Download On the Inside - Plant Physiology

On the Inside
PICKLE and Gibberellin
Response Pathways
sponsivity to GA and elevated levels of
bioactive GAs.
PICKLE (PKL) is a CHD3 chromatinremodeling factor that is necessary for
the repression of embryonic traits in
germinating seedlings. For example, in
Arabidopsis, the primary roots of pkl
seedlings remain swollen and green
(like pickles) after germination and
thus, remain “fixed” in an embryonic
state of development. The application
of gibberellin (GA) to pkl seedlings represses the embryonic state, whereas
decreasing the level of GA in germinating seeds increases the penetrance of
the pickle root phenotype. The adult
shoot phenotype of pkl plants is also
reminiscent of a plant defective in GA
response: pkl plants are dark green,
dwarfed, and require a longer time to
flower. These phenotypes suggest that
PKL itself may play a role in mediating
GA-dependent responses. In this issue,
Henderson et al. (pp. 995–1005) characterize the ability of PKL and GA to
repress embryonic traits, and they reexamine the role of PKL in mediating
GA-dependent responses. They report
that PKL is expressed throughout the
seedling during imbibition and serves
to repress expression of the embryonic
state. Although ABA and GA act antagonistically with respect to germination, the expression of embryonic identity in germinating pkl seeds is much
less responsive to application of ABA.
In addition, they report that although a
mutation of SPY, a well-characterized
negative regulator of GA-dependent
responses, completely suppresses the
germination defect of a plant defective
in GA biosynthesis, it only slightly suppresses the derepression of embryonic
traits that occurs when GA biosynthesis is perturbed in pkl seedlings. These
observations indicate that the GA response pathway that mediates repression of embryonic traits in pkl seedlings
appears distinct from previously characterized GA response pathways. Finally, the authors show that pkl plants
exhibit the phenotypic hallmarks of a
plant that is defective in the ability to
respond to GA, including reduced re-
The inoculation of roots with plant
growth-promoting
rhizobacteria
(PGPR) triggers induced systemic resistance (ISR). ISR by benign microorganisms in close proximity with roots
is distinct from systemic acquired resistance (SAR), in which the response
is triggered by pathogenic microorganisms associated with the aerial
portions of the plant. In this issue,
Ryu et al. (pp. 1017–1026) examine
whether volatile organic compounds
associated with rhizobacteria are involved in the activation of ISR in Arabidopsis. They report that the exposure of Arabidopsis seedlings to
bacterial volatile blends from strains
of Bacillus subtilis and Bacillus amyloliquefaciens significantly reduced the severity of disease caused by the bacterial pathogen Erwinia carotovora.
Chemical analysis of the bacterial volatile emissions revealed the presence
of a series of low-Mr hydrocarbons including the growth promoting chemical 2R,3R-butanediol. The exogenous
application of a racemic mixture of
2,3-butanediol was also found to trigger ISR in Arabidopsis. Transgenic
lines of B. subtilis that emitted lower
levels of 2,3-butanediol were less effective than wild type in protecting
Arabidopsis against pathogen infection. Using transgenic and mutant
lines of Arabidopsis, the authors provide evidence that the signaling pathway activated by volatiles is dependent on ethylene, and independent of
the salicylic acid or jasmonic acid signaling pathways. This study is the
first report of ISR elicited by volatile
chemicals released from PGPR and ascribes a new role for bacterial VOCs in
triggering plant defense responses.
www.plantphysiol.org/cgi/doi/
10.1104/pp.900104.
Alkamides are secondary metabolites comprising over 200 related com-
Bacterial Volatiles Induce
Systemic Resistance
Alteration of Plant
Development by Alkamides
pounds that have been found in as
many as ten plant families. Many of
the plant species containing these
compounds have been used in traditional medicines where they are recognized for their pungent taste and
for causing numbing and salivation.
Insecticidal properties have also been
attributed to some alkamides. Much
less clear, however, is whether alkamides also play a role in plant growth
and differentiation. In this issue,
Ramı́rez-Chávez et al. (pp. 1058–1068)
investigated the effects of affinin, a
naturally occurring alkamide in
plants, and two of its derivatives on
plant growth and early root development in Arabidopsis. They report that
treatments with affinin in the range of
10–6 to 10–4 m alter both shoot and root
biomass production. Developmental
alterations induced by alkamides included greater formation and emergence of lateral roots and increased
root hair elongation. Low concentrations of affinin enhanced primary root
growth and root hair elongation,
whereas higher concentrations inhibited primary root growth by reducing
both cell proliferation and cell elongation. Although the effects of alkamides were similar to those produced
by auxins on root growth and cell parameters, the ability of the root system
to respond to affinin was found to be
independent of auxin signaling. Alkamides appear to be a new class of
plant growth promoting substances
with a significant impact on root
development.
Expression of Potassium
Transport Genes in
Arabidopsis
Although five major families of K⫹
transporters have been identified in
Arabidopsis, the contributions of
many of these transporters to cellular
or whole plant K⫹ homeostasis is unclear. The largest gene family of K⫹
transporters in Arabidopsis, consisting of 13 genes, is the AtKT/KUP family. These transporters were originally
identified in Escherichia coli as K⫹ uptake permeases and named KUPs.
Later homologous genes (HAKs) were
from on
July 31, 2017 - Published
by www.plantphysiol.org
Plant Physiology, March 2004, Vol.Downloaded
134, pp. 881–882,
www.plantphysiol.org
© 2004
American Society of Plant Biologists
Copyright © 2004 American Society of Plant Biologists. All rights reserved.
881
identified in a soil borne fungus. Only
recently have these transporters been
identified and studied in plants. The
manner in which KT/HAK/KUP
transporters contribute to K⫹ transport and homeostasis in plants is not
understood in part because the membrane localization of these proteins is
unknown. It has been suggested that
some of the KT/HAK/KUPs mediate
plasma membrane uptake, while others are involved in vacuolar transport.
Ahn et al. (pp. 1135–1145) have used
RT-PCR to determine the spatial and
temporal expression patterns of each
AtKT/KUP gene across a range of organs. Many AtKT/KUPs were expressed in roots, leaves, siliques, and
flowers of plants grown under K⫹ sufficient conditions in hydroponic culture. AtHAK5 was the only gene in
this family that was up-regulated
upon K⫹ deprivation and rapidly
down-regulated with resupply of K⫹.
Ten AtKT/KUPs were expressed in
root hairs, suggesting an important
role for root hairs in K⫹ uptake. The
growth and Rb⫹ uptake of two mutants lacking root hairs demonstrated
the importance of root hairs in K⫹ uptake. Seven genes encoding AtKUP
transporters were expressed in E. coli,
and their K⫹ transport functions were
confirmed.
Measuring Ca2ⴙ and pH
Dynamics Non-Invasively
Calcium concentrations and pH and
in the apoplast have been measured in
many plants by selective electrodes,
by collecting apoplast fluid or by impermeable fluorescent dyes. However,
little in vivo information on the
change of apoplastic pH and [Ca2⫹] is
available from whole and undisturbed
(i.e. not infiltrated) plants under abiotic stress. Such knowledge can help
to understand the role of the apoplast
in stress responses in more detail. In
this issue, Gao et al. (pp. 898–908)
present a novel approach to this pursuit. To make noninvasive in vivo
measurements of intra- and extracellular ion concentrations, they have
produced transgenic Arabidopsis expressing pH and calcium indicators in
the cytoplasm and in the apoplast.
They expressed modified pHluorins
and aequorin as fusion proteins in the
cytoplasm of Arabidopsis. For studying extracellular ion dynamics, they
also targeted pHluorins and an aequorin variant with low calcium affinity as fusion proteins to the apoplast
by means of an Arabidopsis chitinase
signal sequence. A series of environmental stimuli, particularly salt and
osmotic (mannitol) stress, were used
to demonstrate that the produced selfreporting plants are able to monitor
changes of pH and [Ca2⫹] in the cytoplasm and in the apoplast. Their results suggest that osmotic stress and
salt stress are differently sensed and
processed in plant cells.
Evaluation of Monocot and
Eudicot Divergence Using
the Sugarcane
Transcriptome
Angiosperms originated approximately 200 million years ago (MYA)
and subsequently diverged into several
lineages, which further diversified to
form the approximately 250,000 angiosperm species known today. One approach to obtain information about genome diversity among angiosperms is
through comparative analysis of the
available angiosperm sequences,
which include the Arabidopsis and rice
(Oryza sativa) genomes and the large
amount of expressed sequence tags
(ESTs) that have been produced from
several other monocots and eudicots.
In an effort to understand the monocoteudicot divergence, Vincentz et al.
(pp. 951–959) have compared over
40,000 sugarcane (Saccharum officinarum) consensus sequences assembled
from 237,954 ESTs with the protein and
DNA sequences from other angiosperms, including the genomes of Arabidopsis and rice. A set of sugarcane
sequences was found to be conserved
among angiosperms, but was missing
in Arabidopsis. This finding suggests
that the corresponding genes were
present in the ancestor of monocots
and eudicots and were subsequently
lost in Arabidopsis. These data are consistent with the idea that differential
gene loss is an active process in the
evolution of angiosperm genomes. Approximately two thirds of the sugarcane transcriptome have similar sequences in Arabidopsis. These
sequences may represent a core set of
proteins or protein domains that are
conserved among monocots and eudicots, and probably encode for essential
angiosperm functions. The remaining
sequences represent putative monocotspecific genetic material, half of which
were found only in sugarcane. These
monocot-specific cDNAs represent either novelties, or in many cases, fast
evolving sequences that diverged substantially from their eudicot homologs.
The wide comparative genome analysis presented here provides information on the evolutionary changes that
underlie the divergence of monocots
and eudicots.
Peter V. Minorsky
Department of Natural Sciences
Mercy College
Dobbs Ferry, New York 10522
882
Downloaded from on July 31, 2017 - Published by www.plantphysiol.org
Copyright © 2004 American Society of Plant Biologists. All rights reserved.
Plant Physiol. Vol. 134, 2004