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CLIMATREE
Professor Christiaan van der Schoot and
Research Professor Päivi L H Rinne explain
how recent work on the dormancy cycling of
perennials links with wider concerns about
tree survival in a changing climate
A perennial problem
Can you elaborate on the wideranging impact that climate
change will have on the plant
kingdom?
Can you outline the scope of your group’s
studies at the Norwegian University of Life
Sciences?
We investigate how higher plants orchestrate
their growth, development and architecture. It
is a remarkable feat that in all plants, including
trees, the entire shoot system – everything
visible above the soil – arises from the activity
of a tiny but dynamically organised cell society
which is seated at the terminal growing point.
The ‘shoot apical meristem’ is autonomous
in its formative activity, while sensitive to
inputs from remote plant parts that respond to
important seasonal cues such as photoperiod
and temperature. This responsiveness is critical
to survival, as it enables perennials to arrest
meristem activity at the end of the growing
season and enter a dormant and freezingtolerant state. It also functions in the timely
execution of the flowering process.
To what extent are your research topics
engaging with the threat of climate change?
Climate change has been a motivating factor
all along. This year alone, fluctuations in
temperature have killed vast numbers of
trees in North America, and damaged fruit
trees in Europe. Furthermore, plant survival is
dependent on other species via competition
and cooperation, for example, temperatureinduced precocious flowering may be out of
step with the arrival of pollinators.
Climate models developed by
the Intergovernmental Panel
on Climate Change (IPCC)
predicted a decade ago that
global warming will give rise to
weather instabilities characterised
by unusual fluctuations in
temperature and humidity,
something we witness now. This
increases the susceptibility to pathogens
and hampers the synchronisation of plant
life to the seasons by uncoupling the
ancient interplay between photoperiodand temperature-driven signalling events.
Warm spells in early winter tend to activate
meristems leading to reduced freezingtolerance and vulnerability once frost returns.
Owing to the long generation time of trees, a
rapidly advancing climate change will outpace
the emergence of heritable adaptive traits.
No doubt this will influence their geographic
distribution, with some species profiting and
some declining.
To what extent does your group adopt a
multidisciplinary approach? What benefits has
this yielded?
Dormancy cycling involves phenomena
at all hierarchically interlocking levels of
complexity, and therefore such research is
inherently multidisciplinary. Consequently,
a wide range of technologies must be
employed to obtain all relevant information.
Forward and reverse genetics tools can
be used to investigate gene function, but
its success is dependent on the accurate
characterisation of dormancy as a trait. We
are just at the beginning of understanding
what dormancy cycling is about. We urgently
need to combine genomics, proteomics
and cell biology with cell-cell signalling and
modelling studies.
Furthermore, how do you anticipate your
research findings to be used in the context of
climate change?
In an optimistic scenario our findings have
the potential to ameliorate the behaviour
of tree species in a changing climate. Firstly,
they can help in defining important traits of
trees that contribute to dormancy cycling.
This information can further be used to select
trees for specific breeding programmes and
genetic engineering of appropriate genes
to modify tree behaviour to match the
prevailing climate conditions. Ultimately,
such research has the potential to benefit the
health and yield of domesticated trees and
shrubs, including fruit crops.
Can you explain how your results will help
to form prognoses for how forest trees will
perform during climate change?
In trees growing in nature the mechanisms
that drive the dormancy cycle are often
confounded. For example, the induction of
dormancy by shortening day lengths can be
counteracted by a simultaneously declining
temperature; shortening days induce
dormancy while a declining temperature
promotes release from dormancy. Such
overlap can delay entry into a dormant
state and reduce the depth of dormancy,
thereby reducing the number of chilling hours
required later for dormancy release.
The differential temperature-driven
regulation of the enzymes that control the
accessibility of signals to the communication
channels might be one crucial factor that
needs to be investigated further.
Have any forecasts been made so far?
We cannot currently make explicit forecasts,
but once we know more precisely how the
environment impacts on these enzymes, this
information could be incorporated into more
realistic models.
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CLIMATREE
Against the odds
The ClimaTree and ClimaDorm projects at the Norwegian University of Life Sciences are producing
results which reveal the complex mechanisms behind the survival strategy of perennial dormancy
THE STRATEGY PERENNIALS employ to
overcome winter is both complex and key to
their survival. To understand the regulatory
mechanisms,
molecular
and
genetic
investigations are needed in combination with
cell biological studies and growth analyses.
A sequenced genome is therefore of great
significance. As it happens, however, the first
plant to have its genome fully sequenced was not
a perennial but the annual Arabidopsis (mouse
ear cress). Standardisation of Arabidopsis as the
model plant of choice has resulted in a wealth of
information, but similar in-depth studies of trees
only became possible when the first tree, Populus
trichocarpa (western balsam poplar) had its DNA
sequenced in 2006.
Presently, several other tree genomes are being
sequenced. This is greatly promoting research
on traits that can only be studied in perennials,
such as dormancy. Various teams have embarked
on such studies, including researchers at the
Norwegian University of Life Sciences (UMB).
Initially, their studies were focused on birch, an
attractive species due to its superior tolerance
to low temperatures, economic and ecological
importance, and large variability in growth habits
and distribution range. Poplar is not dissimilar
in this sense but, importantly, the sequencing
of its genome allows detailed investigations
into the mechanism of dormancy cycling. Plans
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INTERNATIONAL INNOVATION
to sequence the birch DNA at the University of
Helsinki, an initiative led by Kangasjärvi, will open
up the possibility for similar studies in birch.
Professor Christiaan van der Schoot and Research
Professor Päivi L H Rinne are buoyed by the
increasing interest in perennials and the unique
opportunities they offer for investigation. Team
members are hopeful that results may be useful
for innovative genetic engineering applications
in tree breeding that aim at better forms, greater
yields, tolerance to pathogens, and adaptability to
seasonal change. There are still challenges facing
tree researchers to obtain funding and overcome
legislative hurdles for the use of transgenic trees.
The team hopes that their initial foray into the
mechanisms of plant dormancy will encourage
further studies, opening up this neglected area.
GROWTH PROBLEMS
In higher plants the shoot apical meristem (SAM)
is central to growth, comprising cells which are
not unlike stem cells in humans. SAM is also
important for the plant’s reaction to changing
weather conditions. To identify genes that
regulate winter bud development, microarrays –
a technology for mapping gene expression – have
been successfully used by various groups. This
type of dataset mostly reveals change in gene
expression related to metabolic variations that
occur downstream of developmental decisions
in a bud. However, it is the upstream switching
points that are of particular importance in
developmental transitions. The complexity of the
developing bud emerges as a result of processes
that occur in different tissues and have their
own dynamics, something a microarray cannot
accurately capture. Inferring causal relation
from a temporal order is also prone to errors. For
example, the team at UMB has been able to show
that products first expressed during dormancy
induction are recruited later in the process during
chilling-induced release from dormancy. This
approach has great potential. As the shorter days
of winter arrive, growth will cease and bud scales
form around an emerging embryonic shoot, a mini
shoot that will give rise to a next round of growth
in the new season. The SAM of the embryonic
shoot is arrested in a dormant state, which is
achieved by a precise deposition of callose in and
around plasmodesmata, cell-cell channels that
meristem cells use to coordinate their activities.
In addition, the transport channels in the stem
that supply the SAM with growth substances
are blocked with winter callose. Remarkably,
the cold of winter subsequently achieves two
things: it releases the bud from dormancy, and
it further increases the level of freezing tolerance
of the bud. Nevertheless, these freezing-tolerant
and non-dormant buds are more susceptible
to deacclimation. They rapidly lose freezing-
INTELLIGENCE
tolerance during long warm spells in winter,
making them highly vulnerable to returning frost.
A surprising outcome of recent poplar research
is the interesting parallel between the annual
Arabidopsis and the perennial poplar. The
transition to flowering in Arabidopsis and the
transition to dormancy in poplar both use the
The link with the photoperiod
induced dormancy and its
temperature dependent depth
is tantalising, but more research
needs to be done before explicit
forecasts are able to be made
gene FLOWERING LOCUS T (FT). In flowering a
leaf-produced signal informs the SAM to flower,
while in dormancy the leaves cease producing
signal, as was first shown by the Nilsson-group in
Umeå. The UMB group confirmed these findings
and in addition showed another similarity. The
gene CENTRORADIALIS LIKE1 (CENL1) plays a
role in dormancy development comparable to
that of its ortholog, TERMINAL FLOWER1 (TFL1),
in flowering of Arabidopsis. These parallels are the
focus of ongoing research.
DEMONSTRABLE RESULTS
The research at UMB has also identified a set of
genes, encoding enzymes of the 1,3-β-glucanase
family, which regulate the conduits that allow
movement of signals from the leaves to the
growing point, as well as within the growing point
itself. These genes appeared to be differentially
regulated by gibberellic acids GA3 and GA4 and
were expressed during different phases of the
dormancy cycle. GA’s are important as they have
the capacity to replace chilling in dormancy release,
and the team hypothesised that they could control
the enzymes to reopen the plasmodesmata.
Localisation of these enzymes at plasmodesmata
was confirmed by transiently over-expressing
selected genes in leaves of N. Benthamiana. Together
with collaborators in Finland and Sweden, the
scientists in Norway are engineering poplars with
these selected genes to investigate if the dormancy
cycle can be modified. The goal is to reveal the
interplay between environment, signalling peptides
and their conduits. Particularly in the context of
global warming and shifting climate patterns, the
link between photoperiod-induced dormancy and
its temperature-dependent depth and release is
tantalising, but more research must be done before
explicit forecasts can be made. It is hoped that this
will lead to a breakthrough.
BEYOND THE PROJECT
The need to continue this research is important,
and the UMB team expects that their work
will have a wide-ranging impact. Their insights
into SAM function, and its importance as the
locus of dormancy, are now widely accepted.
This directs future projects on dormancy in the
direction of meristem biology. In particular, it will
be crucial to establish how SAM cells exchange
signals and shift gene expression in response to
environmental factors. Given the link between
the team’s work and tree domestication, it is
hoped that connections with tree breeders and
biotechnologists will help the group to fully
define the dormancy trait. Ultimately, the aim is
to produce research which will help to ameliorate
forest health, improve productivity, and protect
the ideal of sustainable forest industry.
ClimaTree
TREE RESPONSES TO CLIMATE CHANGE
OBJECTIVES
Plant climate research group ClimaTree
investigates how trees synchronise growth,
development and acclimation to the seasons,
and how decoupling of key environmental
cues by climate change might endanger
survival.
KEY COLLABORATORS
Professor Jaakko Kangasjärvi; Dr Jorma
Vahala, Centre of Excellence in Plant
Signalling, University of Helsinki, Finland
Professor Ove Nilsson, Centre of
Excellence, Umea Plant Science Centre
(UPSC), Sweden
Professor Hilde-Gunn Opsahl HoenSorteberg, Molecular Genetics Group,
Norwegian University of Life Sciences, Ås,
Norway
FUNDING
Research Council of Norway: Strategic
University Program ClimaTree
(nr.155041/140) and two grants from the
competitive grant scheme FRIMOLBIO,
ClimaDorm (nr.171970) and BioDorm
(nr.192013)
CONTACT
Professor Christiaan van der Schoot
Plant Cell & Developmental Biology
Norwegian University of Life Sciences
PO Box 5003, Ås, Norway
T +44 6 496 5633
E [email protected]
CHRISTIAAN VAN DER SCHOOT
pioneered the study of tiny signalling
conduits, plasmodesmata, in the apical
meristem. Since 2000 he has worked as
a professor, investigating how cell-to-cell
communication drives meristem function
and developmental transitions.
PÄIVI RINNE has been a Research Professor
since 2010. Her work focuses on the
perennial nature of trees, particularly the
biology of overwintering.
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