Ros in Gravitropism

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

ecervant@gugu.usal.es

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

medina@inia.es

‘…key role for Nlj16 as a dominant negative

effector that can control the expression of

specific PITPs in nodules.’