). MostCochlearia species are already endangered, as suitable habitats
have become increasingly rare, and two species have become extinct in
the wild (Cochlearia polonica , Cieślak et al., 2007; Cieślak et
al., 2010; C. macrorrhiza , Koch et al., 2003; Koch & Bernhardt,
2004). Most Cochlearia species, having been forced into
cold-adapted and geographically isolated niches, may face extinction if
they are unable to quickly adapt to warming conditions. Evaluating the
different Cochlearia and Ionopsidium taxa in their
physiological responses to freezing temperatures offers the opportunity
to gain insights into the evolution of cold adaptation and tolerance in
the tribe Cochlearieae.
We previously demonstrated that both Cochlearia andIonopsidium show a strong but very similar cold metabolome
response to cold treatment (Wolf et al., 2021). We hypothesized that
adaptations to drought (Mediterranean habitats), salinity (coastal line
habitats), and cold (arctic-alpine environments, cold spring water) may
be achieved via the same primary response in herbaceous (non-woody)
plants, thus posing the question of an evolutionary shared
molecular-physiological trait among Ionopsidium andCochlearia .
The focus of this study was therefore to elaborate on this idea and to
test explicitly for cold tolerance of the living system across
evolutionary scales (genera and species), genomic complexity (diploids
versus polyploids), and biogeographic and bioclimatic gradients
(Mediterranean to the Arctic). We elaborate on the hypothesis thatCochlearia species are caught in its cold-adapted niche, and
successful escape from this bioclimatic cage may require a complete
shift in life history strategies, a shift from perennial/polycarpic to
annual/monocarpic growth, which is the most severe difference betweenCochlearia and Ionopsidium .