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Plant Signaling & Behavior 5:12, 1568-1570; December 2010; ©2010 Landes Bioscience
Two vacuole-mediated defense strategies
in plants
Noriyuki Hatsugai1,2 and Ikuko Hara-Nishimura1,*
Graduate School of Science; Kyoto University; Sakyo-ku, Kyoto; 2Research Center for Cooperative Projects; Hokkaido University; Kita-ku, Sapporo, Japan
1
Key words: plant-pathogen interaction, vacuole, hypersensitive cell death, caspase activity, vacuolar processing enzyme, proteasome
As plants lack immune cells, each cell has to defend itself
against invading pathogens. Plant cells have a large central
vacuole that accumulates a variety of hydrolytic enzymes and
antimicrobial compounds, raising the possibility that vacuoles
play a role in plant defense. However, how plants use vacuoles
to protect against invading pathogens is poorly understood.
Recently, we characterized two vacuole-mediated defense
strategies associated with programmed cell death (PCD). In
one strategy, vacuolar processing enzyme (VPE) mediated the
disruption of the vacuolar membrane, resulting in the release
of vacuolar contents into the cytoplasm in response to viral
infection. In the other strategy, proteasome-dependent fusion
of the central vacuole with the plasma membrane caused the
discharge of vacuolar antibacterial protease and cell deathpromoting contents from the cell in response to bacterial
infection. Intriguingly, both strategies relied on enzymes
with caspase-like activities: the vacuolar membrane-collapse
system required VPE, which has caspase-1-like activity and
the membrane-fusion system required a proteasome that has
caspase-3-like activity. Thus, plants may have evolved a cellular
immune system that involves vacuolar membrane collapse to
prevent the systemic spread of viral pathogens and membrane
fusion to inhibit the proliferation of bacterial pathogens.
Introduction
Plant defense systems against pathogen invasion consist of multiple layers.1 The first line of defense involves physical barriers,
such as the cuticle and cell wall, and constitutively produced antimicrobial compounds (i.e., phytoanticipins). This provides resistance against attack by a wide range of pathogens. However, some
pathogens circumvent these barriers and compounds, and attempt
to colonize the host tissue. Only certain host plants implement
a second line of defense that inhibits pathogen proliferation and
spread. This second defense system, which is enhanced by the
inherited ability to recognize certain pathogens, includes the generation of reactive oxygen species and antimicrobial compounds
*Correspondence to: Ikuko Hara-Nishimura;
Email: [email protected]
Submitted: 08/13/10; Accepted: 08/13/10
Previously published online:
www.landesbioscience.com/journals/psb/article/13319
DOI: 10.4161/psb.5.12.13319
1568
(i.e., phytoalexins). Many of these induced defense responses are
linked to the hypersensitive response (HR), which is accompanied
by rapid and localized programmed cell death (PCD) known as
hypersensitive cell death. The HR is controlled by multiple signal transduction pathways that are initiated upon recognition of
a pathogen avirulence (Avr) factor by a plant resistance (R) gene
product.2 Although the components of the signaling pathways
that lead to hypersensitive cell death have been well-documented,1
little is known about how cell death is brought about, or whether
plants share cell death mechanisms with animals.
The Role of the Vacuole in Plant Defense
Most mature cells from the vegetative tissues of plants have one
large central vacuole that occupies up to 90% of the cell volume
and has a significant impact on the physiology of the organism.
The vacuole has numerous functions, including the maintenance of turgor pressure, protoplasmic homeostasis, storage of
metabolic products, sequestration of xenobiotics and digestion of
cytoplasmic constituents. In addition to its roles in development
and metabolic adjustment, the vacuole is believed to be involved
in plant defense against microbial pathogens and herbivores.3 In
healthy plants, some phytoanticipins, such as cyanogenic glycosides and glucosinolates, are commonly stored in the vacuole as
inactive precursors, but are readily converted into biologically
active antibiotics by plant enzymes in response to pathogen infection or herbivore damage. Since the plant enzymes that activate
these phytoanticipins are already present in healthy plant tissue,
but are separated from their substrates by compartmentalization,
these phytoanticipins can be rapidly activated without requiring
the transcription of new gene products.4,5 On the other hand,
pathogenesis-related (PR) proteins and phytoalexins are not present in healthy plants, but are synthesized in response to pathogen
invasion and stored in vacuoles both at the site of infection and in
uninfected (systemic) tissues, in preparation of secondary infection.4 How these vacuolar-sequestered compounds attack invading pathogens in the cytoplasm or intercellular space remained
elusive until recently.
A Vacuolar Membrane-collapse System
for Plant Immunity
Nicotiana plants that carry the resistance gene to tobacco mosaic
virus (TMV) induce the HR in leaves infected with TMV. An
Plant Signaling & BehaviorVolume 5 Issue 12
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ultrastructural analysis and a viability assay of TMV-infected
leaves showed that disruption of vacuolar membranes occurs prior
to cell death.6-8 The disruption of vacuolar membranes releases
the vacuolar contents, including hydrolytic enzymes, directly
into the cytoplasm and leads to hypersensitive cell death, thereby
preventing viral proliferation. Vacuolar membrane collapse followed by cell death is suppressed in VPE-deficient plants.6-8 This
observation demonstrated that VPE is a key molecule in the
immune system that is associated with vacuolar membrane collapse. VPE was originally identified as a processing enzyme that
is responsible for the maturation of seed storage proteins.9 Studies
indicate that VPE is responsible for the maturation or activation
of vacuolar proteins in plants,10-14 and of lysosomal proteins in
mouse.15 We discussed the pleiotropic functions of VPE family members in a previous review.16 VPE-mediated membranecollapse is also involved in the PCD triggered by mycotoxins.
Some necrotrophic pathogens secrete toxins to kill host cells,
and thereby promote their own growth.17 Fumonisin B1 (FB1),
a mycotoxin produced by Fusarium moniliforme, induces PCD
in host plants.18 FB1-induced cell death is completely abolished
in the Arabidopsis VPE-null mutant, which lacks all four VPE
genes.19 Hypersensitive cell death is a plant defense strategy
against invading pathogens, whereas mycotoxin-induced host
cell death is an infection strategy of necrotrophic pathogens to
overcome plant defense. While these two processes appear to differ from each other, they both involve VPE-mediated vacuolar
membrane collapse.
A Membrane Fusion System for Plant Immunity
Phytopathogenic bacteria do not enter plant host cells, but proliferate in the intracellular space. Recently, we established how vacuolar
antibacterial proteases reach the intercellular space.20 In response
to infection by avirulent bacterial strains of Pseudomonas syringae
and prior to cell death, the large central vacuole of Arabidopsis
plants fused to the plasma membrane,20 resulting in the discharge
of vacuolar antibacterial protease and cell death-promoting agents
from the cells. These molecules suppressed bacterial growth and
caused hypersensitive cell death of the surrounding tissues. A
defect in proteasome function abolished the membrane fusion
associated with both disease resistance and PCD in response to
the bacteria.20 The proteasome is a large protein complex that
consists of three catalytic subunits (ß1, ß2 and ß5), and is responsible for the processing and degradation of intracellular proteins.21
Whereas some studies support the involvement of the ubiquitination pathway in cell death and disease resistance in plants,22,23
there is no direct evidence that the proteasome itself is involved
in plant immunity. Our findings demonstrate that proteasome
function is required for the activation of the immune response
that is associated with vacuolar membrane fusion. Thus, the
proteasome-regulated fusion of vacuolar and plasma membranes
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Two Types of Caspase-like Activities
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Concluding Remarks
We now have a good understanding of vacuole-mediated plant
defense strategies. Plants have evolved immune systems that protect them from invading pathogens. The central vacuole triggers
PCD, rather than just playing a supporting role as a provider of
the proteolytic enzymes that facilitate cell death. Whereas caspase-1-like activity is conferred by VPE in plants, caspase-3-like
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Acknowledgements
This work was supported by PREST and CREST of the Japan
Science and Technology Corporation, by Grants-in-Aid for
Scientific Research (nos. 16085203, 17107002 and 21687003)
from the Ministry of Education, Culture, Sports, Science and
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