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?

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, 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.

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 leafsenescence( 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 (ATG8and ATG18) in the Arabidopsis genome?

Table 1. Microarray analysis of the known or putative autophagyGENEVESTIGATORArabidopsis using .genes from Autophagy genes M. persicae P. syringae -N PCDATG1 AT1g49180 + – – + + +AT2g37840 + + + –AT3g53930 + + + + +AT3g61960 NC + + +ATG3AT5g61500 + + + +ATG4AT2g44140 + + + + – NCAT3g59950 NC + + + +ATG5AT5g17290 + + 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 isought 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 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.

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

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 levelsunder 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 patho-gen is restricted to these lesions and cells immediatelysurrounding 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 infectionmust 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 helicasedomain 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 dependenton a successful innate immune response because Beclin1-silenced 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 includingPI3K/VPS34, ATG3 and ATG7 also resulted in uncon-trolled HR-PCD upon TMV infection. The requirement of Beclin 1 and other autophagy genes tolimit HR-PCD to the infection site during a plant innateimmune response indicates that autophagy may beinvolved in this processes. .

In addition, LysoTracker Red staining and electron micros-copy data showed that autophagy is induced during theN-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.

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: pro-survival or pro-death?

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.

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.

1 、 It is well established that autophagy promotes cell survivalduring nutrient starvation by degrading and recyclingnutrients.

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 ofBcl-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 pathogenssuch as group A Streptococcus (GAS). Autophagy-deficient atg5-/- mutant ES cells areincapable of eliminating GAS, allowing it to survive andProliferate.

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 Bru-ella abortus .2 、 While hiding inside autophagosomes, these pathogens eplicate and either restricts autophagosome maturationor 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