Download 2、role of autophagy in plant innate immunity

Autophagy and plant innate
immunity
穆颖 张晓丽
1、Plant innate(先天的，固有的） 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?
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.
Autophagy is visually characterized by the formation of
a double-membrane bound structure called the
autophagosome (Fig. 1).
PAS:preautophagosomal structure
ATG gene : required for autophagosome formation
and for proper autophygic activity
Formation of the autophagosome
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
Now we use arabidopisis(拟南芥）in example:
In plants, autophagy has been associated with leaf
senescence(叶片衰老） .
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?
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
++
Intriguingly, the Arabidopsis genome retains a homologue of
ATG13 while this gene has been lost in humans, Drosophila (果
蝇）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.
第八张
Despite the strong sequence similarities, some AtATG (拟南芥里
的ATG-like Gene) genes are unable to complement yeast
deletion strains.
This may because that :
the ATG-like genes from affecting autophagy in its respective 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.
Arabidopsis atg3, atg7, atg5, atg9 and atg13 mutants have
identical developmental phenotypes .
For example:
AtATG7 knockout causes a senescence defect and hypersensitivity 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
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
Although autophagy is known to be active at basal levels
under normal physiological conditions, it can be stimulated by
a plethora(过多的） 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(紫丁香属假单胞菌）
or the aphid（蚜虫） 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.
The most extensively studied plant innate immunity involves
recognition of pathogen-encoded avirulence(Avr) proteins by
plant resistance (R) proteins. Often this R–Avr 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 pathogen 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 .
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.
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 HRPCD and restriction of virus spread.
Interestingly silencing of Beclin 1 resulted in uncontrolled HRPCD upon TMV infection (Fig. 2). This phenotype is dependent
on a successful innate immune response because Beclin1silenced plants infected with TMV failed to induce death in the
absence of N.
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.
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 ( 前体自噬体结构）.
Interestingly, silencing of other autophagy genes including
PI3K/VPS34, ATG3 and ATG7 also resulted in uncontrolled 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.
.
In addition, LysoTracker Red staining and electron microscopy data showed that autophagy is induced during the
N-mediated response to TMV not only at the site of HRPCD 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.
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 R–Avr interactions
and general elicitor induced HR-PCD. （就是说这只是
一个例子）
autophagy plays a central role in regulating HR-PCD
that occurs in plant immunity
Autophagy and viability: prosurvival 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.
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(阻止）or preventing apoptosis.
When autophagy is compromised genetically or
pharmacologically（药理学的）, 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.
Aside from affecting apoptosis, autophagy also plays a
more direct role in cell survival.
autophagy itself is a ‘pro-death’ process or results in cell
death by regulating apoptosis.
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.
Autophagy as an antimicrobial
defence mechanism
Mounting evidence indicates that autophagy plays an
important role in the elimination of intracellular and
extracellular pathogens.
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). Autophagydeficient atg5-/- mutant ES cells are
incapable of eliminating GAS, allowing it to survive and
Proliferate.
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.
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 Bruella 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