[Autophagy 3:5, 472-473; September/October 2007]; ©2007 Landes Bioscience
Addendum
Autophagy During Conidiation, Conidial Germination and Turgor Generation in Magnaporthe grisea Xiao-Hong Liu Jian-Ping Lu Fu-Cheng Lin*
Abstract
Key words MgATG1, autophagy, appressorium, turgor generation Acknowledgements
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This study was supported by grants (No. 30270049-30470064 and 30671351) from the National Natural Science Foundation of China, from 863 Program (No. 2002AA245041) of the Ministry of Science and Technology of China, and from a major project (No. 2004C12020-5) of the Zhejiang Science and Technology Bureau of China to F.-C. Lin.
Autophagy is a ubiquitous degradative pathway in eukaryotic cells. It facilitates the bulk degradation of eukaryotic macromolecules and organelles, through degradation within the lysosomal/vacuolar compartment.1 In addition to helping cells survive nutritional starvation, autophagy influences a diverse array of eukaryotic cell functions including turnover of cellular components, adaptation, differentiation and developmental programs.2 Magnaporthe grisea is well known as the causal agent of rice blast, the most serious disease of cultivated rice throughout the world. Recently, this disease has reoccurred every year in China. In 2005, the blast fungus affected 150,000 hectares of rice fields, which led to the loss of 234,000 metric tons of rice in 127 cities, throughout the Sichuan Province of China. Importantly, rice blast has been developed as a model organism for the investigation of fungus‑host interaction in filamentous fungi.3 Recently, we isolated the MgATG1 gene (the orthologue of S. cerevisiae ATG1), encoding a serine/threonine protein kinase, which is composed of a 982 amino acid polypeptide; its sequence is highly conserved among other eukaryotes, including humans and plants. Disruption of the MgATG1 gene influenced the ability to survive starvation, conidiation, conidial germination, lipid turnover, and appressorium turgor. As a result, the DMgatg1 mutant loses its penetration ability and pathogenicity to the host plants.4 Convincing evidence has been obtained in yeast for ATG1 affecting sporulation and viability.5 In Caenorhabditis elegans, unc-51 (equivalent to ATG1) mutant worms are paralyzed, egg‑laying defective, and dumpy and have defects in axonal elongation.6 The Dictyostelium discoideum DdATG1 mutant displayed an inability to form multicellular aggregates, or defects in fruiting body formation.7 How autophagy plays a role in differentiation and development in filamentous fungi remains an open question. Here, we discuss the contribution of autophagy towards the appressorium turgor, differentiation and germination processes in the filamentous fungi M. grisea.
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Previously published online as an Autophagy E-publication: http://www.landesbioscience.com/journals/autophagy/abstract.php?id=4339
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Original manuscript submitted: 04/16/07 Manuscript accepted: 04/24/07
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*Correspondence to: Fu-Cheng Lin; Biotechnology Institute; Zhejiang University; Kaixuan Road 268; Hangzhou 310029 China; Tel.: +86.571.86971185; Fax: +86.571.86971516; Email:
[email protected]
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Biotechnology Institute; Zhejiang University; Hangzhou, China
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Autophagy is a ubiquitous and evolutionarily conserved process found in all eukaryotic cells that allows for the degradation and recycling of old proteins and organelles. Starvation can induce autophagy, and autophagic pathway is an essential process for cellular function under starvation. In Magnaporthe grisea, starvation is one of the key induced factors for the germ tube tip to differentiate into an appressorium. Considering the importance of the rice blast fungus as a primary model for host‑pathogen interaction, the role of autophagy in fungal development, appressorium turgor generation and pathogenicity of M. grisea via its role in organelle and protein turnover is a very significant subject.
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Addendum to:
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Involvement of a Magnaporthe grisea Serine/Threonine Kinase, MgATG1, in Appressorium Turgor and Pathogenesis
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X.-H. Liu, J.-P. Lu, L. Zhang, B. Dong, H. Min and F.-C. Lin
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Eukaryotic Cell 2007; In press
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Autophagy is Necessary for Appressorium Turgor M. grisea can form the typical structure that is called an appressorium similar to many other fungal pathogens. The appressorium is a flattened, hyphal structure that is used to enter host cells during infection; it generates tremendous intracellular turgor pressure (as much as 8.0 Mpa) allowing it to penetrate the leaf cuticle.8,9 This enormous turgor in the appressorium is a consequence of the accumulation of very large quantities of glycerol in the cell, and potential sources of its biosynthesis are lipid, and glycogen, as well as the sugars, trehalose and mannitol, in the conidium.10 These substances are transformed in the conidium to generate enough strength in the appressorium to rupture the cuticle of the host. In this turnover of cellular components in the appressorium, what is the role of autophagy? Autophagy
2007; Vol. 3 Issue 5
Autophagy During Appressorium Turgor Generation in Magnaporthe grisea
In our research, the DMgatg1 mutant could form the normal appressorium in shape, but the substances within the conidium could not be transformed to allow the appressorium to maintain normal turgor. Transmission electron microscope data confirm that the MgAtg1 null mutant is defective in turnover of its cytoplasmic contents, similar to the Saccharomyces cerevisiae atg1 null and other autophagy mutants. In conclusion, autophagy did not affect appressorial differentiation, but rather affected the substance transformation in M. grisea and turgor generation. Appressorial turgor generation is controlled by many genetic pathways, such as the PMK1 or MPS1 mitogen-activated protein kinase cascades, and the fatty acid pathway.10 Previously, the role of autophagy in turgor generation was unknown. By deleting the genes that code for M. grisea homologues of yeast ATG2, ATG4, ATG5, ATG9 and ATG18, we have shown that autophagy is essential for turgor generation in the appressorium in M. grisea, and the corresponding null mutants (available in our lab, pending publication) fail to rupture the cuticle of the host. These results appear to be consistent with the DMgatg8 mutants, which were unable to infect plants efficiently via the appressorium.11 Therefore, it seems obvious that a block in autophagy influences the turgor generation of appressorium.
10. Thines E, Weber RW, Talbot NJ. MAP kinase and protein kinase A‑dependent mobilization of triacylglycerol and glycogen during appressorium turgor generation by Magnaporthe grisea. Plant Cell 2000; 12:1703‑18. 11. Veneault‑Fourrey C, Barooah M, Egan M, Wakley G, Talbot NJ. Autophagic fungal cell death is necessary for infection by the rice blast fungus. Science 2006; 312:580‑3. 12. Kikuma T, Ohneda M, Arioka M, Kitamoto K. Functional analysis of the ATG8 homologue Aoatg8 and role of autophagy in differentiation and germination in Aspergillus oryzae. Eukaryot Cell 2006; 5:1328‑36.
Autophagy is Necessary for Conidiation, Conidial Germination and Sexual Reproductive Ability The role of autophagy has been established in filamentous fung; a block in autophagy changes conidiation and conidial germination. In M. grisea, conidiation decreased significantly, and conidial germination was delayed in autophagy mutants. In Aspergillus oryzae, disruption of the AoATG8 gene (the orthologue of S. cerevisiae ATG8) influences the formation of aerial hyphae, conidiation, and germination.12 Similarly, a block in autophagy changes the sexual reproductive ability in M. grisea; the ascocarp (fruiting body) cannot form in the DMgatg1 mutant. In the current work, we have characterized the serine/threonine kinase MgAtg1. Our results suggest that MgATG1 is important to autophagy and to conidiation, conidial gemination and turgor generation in M. grisea. Investigation of the network of the autophagy genes will definitely lead to an understanding of the molecular machinery of infection mechanisms in M. grisea, a filamentous fungal pathogenic model. References 1. Klionsky DJ. The molecular machinery of autophagy: Unanswered questions. J Cell Sci 2005; 118:7‑18. 2. Levine B, Klionsky DJ. Development by self‑digestion: Molecular mechanisms and biological functions of autophagy. Dev Cell 2004; 6:463‑77. 3. Ou SH. Rice diseases. 2nd ed. Kew, UK: Commonwealth Mycological Institute, 1985. 4. Liu XH, Lu JP, Zhang L, Dong B, Min H, Lin FC. Involvement of a Magnaporthe grisea serine/threonine kinase gene, MgATG1, in appressorium turgor and pathogenesis. Eukaryot Cell 2007; In press. 5. Matsuura A, Tsukada M, Wada Y, Ohsumi Y. Apg1p, a novel protein kinase required for the autophagic process in Saccharomyces cerevisiae. Gene 1997; 192:245‑50. 6. Ogura K, Wicky C, Magnenat L, Tobler H, Mori I, Muller F, Ohshima Y. Caenorhabditis elegans unc-51 gene required for axonal elongation encodes a novel serine/threonine kinase. Genes Dev 1994; 8:2389‑400. 7. Tekinay T, Wu MY, Otto GP, Anderson OR, Kessin RH. Function of the Dictyostelium discoideum Atg1 kinase during autophagy and development. Eukaryot Cell 2006; 5:1797‑806. 8. Howard RJ, Ferrari MA, Roach DH, Money NP. Penetration of hard substrates by a fungus employing enormous turgor pressures. Proc Natl Acad Sci USA 1991; 88:11281‑4. 9. de Jong JC, McCormack BJ, Smirnoff N, Talbot NJ. Glycerol generates turgor in rice blast. Nature 1997; 389:244.
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