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IN BRIEF
When a Tree Falls in the Woods: The Gravitropic Response in Poplar
If a tree survives a fall in the woods, the main
stem grows upward from its new position. In
angiosperm trees, this growth reorientation
is achieved via differential activity in the
cambium, which produces tension wood
on the side of the stem now facing upward
and opposite wood on the bottom side
(Ruelle, 2014). Whereas opposite wood is
phenotypically similar to “normal” wood
formed by upright stems, tension wood contains characteristic tension wood fibers with
an additional, tertiary cell wall, the gelatinous
layer (G-layer). The G-layer has been proposed to be responsible for the tensile force
that causes the stem to bend upright as it
grows (Mellerowicz and Gorshkova, 2012).
Tension wood development likely involves
hormones including auxin, ethylene, and gibberellic acid, although their precise roles have
been difficult to ascertain. Cell wall- and
hormone-related genes are differentially expressed in tension wood, but the underlying
transcriptional regulation networks are not
clear. New work from Gerttula et al. (2015)
integrates experimental and computational
data to describe the genetic and molecular
processes underlying these fascinating developmental responses.
The poplar (Populus spp) transcription
factor ARBORKNOX2 (ARK2) is expressed
in the cambium and affects wood development (Du et al., 2009). Here, Gerttula et al.
established that ARK2 is important in the
gravibending response. When placed on
their sides, poplar saplings with reduced
ARK2 levels (miRNA-ARK2) bent upward
later and to a lesser degree compared
with the wild type, whereas those overexpressing ARK2 (OE-ARK2) showed greater
gravibending (see figure). Interestingly,
the degree of gravibending was not associated with the number of tension wood fibers
formed, but instead with their rate of maturation, particularly of the G-layer. Thus, increased
ARK2 expression confers earlier maturation of
tension wood fibers, which leads to greater
bending.
Gerttula et al. found that the PINFORMED3
(PIN3) auxin transporter was uniformly
www.plantcell.org/cgi/doi/10.1105/tpc.15.00824
Gravibending response in poplar. Two weeks after being placed on their sides, poplar downregulated
in ARK2 (A) show weaker gravibending than wild-type (B) and ARK2-overexpressing (C) saplings.
Bars ¼ 10 cm. (Reprinted from Gerttula et al. [2015], Figure 1.)
distributed in the plasma membrane of
endodermal cells in the inner cortex of young
upright stems, as well as in the secondary
phloem in older stems. These same cells contained amyloplasts, suggesting that they could
act as gravisensors. When the trees were
turned on their sides, PIN3 relocated to the
ground-facing side of the cells. Importantly,
this PIN3 localization would lead to auxin
movement toward the cambium on the tension
wood side and away from the cambium on the
opposite wood side, which could trigger the
differential cambium activity that leads to opposite and tension wood production.
The authors analyzed transcriptome data
from normal wood, tension wood, and opposite wood, the latter of which has typically been
thought a passive participant in the gravitropic
responses. Their comprehensive gene regulatory network analysis placed genes into modules correlated with tension wood traits that
could be further dissected to identify putative
regulators of different aspects of graviresponse and wood formation.
This work from Gerttula et al. beautifully
applies a range of molecular techniques to
poplar to address several key questions in
wood formation and gravitropic responses in
trees. The results suggest that endodermis and
secondary phloem cells serve as the gravisensors and support a simple model in which this
sensing leads to lateral auxin transport that
underlies different responses to gravity in dif-
ferent sides of the stem. Furthermore, the
network analysis provides candidate genes
for future study and predictive models for hypothesis generation and testing in a long-lived
forest tree species.
Nancy R. Hofmann
Science Editor
[email protected]
ORCID ID: 0000-0001-9504-1152
REFERENCES
Du, J., Mansfield, S.D., and Groover, A.T.
(2 009). The Populu s homeobox gene
ARBORKNOX2 regulates cell differentiation
during secondary growth. Plant J. 60: 1000–
1014.
Gerttula, S., Zinkgraf, M., Muday, G., Lewis,
D., Ibatullin, F.M., Brumer, H., Hart, F.,
Mansfield, S.D., Filkov, V., and Groover, A.
(2015). Transcriptional and hormonal regulation
of gravitropism of woody stems in Populus.
Plant Cell 27: 10.1105/tpc.15.00531.
Mellerowicz, E.J., and Gorshkova, T.A. (2012).
Tensional stress generation in gelatinous fibres:
a review and possible mechanism based on cellwall structure and composition. J. Exp. Bot. 63:
551–565.
Ruelle, J. (2014). Morphology, anatomy and
ultrastructure of reaction wood. In The Biology
of Reaction Wood, B. Gardiner, J. Barnett,
P. Saranpää, and J. Gril, eds (Berlin, Heidelberg:
Springer-Verlag), pp.13–35.
The Plant Cell Preview, www.aspb.org ã 2015 American Society of Plant Biologists. All rights reserved.
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When a Tree Falls in the Woods: The Gravitropic Response in Poplar
Nancy R. Hofmann
Plant Cell; originally published online September 29, 2015;
DOI 10.1105/tpc.15.00824
This information is current as of June 16, 2017
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