Tree Physiology 36, 1–5
doi:10.1093/treephys/tpv120
Commentary
How regulation of phloem transport could link potassium
fertilization to increased growth
Johannes Liesche1,2
1College
of Life Sciences, Northwest A&F University, Yangling, Shaanxi 712100, China; 2Corresponding author ([email protected])
Received September 10, 2015; accepted October 13, 2015; published online November 26, 2015; handling Editor Danielle Way
Identifying the factors that limit growth is a pre-requisite for
using plant resources more efficiently, especially under changing
climate conditions. For the optimization of tree growth in plantations or re-forestation areas, a detailed understanding of this
complex regulation is required. In this issue, ­Epron et al. (2015)
make a significant contribution toward this goal by demonstrating how potassium (K) fertilization acts to increase growth of
field-grown Eucalyptus grandis trees. Their results point to the
central role that carbon transport plays in the regulation of tree
growth. In the same way, previous studies provided evidence
that ozone (­K asurinen et al. 2012), elevated CO2 (­Mikan et al.
2000) and nitrogen (­Högberg et al. 2010) affect growth by
altering the flux of carbon to a particular plant compartment, as
do stress and other factors that increase the carbon storage rate
(­Babst et al. 2005, ­Sala et al. 2012). Nevertheless, we still have
little knowledge of the specific conditions under which carbon
transport limits growth or how it could be targeted to increase
carbon-use efficiency.
Carbon, in the form of carbohydrates, is transported through
the highly specialized cells of the phloem from source organs
like leaves to sink organs like roots or fruit. The osmotic pressures resulting from different concentrations of carbohydrates
in source and sink phloem translate into a hydrostatic pressure
gradient that drives mass flow of molecules toward sink tissues. In trees, the phloem is often seen as a passive distribution network, whose function depends only on supply and
demand in the different parts of the plant. However, the identification of sucrose transporters that can influence phloem
loading (­Payyavula et al. 2011) and the demonstration of plasticity in phloem transport speed and volume (­Windt et al.
2006) indicate the potential for an active regulation of phloem
transport.
In order to assess the effect of K on whole-plant carbon allocation and especially phloem transport, ­Epron et al. (2015)
­followed the distribution of isotopic carbon in K-fertilized and
unfertilized E. grandis trees over one growth season. At the
same time, the influence of water availability was tested by
excluding rainwater throughfall for half of the trees. A 13CO2
pulse was applied to leaves and the 13C-signal traced in the
12 m high field-grown trees by detectors positioned along the
stem and at the roots. The resulting data on transport speeds
and residence times were complemented by quantification of
growth parameters, carbon dynamics in different parts of the
plant and the analysis of phloem anatomy. Potassium availability
was found to have a strong impact on phloem transport as the
increased growth of K-fertilized trees was accompanied by
higher carbon export from leaves and increased speed and volume of transport in the phloem. However, also the carbon supply in the foliage was increased through photosynthetic
carbohydrate production and the measurement of respiration in
the roots indicated higher sink demand. This leads to the question of whether K impacts phloem transport directly with the
consequence of changes in source and sink or if it is the other
way around, the effect of K on source and sink activity leading
to altered phloem transport. While providing a complete picture of whole-plant carbon allocation, current isotope tracing
approaches do not provide the necessary detail at the cellular or
molecular level to solve this question. Nevertheless, some of the
observations reported by ­Epron et al. (2015) are indicative of a
direct control of phloem function by K availability. In the following, the results are discussed with regard to the potential regulatory mechanisms of phloem transport.
The higher initial carbon export rate from leaves of K-fertilized
trees could be caused by a number of processes either
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2 Liesche
­ pregulating the driving force for loading of carbohydrates into
u
the phloem or decreasing the resistance for phloem loading
(­Figure 1). The loading of carbohydrates into the long-distance
transport tissue, the phloem, is assumed to be passive symplasmic in Eucalypts and most angiosperm and gymnosperm tree
species (­Davidson et al. 2011, ­Liesche et al. 2011). This loading type is characterized by a high degree of symplasmic coupling between the cells of the pre-phloem pathway and the
absence of active carbohydrate accumulation in the companion
cell/sieve element complex. To move carbohydrates into the
phloem, a higher concentration in mesophyll cells compared
with the sieve elements is thought to enable diffusion of carbohydrates into the phloem (­Zhang et al. 2014).
This means that processes that alter the mesophyll–sieve element concentration gradient can regulate phloem loading and,
thereby, carbon export. In the study by ­Epron et al. (2015) the
effect could be the result of higher carbohydrate production in
the leaf mesophyll and/or higher consumption in the carbon
sinks. However, it could also be speculated that increased carbon export is actually the cause of changes in source and sink,
and the higher K availability influences other factors that regulate carbon export (Figure 1). While termed passive loading,
there are indications that active transport processes are involved
in the control of the cytosolic sucrose concentration in the
­different cells. In poplar, a vacuolar sucrose transporter was
shown to have a strong effect on the phloem loading capability
(­Payyavula et al. 2011). Phloem loading could also be controlled by dynamic regulation of plasmodesmata, thereby
changing the transport capacity of the symplasmic pathway,
which has been demonstrated in phloem unloading in pea roots
(­Schulz 1995) but has not been shown in relation to phloem
loading. Nevertheless, the conspicuous plasmodesmata seen,
for example, in Populus deltoides Bartr. ex Marsh. (­Russin and
­Evert 1985), are indicative of the potential for permeability
regulation (­Schulz 1999). Sucrose conversion to and from
starch in the plastids could also help to balance cytosolic
sucrose concentration, although starch accumulation follows
strictly diurnal patterns and offers therefore no option for shortterm acclimation. Furthermore, water movement could have a
major impact on loading strength if it moves intracellularly in the
opposite direction to carbohydrate diffusion (­Münch 1930,
­Schulz 2015). Aquaporins could be targets to control the distribution of water going from vasculature to stomata between
the apoplasmic pathway in the cell-wall space and the symplasmic pathway (­Laur and ­Hacke 2014).
Epron et al. also report a major effect of K fertilization on
carbon transport inside the phloem. The carbon transport speed
in the stem phloem was twice as high for K-fertilized trees compared with K-deficient control trees. In addition, the authors
observed a higher cross-sectional area of sieve elements in the
stems of K-fertilized trees, which indicates a higher transport
capacity. Since carbon concentration in the phloem appears
Tree Physiology Volume 36, 2016
unchanged (­Battie-Laclau 2014, ­Epron et al. 2015), the higher
speed and capacity should translate into a significantly higher
amount of carbon being transported from source to sink. Absolute quantitative data would be necessary to show how much
carbon actually reaches the sink since not all sieve elements are
necessarily active. Trees, like herbs, seem to keep a surplus of
transport capacity by not using all available sieve elements
(­Windt et al. 2006, ­De S
­ chepper et al. 2013).
Phloem transport speed in trees has been in focus recently
with data now available for a wide range of species (­Liesche
et al. 2015). However, few reports showed how the speed in
trees of the same species is influenced by growth conditions, as
done by ­Epron et al. (2015) for K availability. What causes the
higher speed observed in K-fertilized trees? The viscosity of
phloem sap, which could change with different K concentrations,
does not seem to play a role, since overall concentration of
osmotically active compounds in the phloem sap was not
changed significantly between K-fertilized and K-deficient trees
(­Battie-Laclau 2014). Different sieve element length and/or
diameter would also influence transport speed (­Jensen et al.
2012a), but anatomical analysis indicates that mostly the number of sieve elements and not their structure vary within one
species (­Jyske and ­Hölttä 2015, ­Ronellenfitsch et al. 2015).
This indicates that the higher transport speed is caused by an
increase of the hydrostatic pressure potential in the phloem,
either by K stimulating carbohydrate utilization in roots, or by
increasing the amount of carbohydrates loaded into the phloem
along the transport path (Figure 1). While the influence of sink
strength on phloem function in trees has been demonstrated, the
role of re-loading is still under discussion.
As in herbaceous plants, experiments in trees have shown that
up to 90% of carbohydrate molecules are unloaded from the
stem phloem while others are re-loaded (­Babst et al. 2005, ­De
­Schepper et al. 2013, ­Epron et al. 2015). This mechanism is
usually referred to as a leakage-retrieval mechanism, although
‘leakage’ is a somewhat misleading term for carbohydrates
­leaving the sieve element–companion cell complex either by
crossing the plasma membrane with the help of passive channels
or active transporters, or diffusion through plasmodesmata.
Sucrose transporters, which were found to be responsible for
re-­loading of carbohydrates into the phloem of herbaceous
plants (­Gould et al. 2012), were also found in the stem of s­ everal
tree species although their activity has not been directly linked to
phloem function so far (­Decourteix et al. 2006, ­Payyavula et al.
2011). If they were involved in re-loading, higher K levels could
potentially lead to their upregulation, which was shown to be the
case in Arabidopsis thaliana (­Gajdanowicz et al. 2011). Indeed,
an active transporter-mediated re-loading of carbohydrates was
also hypothesized to amplify the source–sink concentration
potential for efficient phloem transport over the long distances
encountered in trees (­Steppe et al. 2015). However, the available
experimental and theoretical data rather indicate that trees do not
Regulation of phloem transport 3
Figure 1. Potential factors in the regulation of carbohydrate loading into the phloem and transport along the phloem path. In many tree species, the
sieve element–companion cell complex is symplasmically coupled to their surrounding cells via plasmodesmata. The amount of carbohydrates being
loaded into the phloem is directly influenced by processes that change the diffusion potential between the mesophyll in the leaf or parenchyma cells in
the stem and the phloem. This means that an increase in cytosolic carbohydrate concentration in these cells, through carbohydrate production (1) or
re-mobilization from storage (2), lead to an increase in phloem loading. In addition, the loading capacity could be dynamically regulated via changes
in the plasmodesmata-mediated cell-wall permeability (3) between the cells of the pre-phloem pathway. The mechanism of unloading and re-loading
along the phloem path remains to be elucidated. Sucrose transporters could pump carbohydrates into the phloem, while unloading happens through
(passive?) membrane carriers and/or plasmodesmata. Upregulation of sucrose importers (4) or reduction in plasmodesmata permeability and sucrose
exporter activity (5) would lead to increased carbohydrate levels in the phloem. The increased osmotic potential would lead to additional water uptake,
thereby enhancing phloem sap flow. Higher water availability itself is a factor that can increase phloem transport by facilitating hydrostatic pressure
build-up (6). In the leaf, the aquaporins in the plasma membrane could control whether water moves intracellularly from vasculature to stomata, thereby
slowing down carbohydrate diffusion toward the phloem, or, instead, moves in the cell-wall space (7). Arrows indicate how carbohydrate and water
movement could increase phloem loading/re-loading.
depend on additional driving force along the pathway (­Münch
1930, ­Sevanto et al. 2003, ­Jensen et al. 2012b). ­Furthermore,
while the sieve element–companion cell complex in herbaceous
plants is symplasmically isolated, a pre-condition for efficient reloading, this is not necessarily the case for trees. At least in a
large number of gymnosperms, the a­ bundance of plasmodes-
mata at the interface between sieve elements, Strasbuger cells
and ray parenchyma cells indicates a symplasmic phloem unloading pathway (­den ­Outer 1967, ­Sauter et al. 1976). Detailed anatomical data to assess sieve element connectivity in the stem of
angiosperm trees are lacking. Ideally, live-cell measurements to
quantify how much carbohydrate can actually move out of/into
Tree Physiology Online at http://www.treephys.oxfordjournals.org
4 Liesche
the phloem would provide evidence whether leakage-retrieval in
trees is a mostly passive process or facilitated by membrane
transporters. This would not only be important for the identification of regulatory factors, but also for our general understanding
of the mechanism of phloem transport in trees. If indeed carbohydrates can freely diffuse between phloem cells and living cells
in the bark and wood, then, resembling the situation in the leaf,
parenchyma cells would need to have a cytosolic concentration
at least matching that of the phloem. These cells would thereby
provide an enormous buffer between source and far-away sinks
that might eliminate the need for a high leaf-to-root osmotic pressure potential.
The identification of factors that regulate carbon transport in
the phloem and the amount transported to a specific sink
requires more detailed carbon tracing experiments. To investigate the contribution of active re-loading along the phloem,
sucrose transporter inhibitors (­Giaquinta 1976) or cold girdling (­De ­Schepper et al. 2013) can be used, or experiments
­performed on plants with reduced sucrose transporter expression. Importantly, field studies have to be complemented with
high sample number experiments under controlled conditions
(­Liesche et al. 2013). In addition to carbon tracing experiments,
magnetic resonance imaging (MRI) is a powerful tool to measure
the phloem transport speed and the volume in vivo (­Windt et al.
2006). Magnetic resonance imaging has the advantage of providing visual information of flow, directly indicating the crosssectional area of actively translocating sieve elements at the time
of analysis (­Windt et al. 2006). Furthermore, how the transport
capacity of the phloem is influenced by specific factors can be
investigated by studying the formation of sieve elements with
synchrotron X-ray microtomography in connection with metabolic analysis (­Jyske et al. 2015). In order to find out if symplasmic transport capacity between phloem and surrounding cells is
dynamically regulated, live-cell microscopy can be used measure
diffusion coefficients for inter-cellular transport through plasmodesmata (­Liesche and ­Schulz 2012).
Without doubt, the rapidly expanding genomic resources and
molecular techniques available for tree research will greatly
facilitate the identification of proteins with a function in the
regulation of phloem loading and transport. Continuing investigation of candidate proteins and eventual forward genetic
screens will uncover complete signaling cascades that provide
the molecular link between a factor like increased K availability
and the observed response of higher carbon transport rate. The
combination of molecular work and large-scale studies, like the
one performed by ­Epron et al. (2015), will provide the link
between factors such as nutrient or water availability and
growth. Data from future studies will not only inform biotechnological approaches to optimize carbon-use efficiency in trees,
but also help to uncover the basic principles behind carbon
transport in trees.
Tree Physiology Volume 36, 2016
Acknowledgments
I thank Robert Turgeon and Alexander Schulz for helpful comments.
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