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Establishing feeding sites
Physiological and Molecular Plant Pathology 67 (2005) 1 www.elsevier.com/locate/pmpp
Editorial
Establishing feeding sites Plant pathogens are heterotrophic organisms relying on an external source of food for survival [1]. Some pathogens are necrotrophic and readily feed off of dead organic matter [1,5]. However, many are either biotrophic or hemibiotrophic and keep invaded host cells and tissues alive for at least part of the pathogenesis process [1,4,5]. Classical examples of highly biotrophic pathogens are the obligately parasitic rusts, powdery mildews and downy mildews [1]. These fungal and oomycete pathogens establish a very specific feeding relationship with host cells that involve the formation of a haustorium that allows for the acquisition of nutrients from the host cell without killing the host. Several recent studies on the structure and function of haustoria have started to reveal how this feeding association develops and is regulated [4]. Obligately parasitic fungal and oomycete pathogens are not the only multicellular plant pathogens that must establish very specific feeding relationships with their host. The plant parasitic nematodes have also developed feeding interactions with their hosts that requires specialized structures [1,3]. In some nematode-plant interactions, the nematode feeds directly out of host cells. The migratory nematodes remove cellular contents and frequently cause death of the host cell while the sedentary nematodes, those in the genera Meloidogyne or Heterodora, cause significant changes in host cell/tissue structure to occur in response to the nematode and these plant structures serve as the food source for the parasite [3]. For example, the root knot nematodes of the genus Meloidogyne induce the formation of giant cells [1,3]. The cyst nematodes in the genera Heterodora and Globodora also induce the formation of specialized feeding cells known as a syncytium [1,3]. Common to all of the plant parasitic nematodes is a specialized feeding structure known as a stylet [1,3]. The stylet is a hollow structure that is used by the nematode to penetrate the plant cell wall and, prior to feeding, inject salivary secretions into the host cell [3]. These secretions are thought to be involved in the stylet penetration process and, in the case of root knot and cyst nematodes, in the establishment of the feeding sites [3]. Understanding the biochemical nature and function of the proteins in these secretions is, therefore, key to understanding the nature of parasitism in the plant parasitic nematodes. In this issue, Blanchard et al. [2], examine a gene expressed by the potato cyst nematode Globodora pallida that may be 0885-5765/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.pmpp.2005.12.006
involved in the establishment of the syncitium. The authors isolated a gene with homology to ranbpm (‘Ran binding protein to microtubules’) from G. pallida second stage juveniles. Using in situ hybridization, expression of the gene was localized in the dorsal glands of the second stage juveniles, and this suggests that the protein product of this gene is possibly involved in the establishment of the feeding site. Examination of several developmental stages revealed strongest expression of the gene in hatched second stage juveniles and in second stage juveniles that had penetrated into root tissue. No expression of the gene was observed in either males or females. The lack of expression in the adults is consistent with the hypothesis that the gene is associated with the establishment of the feeding sites. Cloning of the rbp-1 gene from G. pallida resulted in the isolation of the gene Gp-rbp-1 that is predicted to code for a 29 kDa protein that is small enough to be passed through the nematode stylet. A signal sequence characteristic to other secreted proteins was also identified. A similar gene was characterized from another potato cyst nematode, G. ‘mexicana’. Thus, the size and nature of this protein along with timing of its expression provides support for it role in pathogenesis. How this protein interacts with the invaded host cell to help facilitate the formation of the feeding site is not known. However, the information from this work [2] and similar findings from Heterodora species should allow for better understanding of how these nematodes establish specialized feeding sites in their hosts. References [1] Agrios G. Plant pathology. 5th ed. San Diego, CA: Elsevier; 2005. [2] Blanchard A, Esquibet M, Fouville D, Grenier E. Ranbpm homologue genes characterized in the potato cyst nematode Globodora pallida and Globodora ‘mexicana’. Physiol Mol Plant Pathol; in press. [3] Davis EL, Hussey RS, Baum TJ, Bakker J, Schots A, Rosso M-N, et al. Nematode parasitism genes. Annu Rev Phytopathol 2000;38:365–96. [4] Mendgen K, Hahn M. Plant infection and the establishment of fungal biotrophy. Trends Plant Sci 2002;7:352–6. [5] Oliver RP, Ipcho SVS. Arabidopsis pathology breathes new life into the necrotrophs-vs.-biotrophs classification of fungal pathogens. Mol Plant Pathol 2004;5:347–52.
R. Hammerschmidt* Department of Plant Pathology, Michigan State University, 107 CIPS Building, East Lansing, MI 48824-1311, USA E-mail address: [email protected] * Tel.: C1 517 353 8624; fax: C1 517 353 1781.
Editorial
Establishing feeding sites Plant pathogens are heterotrophic organisms relying on an external source of food for survival [1]. Some pathogens are necrotrophic and readily feed off of dead organic matter [1,5]. However, many are either biotrophic or hemibiotrophic and keep invaded host cells and tissues alive for at least part of the pathogenesis process [1,4,5]. Classical examples of highly biotrophic pathogens are the obligately parasitic rusts, powdery mildews and downy mildews [1]. These fungal and oomycete pathogens establish a very specific feeding relationship with host cells that involve the formation of a haustorium that allows for the acquisition of nutrients from the host cell without killing the host. Several recent studies on the structure and function of haustoria have started to reveal how this feeding association develops and is regulated [4]. Obligately parasitic fungal and oomycete pathogens are not the only multicellular plant pathogens that must establish very specific feeding relationships with their host. The plant parasitic nematodes have also developed feeding interactions with their hosts that requires specialized structures [1,3]. In some nematode-plant interactions, the nematode feeds directly out of host cells. The migratory nematodes remove cellular contents and frequently cause death of the host cell while the sedentary nematodes, those in the genera Meloidogyne or Heterodora, cause significant changes in host cell/tissue structure to occur in response to the nematode and these plant structures serve as the food source for the parasite [3]. For example, the root knot nematodes of the genus Meloidogyne induce the formation of giant cells [1,3]. The cyst nematodes in the genera Heterodora and Globodora also induce the formation of specialized feeding cells known as a syncytium [1,3]. Common to all of the plant parasitic nematodes is a specialized feeding structure known as a stylet [1,3]. The stylet is a hollow structure that is used by the nematode to penetrate the plant cell wall and, prior to feeding, inject salivary secretions into the host cell [3]. These secretions are thought to be involved in the stylet penetration process and, in the case of root knot and cyst nematodes, in the establishment of the feeding sites [3]. Understanding the biochemical nature and function of the proteins in these secretions is, therefore, key to understanding the nature of parasitism in the plant parasitic nematodes. In this issue, Blanchard et al. [2], examine a gene expressed by the potato cyst nematode Globodora pallida that may be 0885-5765/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.pmpp.2005.12.006
involved in the establishment of the syncitium. The authors isolated a gene with homology to ranbpm (‘Ran binding protein to microtubules’) from G. pallida second stage juveniles. Using in situ hybridization, expression of the gene was localized in the dorsal glands of the second stage juveniles, and this suggests that the protein product of this gene is possibly involved in the establishment of the feeding site. Examination of several developmental stages revealed strongest expression of the gene in hatched second stage juveniles and in second stage juveniles that had penetrated into root tissue. No expression of the gene was observed in either males or females. The lack of expression in the adults is consistent with the hypothesis that the gene is associated with the establishment of the feeding sites. Cloning of the rbp-1 gene from G. pallida resulted in the isolation of the gene Gp-rbp-1 that is predicted to code for a 29 kDa protein that is small enough to be passed through the nematode stylet. A signal sequence characteristic to other secreted proteins was also identified. A similar gene was characterized from another potato cyst nematode, G. ‘mexicana’. Thus, the size and nature of this protein along with timing of its expression provides support for it role in pathogenesis. How this protein interacts with the invaded host cell to help facilitate the formation of the feeding site is not known. However, the information from this work [2] and similar findings from Heterodora species should allow for better understanding of how these nematodes establish specialized feeding sites in their hosts. References [1] Agrios G. Plant pathology. 5th ed. San Diego, CA: Elsevier; 2005. [2] Blanchard A, Esquibet M, Fouville D, Grenier E. Ranbpm homologue genes characterized in the potato cyst nematode Globodora pallida and Globodora ‘mexicana’. Physiol Mol Plant Pathol; in press. [3] Davis EL, Hussey RS, Baum TJ, Bakker J, Schots A, Rosso M-N, et al. Nematode parasitism genes. Annu Rev Phytopathol 2000;38:365–96. [4] Mendgen K, Hahn M. Plant infection and the establishment of fungal biotrophy. Trends Plant Sci 2002;7:352–6. [5] Oliver RP, Ipcho SVS. Arabidopsis pathology breathes new life into the necrotrophs-vs.-biotrophs classification of fungal pathogens. Mol Plant Pathol 2004;5:347–52.
R. Hammerschmidt* Department of Plant Pathology, Michigan State University, 107 CIPS Building, East Lansing, MI 48824-1311, USA E-mail address: [email protected] * Tel.: C1 517 353 8624; fax: C1 517 353 1781.