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8/10/2019 Ros in Gravitropism
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The role of auxins in gravitropism was
discovered during early molecular
research into phytohormones. Further
research led to the conclusion that auxin
re-distribution is responsible for the
gravitropic response in roots. This
auxin effect, first proven in monocots
was found later in dicots, including
Arabidopsis . In a recent article, Jung-Hee
Joo and colleagues1 investigated the
role that reactive oxygen species (ROS)
have in this process in the monocot
Zeamays . Their results strongly suggest
that ROS are key mediators of the auxin
effect in gravitropism.
First, Joo et al . show that ROS are
produced following the gravitropic
stimulus. When maize roots are placed
horizontally to stimulate the gravitropic
response, the generation of ROS is
detected in the apex. Production of ROS is
asymmetric and occurs in the lower cortex.
The production of ROS is responsible for
the gravitropic response because the
gravitropic response is inhibited by
treatment with ROS scavengers, such as
N -acetyl cysteine (NAC). Second, the
application of auxin to roots stimulates
ROS production in root cells in planta and
in root protoplast cultures. Further
indication of a directional cause–effect
relationship between auxin–ROS and
gravitropism comes from roots treated
with N-1-naphthylphthalamic acid (NPA, an
inhibitor of auxin transport). NPA treatmentresults in inhibition of gravitropism.
Adding H2O
2can reverse the effect of NPA.
Joo et al . indicate that ROS might
work by activating kinase, but they also
take into account that other mediators of
the gravitropic response might exist,
such as calcium and inositol
(1,4,5)-trisphosphate.
The generation of ROS has already
been identified in several plant processes,
in particular, their involvement in defence
responses. Recently, calcium-dependent
protein kinases have been reported to be
important in plant defence responses. In
recent years, we have witnessed the
convergence of transduction chains from
diverse plant hormones towards common
points. A question provoked by Joo et al.’s
work is: do other plant hormones apart
from auxin have ROS as mediators of
their action?
1 Joo, J-H.et al . (2001) Role of auxin-induced
reactive oxygen species in root gravitropism.
Plant Physiol. 126, 1055–1060
Emilio Cervantes
TRENDS in Plant Science Vol.6 No.12 December 2001
http://plants.trends.com 1360-1385/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved.
556 News&Comment
Journal Club
ROS in root gravitropism: the auxin messengers?
‘…ROS are key mediators of the auxin
effect in gravitropism.’
Plant SEC14-like proteins controlling a phosphoinositide traffic jamPhosphatidylinositol transfer proteins
(PITPs) are defined by their ability to
transfer phosphatidylinositol or aphosphatidylcholine monomer between
membrane bilayers in vitro . Studies in
mammalian systems have suggested that
PITPs play a key role in controlling the
activities of various inositol lipid-
signalling pathways by regulating the
production of some phosphoinositides.
These inositol-containing phospholipids
belong to a large and heterogeneous
group of lipids with structural and key
regulatory roles in a wide range of cellular
signalling events. Recently, several plant
proteins similar to yeast PITP (SEC14)
have also been identified and characterized.
Now, Phillip Kapranov et al.1 report an
important insight into the role of PITP in
nodule development in Lotus japonicus .
With the objective of identifying new
sequence tags correlated with late stages of
symbiotic root nodule development, the
group previously characterized a novel
nodule-specific cDNA, LjNOD16. Now, they
report that the predicted Nlj16 amino acid
sequence might represent a truncated
version of a much longer protein. They
report an unanticipated relationship
between Nlj16 and the predicted protein
products of four members of a previouslyundescribed gene family that encode novel
PITP-like proteins. All the members of this
protein family share a two-domain structure
consisting of an N-terminal PITP-like
domain and a C-terminal domain with a
high level of homology to nodulin Nlj16
that is thought to work as a specific plasma
membrane-targeting domain. LjNOD16
expression in nodule tissues results from
an unusual transcriptional control that is
modulated by a nodule-specific promoter
located in an intron of the LjPLP gene.
Kapranov et al . have identified the
presence of a novel family of
developmentally regulated genes
encoding SEC14-like proteins (LjPLP) and
suggest a new key role for Nlj16 as a
dominant negative effector that can control
the expression of specific PITPs in nodules.
These data suggest a new scenario where
LjPLPs might promote phosphoinositide-
driven signalling cascades initiated at the
plant cell plasma membrane. Because theputative down regulation of LjPLP function
in nodules might be relevant to the
symbiosis between legumes and their
endosymbionts, it would be important to
uncover the range of responses involving
LjPLP genes. Does PITP perform various
functions using different molecular
mechanisms? Does LjPLP over-expression
affect nodule development? Kapranov
etal. have identified a new family of PITPs
that have the potential to modulate
inositol lipid-signalling pathways involved
in the process of symbiosis between
legumes and their endosymbionts in
Lotus japonicus , an important step in
understanding how plants coordinate
developmental processes related to
symbiotic nitrogen fixation.
1 Kapranov, P. et al. (2001) Nodule-specific
regulation of phosphatidylinositol transfer
protein expression inLotus japonicus .
Plant Cell 13, 1369–1382
Joaquín Medina
‘…key role for Nlj16 as a dominant negative
effector that can control the expression of
specific PITPs in nodules.’
