Tree Physiology 34, 915–918
doi:10.1093/treephys/tpu082
Commentary
Physiological trade-offs in the complexity of pine tree
defensive chemistry
Luis Sampedro1
Misión Biológica de Galicia—Consejo Superior de Investigaciones Científicas, E36080 Pontevedra, Galicia, Spain (genecolpines.weebly.com); 1Corresponding author
([email protected])
Received July 27, 2014; accepted August 24, 2014; handling Editor Danielle Way
Like all organisms on Earth, trees must finely tune the relative
allocation of resources to their living functions (namely growth,
maintenance, defence and reproduction), seeking to optimize
the costs and benefits (Bazzaz et al. 1987). As resources are
limited and those allocated to one trait cannot be allocated to
another, conflicts in resource allocation may result in tradeoffs among different functions or traits (Agrawal et al. 2010).
Such trade-offs might emerge as negative phenotypic correlations between pairs of traits with a shared source (reviewed
by Saeki et al. 2014). Thus, patterns of phenotypic covariation
(i.e., individual-based correlations) between traits help reveal
possible conflicts in resource allocation and shared regulatory
processes. One example in tree ecology and physiology is
the relative allocation of resources to chemical defences (e.g.,
Keinanen et al. 1999, Koricheva et al. 2004, Donaldson et al.
2006, Agrawal 2011). Because of their life-history characteristics (such as being long-lived, large and forming extensive
and stable populations) trees usually support a particularly
diverse, extensive and temporally variable community of herbivores and pathogens. The selective pressure imposed by their
antagonists has led to the evolution of effective resistance
mechanisms, which include both constitutive and inducible
defences (Zangerl and Bazzaz 1992). Constitutive defences,
which are permanently expressed irrespective of the incidence of herbivores and pathogens, represent the first line of
resistance. In contrast, induced resistance traits are activated,
synthesized or mobilized in response to biotic challenges or
cues of biotic damage. Plant resistance based on inducible
defences, although energy saving, is a risky strategy as its
benefits are based on the reliable identification of biotic cues
(Karban 2011). Moreover, a plant could remain vulnerable for
a period of time while induced defences are not activated or
synthesized (e.g., Gómez et al. 2010). Therefore, plants need
to combine a defensive system based on constitutive defences
(which are omnipresent but have high associated costs) with
induced defences (where the delay in activation is compensated for by the reduced cost associated with its production).
The overlap of both strategies in space and time is assumed
to be crucial for an efficient defence strategy (Cipollini and
Heil 2010). Although there is an increasing body of theoretical predictions about this framework, little is known about the
phenotypic integration of defensive investment in long-lived
plants, whose life-history determinants (e.g., maintenance
costs, delayed reproduction, multiple reproductive events and
extent of life cycles) largely differ from those of herbaceous
and annual plants examined to date.
In this issue, Villari et al. (2014) present a case study of
defence allocation patterns in pine trees. Constitutive and
inducible defensive chemistry in pine trees is based on high
concentrations of a variable array of carbon-based secondary
compounds of a diverse chemical nature, namely terpenoids
and phenolic compounds (Krokene et al. 2003, Keeling and
Bolhmann 2006, Mumm and Hilker 2006). Villari et al. challenged individuals of a mature Scots pine alpine population
growing under extreme environmental stress with bark plugs
inoculated with two fungi associated with pine bark beetles
(see Villari et al. 2012). Three weeks after inoculation, samples
of bark and phloem tissues of induced and control trees were
subjected to metabolite profiling of terpenoids and phenolics.
The authors looked for phenotypic, individual-based correlations that provide information on possible resource-derived
trade-offs between investment in secondary chemistry and
© The Author 2014. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
916 Sampedro
growth rates, between different chemical compounds and
between constitutive and inducible variation in defensive
chemistry.
Phenotypic correlation between growth rate, age
and investment in defensive chemistry
A major finding of Villari et al.’s study is that the tree-ring growth
rate over the past 10 years was a good predictor of constitutive investment in terpenoids (for both total terpenoids and
most of the individual terpenoids identified). However, tree-ring
growth was not related to the concentration of total phenolics
or individual phenolic compounds. In addition, induced variation
of most individual and total terpenoids and phenolics was not
related to tree-ring growth. In other words, Villari et al. (2014)
found no evidence of allocation conflicts between primary and
secondary metabolism, but rather, they uncovered a positive
relationship between growth and defensive investment under
those growth-limiting conditions, as predicted by the extended
Growth–Differentiation Balance Hypothesis (GDBH) (Herms
and Mattson 1992). From an evolutionary point of view, the
theoretical background predicts that optimal relative investment
in defences should be greater in slow-growing plant species
or lineages adapted to stressful, growth-limiting environments
due to greater construction costs (as posed by the Resource
Availability Hypothesis by Coley et al. 1985, reviewed by Endara
and Coley 2011). However, from a physiological point of view,
the GDBH predicts that optimal investment in defences at the
individual level is not a linear function of carbon assimilation
and growth rates across the entire growth rate range of a species (Herms and Mattson 1992).
Pine trees are a good model for studying these relationships,
as they have differentiated secretory organs for production
and storage of chemical defences. Terpenoids and phenolic
compounds are secreted and stored in specialized tissues: the
resin duct network of the xylem and cortex and the polyphenolic parenchyma cells in the phloem, respectively (Franceschi
et al. 2005). Damage signalling triggers physiological processes and changes in the cambium, leading to swelling and/or
differentiation of more or greater xylem resin ducts and polyphenolic parenchyma cells (Krokene et al. 2003, Hudgins and
Franceschi 2004). These tissues remain functional and active
for a time, serving as a footprint of previous induced responses
or past damage. This is advantageous for retrospective studies
linking growth rates and net (current + past) defensive investment. In this sense, the results of Villari et al. agree with recent
papers suggesting that there is no evidence of phenotypic
resource-based trade-offs between growth and resin-based
defences in pine trees (Ferrenberg et al. 2014, RodríguezGarcía et al. 2014).
On the other hand, studies characterizing the concentrations
and profiles of both terpenoids and phenolics are scarce, making
Tree Physiology Volume 34, 2014
this paper particularly welcome. While numerous studies have
focused on terpenoids and phenolic compounds in conifers,
most of these studies have been devoted to examining the chemical ecology of a single type of compound (e.g., single groups of
terpenes, phenolics or alkaloids). The relative lack of more comprehensive studies may be due to the lack of facilities needed for
the analysis of chemicals of different nature, as well as the expertise required for data analysis and interpretation of results. The
complexity and diversity of defensive chemistry in trees is vast,
and papers covering the entire array of chemicals (such as that
from Villari et al.) are valuable for understanding the integration of
tree defences and their environmental determinants.
An important point for fully understanding resource-derived
trade-offs is that the limiting source of carbon constraining
the allocation must be clearly identified (recently reviewed by
Saeki et al. 2014). In the case of pine defensive chemistry,
the energy source for defensive chemistry is likely a combination of current assimilates and non-structural stored carbohydrates. Identifying the relative contribution of these two
sources to pine defences may have important implications for
plant physiology and ecology (Martinez-Vilalta 2014, Saffell
et al. 2014). Although this topic has been studied at the cellular
level (e.g., Affek and Yakir 2003, Schnitzler et al. 2004), more
research is needed about the relative contribution of current
carbon assimilation and stored carbohydrates to the synthesis
of both constitutive and induced phenolics and terpenoids at
the whole tree level (but see Goodsman et al. 2013). Advances
in our understanding of this topic would help us to understand
how environmental factors such as those leading to carbon
starvation and altered carbon storage (e.g., water availability
and winter temperature) will affect current and future defensive investment and capabilities in pine trees, subsequently
impacting tree survival and performance in a changing climate.
Phenotypic correlations between constitutive
and inducible variation
Villari et al. (2014) also explored the functional relationships
between the constitutive concentration and inducible variation
of pairs of compounds. Their analyses resulted in the classification of five response types, depending on the sign, shape
(linear or quadratic) and slope of the response to their experimental treatments. Studying the pattern of phenotypic covariation between pairs of compounds is a valuable approach, as
the functional linkage between resource-related traits is not
necessarily linear, because gains in one trait could lead to multiplicative effects in the other. Thus, non-linear relationships are
expected and must be explored to characterize the physiological trade-offs, as stressed by Saeki et al. (2014) in their recent
review about concepts and methodology on this topic.
As constitutive and induced resistance cannot be maximized at the same time, and lineages expressing high levels
Physiological trade-offs in pine tree defensive chemistry 917
of constitutive resistance would obtain limited fitness benefits
from expressing induced resistance, an evolutionary trade-off
between both strategies is predicted (e.g., Kempel et al. 2011,
but see Morris et al. 2006 for a critical review of methodological approaches). This type of negative genetic correlation
between traits may impose evolutionary constraints on the
simultaneous improvement of both strategies and thus influence evolutionary trajectories within populations, and as well
as at the macroevolutionary level (Sgrò & Hoffmann 2004, but
see Moreira et al. 2014). For this purpose, studies based on
phenotypic correlations, although valuable, allow for only limited evolutionary inference compared with studies based on
genotypic or family correlations, which provide information
on heritable, genetic-based trade-offs (reviewed by Agrawal
et al. 2010). In long-lived trees, this type of research has been
mainly performed using young individuals (e.g., Sampedro
et al. 2011, Carillo-Gavilán et al. 2012, Moreira et al. 2013),
while the results from Villari et al. (2014) should encourage
further research on this topic at the adult stage.
Covariation between individual secondary
compounds
Villari et al. (2014) found no phenotypic correlations between
the concentration of total phenolics and total terpenoids, and
they detected no significant correlations between individual
chemical compounds. Importantly, both results suggest that
there are no conflicts or substrate competition in the alternative allocation to the phenylpropanoid and isoprenoid defensive
pathways at the individual level. Such an absence of negative
correlations between defensive chemicals at the individual
level, or even a positive covariation, is not uncommon in plants
(Koricheva et al. 2004).
Due to the vast diversity in quality and concentration of pine
chemical defences (i.e., Iason et al. 2011), and provided that
defensive chemicals do not function in isolation, it seems that
the next step is to take advantage of multivariate bioinformatic
tools for the analysis of these types of large databases. Further
efforts that combine the analytical approaches of evolutionary
ecology, chemical ecology and tree physiology are required.
The analysis of databases covering hundreds of compounds
would greatly benefit from network analysis and whole plant
metabolome approaches. A recent article by Harding et al.
(2014) and some recent reviews on this topic (Kliebenstein
2014, Moore et al. 2014) illustrate these points, and they
encourage new analytical approaches to understanding the
complexity and common regulatory networks in constitutive
and induced allocation of conifer defences. Advanced analysis
of exhaustive chemical studies of adult trees in forest genetic
trials with family or population structure would be crucial for
understanding the multivariate structure of plant defences in
long-lived plants, as well as the metabolic integration of costs
and benefits of individual chemical species and their patterns
of context dependency.
In summary, Villari et al. (2014) provide new ideas and a valuable model for new approaches that should motivate scientists to
put more effort into the fine-scale characterization of the diversity of secondary metabolites in response to biotic challenges.
These approaches would benefit from advanced multivariate
analyses and experimental designs using plant material with a
known genetic background. This would help advance our knowledge of the phenotypic integration and physiological regulation
of tree defensive chemistry, as well as the evolutionary ecology of defences in long-lived plants. This type of research has
usually been performed on young individuals under controlled
conditions, and it is clear that more effort is needed to examine
adults under field conditions. Such research would be challenging, given the difficulties in establishing and maintaining mature
field stands of known genetic backgrounds. Thus, we should
encourage more intense interactions between forest tree breeders and tree physiologists, as future research in this field should
utilize the long-term networks of progeny and provenance trials
established by national and provincial forest services.
Acknowledgments
The author thank Rafael Zas and Xoaquin Moreira, Jose
Climent, Luis Santos, Jordi Voltas and other colleagues in the
FENOPIN group (http://www.genecolpines.weebly.com) for creative debating, questions and answers about defensive strategies in pine trees. The author also thanks the editor and two
reviewers for their suggestions.
Conflict of interest
None declared.
Funding
Financial support was obtained from the Spanish National
Research Grants (competitive grant AGL2012-40151 –
FENOPIN) co-financed by EU-FEDER.
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