Autophagy and plant innate immunity

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Autophagy and plant innate immunity

• ?? ???

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1?Plant innate(???,???) immunity is often
associated with specialized programmed cell death
at or near the site of pathogen infection.?
2?role of autophagy in plant innate
immunity? autophagy is observed in healthy and
dying plant cells,whether autophagy plays a
protective or a destructive role during an immune
response?
3
autophagy, an evolutionarily conserved process of
bulk protein and organelle turnover, was shown to
play an important role in limiting cell death
initiated during plant innate immune responses.
Autophary For a cell to maintain
homeostasis, there needs to be constant turnover
of macromolecules to adjust to the cellular
changes necessitated by responses to the
immediate environment. Autophagy, a Greek word
meaning to eat oneself, is needed for
non-specific protein and organelle turnover.
It can be induced by multiple stress factors
including cellular damage, lack of nutrients or
pathogen attack . There are different types
of autophagy, including biosynthetic autophagy as
seen in yeast in the trafficking of the vacuolar
protein aminopeptidase during cytoplasm-to-vacuole
transport (CVT) and several degradative types
including macroautophagy . We will use
autophagy to refer to bulk degradation mediated
by macroautophagy.
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Autophagy is visually characterized by the
formation of a double-membrane bound structure
called the autophagosome (Fig. 1).
PASpreautophagosomal structure ATG gene
required for autophagosome formation and for
proper autophygic activity
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Formation of the autophagosome
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Autophagy in plant
Regulation of PCD(programmed cell death)
initinated during plant innate immunity by
autophage
Autophagy and viability pro-survial or pro-death
Autophagy as an antimicrobial defence mechanism
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Now we use arabidopisis(???)in example
In plants, autophagy has been associated with
leaf senescence(????) .
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Extensive analysis of the Arabidopsis genome has
identified at least 36 genes with significant
homology to yeast ATG genes (Table 1). Two
main questions arise when examining the
Arabidopsis ATG-like (AtATG) genes 1. Why
have plants retained ATG genes (for example
ATG13) not required for autophagy in other higher
eukaryotes 2. why are there multiple copies of
ATG-like genes (ATG8 and ATG18) in the
Arabidopsis genome?
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Table 1. Microarray analysis of the known or
putative autophagy GENEVESTIGATOR Arabidopsis
using . genes from Autophagy genes M. persicae
P. syringae -N
PCD ATG1
AT1g49180

AT2g37840

AT3g53930

AT3g61960 NC

ATG3 AT5g61500

ATG4 AT2g44140

NC AT3g59950 NC

ATG5 AT5g17290

NC
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Intriguingly, the Arabidopsis genome retains a
homologue of ATG13 while this gene has been lost
in humans, Drosophila (??)and Caenorhabditis
elegans. The association/disassociation of
ATG13 with ATG1 is ought to play a key role in
initiation of autophagy in yeast Some
higher eukaryotes including humans have retained
the ability to form autophagosomes and regulate
autophagy despite lacking ATG13. This
suggests that ATG13 is not needed for autophagy
outside of yeast or that an unrelated protein
retains the function of ATG13 in these higher
eukaryotes.
???
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Despite the strong sequence similarities, some
AtATG (?????ATG-like Gene) genes are unable to
complement yeast deletion strains. This may
because that the ATG-like genes
from affecting autophagy in its respec- tive
organism. It has been demonstrated
that some mammalian ATG-like genes are
functionally equivalent to their respective yeast
counterparts and are required for autophagy in
mammals despite not being able to complement
yeast mutants.
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Arabidopsis atg3, atg7, atg5, atg9 and atg13
mutants have identical developmental phenotypes
. For example AtATG7 knockout causes
a senescence defect and hyper- sensitivity to
nitrogen and carbon starvation, consistent with a
requirement for autophagy during nutrient
limitation in yeast and other higher eukaryotes
. The senescence phenotype can be rescued
with a wild-type copy of AtATG7 but not with
AtATG7 mutant that effect catalytic activity and
autophagosome formation
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Regulation of PCD initiated during plant innate
immunity by autophagy

• Recent studies from plants and animals
  indicate that the autophagic machinery is
  involved in innate and adaptive immunities

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Although autophagy is known to be active at
basal levels under normal physiological
conditions, it can be stimulated by a
plethora(???) of stresses including cellular
damage, nutrient starvation and pathogen
infection . plant ATG genes are upregulated
during nitrogen starvation consistent with their
function in senescence and hypersensitivity to
nutrients Interestingly, these genes are also
regulated during defence responses to pathogens.
Several AtATG genes are upregulated following
infection with the bacterium Pseudomonas
syringae(????????) or the aphid(??) Myzus
persicae (Table 1) . With few exceptions, it
is interesting to note that most of the AtATGs
are upregulated during both PCD and P. syringae
infection.
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The most extensively studied plant innate
immunity involves recognition of pathogen-encoded
avirulence(Avr) proteins by plant resistance (R)
proteins. Often this RAvr interaction leads to
the hypersensitive response (HR), a form of PCD,
at the site of pathogen infection. HR-PCD is
often observed as necrotic lesions and the
patho- gen is restricted to these lesions and
cells immediately surrounding it.
Therefore, it is thought that the HR-PCD
functions to limit the spread of pathogen from
infection sites into adjacent healthy tissue .
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The HR-PCD initiated at the site of pathogen
infection must be meticulously controlled through
specific mechanisms and checkpoints to minimize
damage to the rest of the plant.
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Recent evidence indicates that autophagy is
one mechanism by which HR-PCD initiated during
plant innate immunity is controlled
The tobacco N protein belongs to the TIR-NB-LRR
class of R proteins and confers resistance to
tobacco mosaic virus (TMV) . The N protein
specifically recognizes the 50 kDa
helicase domain of the TMV replicase protein to
trigger induction of HR-PCD and restriction of
virus spread. Interestingly silencing of
Beclin 1 resulted in uncontrolled HR-PCD upon
TMV infection (Fig. 2). This phenotype is
dependent on a successful innate immune response
because Beclin1-silenced plants infected with TMV
failed to induce death in the absence of N.
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Fig. 2. Beclin 1 is required to limit the spread
of HR-PCD induced by TMV. Representative
photographs of leaves from non-silenced
control (VIGS-Vector) and Beclin 1-silenced
(VIGS-Beclin 1) plants infected with GFP-tagged
TMV. Red colour in the background of GFP
fluorescence in the UV illumination photographs
is due to auto-fluorescence from chlorophyll.
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Mammalian Beclin 1, was first identified as an
interactor of the antiapoptotic protein Bcl-2 .
Beclin 1 is part of a class III PI3K/VPS34
complex required for autophagosome formation and
recruitment of other ATG proteins into PAS (
???????). Interestingly, silencing of other
autophagy genes including PI3K/VPS34, ATG3 and
ATG7 also resulted in uncon- trolled HR-PCD upon
TMV infection. The requirement of Beclin 1
and other autophagy genes to limit HR-PCD to the
infection site during a plant innate immune
response indicates that autophagy may be involved
in this processes. .
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In addition, LysoTracker Red staining and
electron micros- copy data showed that autophagy
is induced during the N-mediated response to TMV
not only at the site of HR- PCD but also in the
adjacent healthy tissue. autophagy is not
required for execution of PCD, but rather it is
required to limit PCD to the infection site.
These results imply that there is a
pro-death signal(s) moving out of the pathogen
infected area into adjacent tissues that is
negatively regulated by autophagy.
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However, in the Beclin 1-silenced plants, the
induction of autophagy was compromised upon TMV
infection. Autophagy is not only required to
limit N-TMV induced HR-PCD but also other RAvr
interactions and general elicitor induced HR-PCD.
(??????????) autophagy plays a central role in
regulating HR-PCD that occurs in plant immunity
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Autophagy and viability pro-survival or
pro-death?
Compounding the intrigue was the discovery that
Beclin-1 interacts with Bcl-2, suggesting
surprising novel crosstalk between two potential
cell death pathways . However, it is still
unclear exactly if and how Bcl-2 affects
autophagy or how autophagy affects antiapoptotic
Bcl-2 family proteins.
two hypothesis 1? autophagy plays a crucial
role in cell survival during PCD.
2? autophagy itself is a pro-death process
or results in cell death by regulating
apoptosis.
Recent evidence suggests that both
hypotheses may be correct depending on the cell
type, stimuli and developmental stage.
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1?It is well established that autophagy
promotes cell survival during nutrient starvation
by degrading and recycling nutrients.
2?Autophagy may also indirectly promote cell
survival by retarding(??)or preventing apoptosis.

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When autophagy is compromised genetically or
pharmacologically(????), HeLa cells die via an
apoptotic pathway. This death is
abolished using Bcl-2 or caspase inhibitors
suggesting that autophagy prolongs survival by
deterring the onset of apoptosis.
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Aside from affecting apoptosis, autophagy also
plays a more direct role in cell survival.
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autophagy itself is a pro-death process or
results in cell death by regulating apoptosis.
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Overexpression of Bcl-2 or Bcl-X in wild-type
mouse embryonic fibroblasts treated with
etoposide, a common apoptotic reagent, resulted
in increased autophagosome formation. Autophagy
mediated death seems to depend on Bcl-Xbecause
elimination of Bcl-X reduced the formation of
autophagosomes and death. In addition,
Bax/Bak double-knockout mice lines were unable to
induce apoptosis, yet when treated with
etoposide, the cells were still able to die.
The non-apoptotic death was dependent on
functional autophagic mechanisms.
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Autophagy as an antimicrobial defence mechanism
Mounting evidence indicates that autophagy plays
an important role in the elimination of
intracellular and extracellular pathogens.
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In animals, recent findings indicate that
autophagy is also used to combat against
bacterial pathogens. Autophagy induced by
nutrient starvation or by treatment with
rapamycin actively inhibits the survival of the
facultative intracellular pathogen Mycobacterium
tuberculosis Similarly, autophagic vesicles
effectively engulf and destroy invading
extracellular pathogens such as group A
Streptococcus (GAS). Autophagy-deficient atg5-/-
mutant ES cells are incapable of eliminating GAS,
allowing it to survive and Proliferate.
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These results indicate that autophagy functions
as an antiviral defence mechanism. The increase
in virus accumulation in autophagy deficient
cells suggests that ATG proteins might target
cellular factors or pathways required for virus
replication and spread.
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It seems some bacteria have evolved effective
counter-defence strategies to subvert autophagy
and promote successful infection. 1?Some
bacterial and viral pathogens have evolved to
utilize the autophagy machinery for replication
or survival inside the host cell
Autophagosomal-like vesicles provide a
replicative niche for a variety of pathogens
including Legionella pneumophila, and Bru- ella
abortus . 2?While hiding inside autophagosomes,
these pathogens eplicate and either restricts
autophagosome maturation or delay fusion with the
lysosome. 3?The invasive pathogen Porphyromonas
gingivalis even stimulates autophagosome
formation and uses them to enter the host
cells. 4?Similarly, poliovirus and mouse
hepatitis virus (MHV) induce the formation of
autophagosome-like double-membrane vesicles (DMV)
and use them as a replicative niche. eg In
atg5-/-ES cells infected with MHV, formation of
DMV is inhibited and replication of the virus is
drastically reduced indicating thatautophagy is
required for efficient replication of MHV