- Email: [email protected]
The Sarajevo Botanic Garden
research
news
The With the support of the Cambridge University Botanic Garden, a group has launched a scheme to help in the reconstruction of the Botanic Garden of Sarajevo, the capital city of Bosnia. The site of the Garden survives, but the live collections have been largely lost over the last three years. One of the most remarkable living collections was that of endemic Balkan plants, which Dr ~edomil ~ili~ ~who has recently retired from the post of Garden Director) had built up over the preceding 30 years. The local flora was the subject of a book by Dr ~ili~, which beautifully illustrated the endemic flowers of this region 1. The Sarajevo Garden also provided a detailed seed list every year until 1990. Many gardens will have been beneficiaries of seeds on these lists under the garden's international seed exchange programme. Since many of the plants on offer were collected in the wild, they were a remarkable and unique record of the local flora.
The Cambridge group has been able to trace 12 such 'Sarajevo plants', which continue to grow in an alpine house and rock garden. Two examples are Alyssum
as a source of conserved stock either as rooted plants, cuttings or seedsI to help restore the Sarajevo Garden to its former state. It may well be that other gardens throughout the world hold similar collections or specimens of Sarajevo plants and would be able to help in this restoration project. Other schemes, including the possibility of supporting workers in the garden, and help with general reconstruction costs, are also under active development. Further details of how to assist can be obtained by contacting me at the address below.
S.M. Waiters
moellendorfianum, a silvery-haired, yellowflowered crucifer, and FritiUaria messanensis subsp, gracitis (which is part of the National Collection held in Cambridge). These plants can now be used
c/o Cambridge University Botanic Garden. Cory Lodge, Bateman Street. Cambridge. UK CB2 1JF Fax +44 1223 315964 (101523.527 @compuserve.com~
References 1 C. Sili~ 1990 Endemi~neBiIjke, S~jetlost
Bacterial avirulence proteins: where's the action? Known as triggers of plant defense reactions and residents of the bacterial cytoplasm, the avirulence proteins (Avr proteins) of plant pathogenic bacteria have been unlikely suspects in the search for molecular agents of pathogenesis. However, growing evidence that avirulence may be an alias, and the bacterial cell a false address, is raising suspicions that these proteins could actually be the ringleaders of parasitic attack. The Avr proteins are now known primarily through their ability to control the cultivar-specific interactions of races of Pseudomonas syringae and Xanthomonas spp., which they do according to the rules of Flor's gene-for-gene hypothesis~. For example, the avrB gene in P. syringae pv. gIycinea interacts with the RPG1 resistance gene in soybean (Glycine max): when inoculated into the soybean cultivars carrying RPG1, any strains of the pathogen that carry avrB will elicit the defense-associated hypersensitive response (HR) rather than elicit bacterial blight disease 2. Over 30 bacterial avr genes have been cloned and characterized, but inspection of the menagerie of corresponding Avr proteins has yielded no unilying explanation for how they act or why bacteria produce them 3. Now, three recent (or pending) papers provide new insights into the requirements, location and possible nature of Avr protein activity~ .
The importance of the Hrp secretion system for delivering avirulence signals The paper by Pirhonen et al. 4 concerns the minimum requirements for elicitation of a genotype-specific HR. It has been known since 1989 that expression ofavr genotypes © 1996 Elsevier Science Ltd
is dependent upon the hrp (hypersensitive response and pathogenesis) genes of plant pathogenic bacterial For P. syringae, this is partly caused by coregulation of hrp and avr genes3,~,8. However, the use of nonregulatory hrp mutations, or the expression of avr genes using foreign promoters (i.e. to which the genes would not normally be connected), fails to obviate this requirement. The discovery that many hrp genes encode components of a type III protein secretion pathway has led to speculation that Avr proteins may be secreted 9,1°. Although translocation of any Avr protein out of the bacterial cytoplasm has yet to be observed, Pirhonen et aI. provide compelling evidence that the prorein secretion capability of the Hrp system is involved in Avr signal delivery, by exploiting the observation that nonpathogenic Escherichia coli cells carrying a functional cluster of hrp genes from a pathogenic bacterium can trigger a genotype-specific HR. This hip cluster, from P. syringae pv. syringae 61, is carried on cosmid pHIR11, and enables nonpathogenic bacteria such as E. coli or Pseudomonas fluorescens to elicit the HR in tobacco (Nicotiana tabacum) and several other plants 11. Elicitation of a typical 'strong' HR by E. coli carrying pHIR11 requires at least four ctasses of genes: * Regulatory genesS. . Secretory genes 12. . A harpin-encoding gene 13. o An apparent avr gene (hrmA is required, but is not in the other three classes, and has many characteristics of an avr geneW. The HrpZ harpin protein can elicit the HR when infiltrated into tobacco leaves, but is neither sufficient nor absolutely
required for bacteria to elicit at least partial necrosis 13. Members of the other three classes are required for the HR, but have no elicitor activity of their own when infiltrated into leaves. Pirhonen et al. demonstrate that E. coli can elicit a genotype-specific HR when carrying three components: pHIRll; a plasmid strongly expressing the HrpL sigma facter14; and a plasmid expressing either AvrB or one of several other wellcharacterized Avr proteins. Most importantly, this activity is abolished by mutations in either of two secretion-related hrp genes, but not by nonpolar deletions in the hrpZ gene. Thus, an Hrp secretion system, expressed in a species that would not normally contain it, is both sufficient and necessary for the delivery of Avr signals. The contribution of individual secretion pathway components to this process can now be efficiently dissected.
Action of the avirulence protein AvrB in the plant cell cytoplasm The paper by Gopalan et al. 5 independently explores the requirements for %eterologous' AvrB signal delivery- using the pHIR11 hrp cluster in P. fluorescens (a nonpathogenic bacterium closely related to P. syringae), and it addresses the site of Avr protein action. By using 'FLAG'-epitope-tagged AvrB and promoters of different strength, they demonstrafe that overproduction of AvrB in bacterial cells, or infiltration of high levels of purified AvrB into the leaves of soybean and Arabidopsis carrying the corresponding resistance genes, cannot obviate the requirement for a functional Hrp system in the elicitation of a genotype-specific HR. July 1996,Vol. 1, No. 7
209
news
The With the support of the Cambridge University Botanic Garden, a group has launched a scheme to help in the reconstruction of the Botanic Garden of Sarajevo, the capital city of Bosnia. The site of the Garden survives, but the live collections have been largely lost over the last three years. One of the most remarkable living collections was that of endemic Balkan plants, which Dr ~edomil ~ili~ ~who has recently retired from the post of Garden Director) had built up over the preceding 30 years. The local flora was the subject of a book by Dr ~ili~, which beautifully illustrated the endemic flowers of this region 1. The Sarajevo Garden also provided a detailed seed list every year until 1990. Many gardens will have been beneficiaries of seeds on these lists under the garden's international seed exchange programme. Since many of the plants on offer were collected in the wild, they were a remarkable and unique record of the local flora.
The Cambridge group has been able to trace 12 such 'Sarajevo plants', which continue to grow in an alpine house and rock garden. Two examples are Alyssum
as a source of conserved stock either as rooted plants, cuttings or seedsI to help restore the Sarajevo Garden to its former state. It may well be that other gardens throughout the world hold similar collections or specimens of Sarajevo plants and would be able to help in this restoration project. Other schemes, including the possibility of supporting workers in the garden, and help with general reconstruction costs, are also under active development. Further details of how to assist can be obtained by contacting me at the address below.
S.M. Waiters
moellendorfianum, a silvery-haired, yellowflowered crucifer, and FritiUaria messanensis subsp, gracitis (which is part of the National Collection held in Cambridge). These plants can now be used
c/o Cambridge University Botanic Garden. Cory Lodge, Bateman Street. Cambridge. UK CB2 1JF Fax +44 1223 315964 (101523.527 @compuserve.com~
References 1 C. Sili~ 1990 Endemi~neBiIjke, S~jetlost
Bacterial avirulence proteins: where's the action? Known as triggers of plant defense reactions and residents of the bacterial cytoplasm, the avirulence proteins (Avr proteins) of plant pathogenic bacteria have been unlikely suspects in the search for molecular agents of pathogenesis. However, growing evidence that avirulence may be an alias, and the bacterial cell a false address, is raising suspicions that these proteins could actually be the ringleaders of parasitic attack. The Avr proteins are now known primarily through their ability to control the cultivar-specific interactions of races of Pseudomonas syringae and Xanthomonas spp., which they do according to the rules of Flor's gene-for-gene hypothesis~. For example, the avrB gene in P. syringae pv. gIycinea interacts with the RPG1 resistance gene in soybean (Glycine max): when inoculated into the soybean cultivars carrying RPG1, any strains of the pathogen that carry avrB will elicit the defense-associated hypersensitive response (HR) rather than elicit bacterial blight disease 2. Over 30 bacterial avr genes have been cloned and characterized, but inspection of the menagerie of corresponding Avr proteins has yielded no unilying explanation for how they act or why bacteria produce them 3. Now, three recent (or pending) papers provide new insights into the requirements, location and possible nature of Avr protein activity~ .
The importance of the Hrp secretion system for delivering avirulence signals The paper by Pirhonen et al. 4 concerns the minimum requirements for elicitation of a genotype-specific HR. It has been known since 1989 that expression ofavr genotypes © 1996 Elsevier Science Ltd
is dependent upon the hrp (hypersensitive response and pathogenesis) genes of plant pathogenic bacterial For P. syringae, this is partly caused by coregulation of hrp and avr genes3,~,8. However, the use of nonregulatory hrp mutations, or the expression of avr genes using foreign promoters (i.e. to which the genes would not normally be connected), fails to obviate this requirement. The discovery that many hrp genes encode components of a type III protein secretion pathway has led to speculation that Avr proteins may be secreted 9,1°. Although translocation of any Avr protein out of the bacterial cytoplasm has yet to be observed, Pirhonen et aI. provide compelling evidence that the prorein secretion capability of the Hrp system is involved in Avr signal delivery, by exploiting the observation that nonpathogenic Escherichia coli cells carrying a functional cluster of hrp genes from a pathogenic bacterium can trigger a genotype-specific HR. This hip cluster, from P. syringae pv. syringae 61, is carried on cosmid pHIR11, and enables nonpathogenic bacteria such as E. coli or Pseudomonas fluorescens to elicit the HR in tobacco (Nicotiana tabacum) and several other plants 11. Elicitation of a typical 'strong' HR by E. coli carrying pHIR11 requires at least four ctasses of genes: * Regulatory genesS. . Secretory genes 12. . A harpin-encoding gene 13. o An apparent avr gene (hrmA is required, but is not in the other three classes, and has many characteristics of an avr geneW. The HrpZ harpin protein can elicit the HR when infiltrated into tobacco leaves, but is neither sufficient nor absolutely
required for bacteria to elicit at least partial necrosis 13. Members of the other three classes are required for the HR, but have no elicitor activity of their own when infiltrated into leaves. Pirhonen et al. demonstrate that E. coli can elicit a genotype-specific HR when carrying three components: pHIRll; a plasmid strongly expressing the HrpL sigma facter14; and a plasmid expressing either AvrB or one of several other wellcharacterized Avr proteins. Most importantly, this activity is abolished by mutations in either of two secretion-related hrp genes, but not by nonpolar deletions in the hrpZ gene. Thus, an Hrp secretion system, expressed in a species that would not normally contain it, is both sufficient and necessary for the delivery of Avr signals. The contribution of individual secretion pathway components to this process can now be efficiently dissected.
Action of the avirulence protein AvrB in the plant cell cytoplasm The paper by Gopalan et al. 5 independently explores the requirements for %eterologous' AvrB signal delivery- using the pHIR11 hrp cluster in P. fluorescens (a nonpathogenic bacterium closely related to P. syringae), and it addresses the site of Avr protein action. By using 'FLAG'-epitope-tagged AvrB and promoters of different strength, they demonstrafe that overproduction of AvrB in bacterial cells, or infiltration of high levels of purified AvrB into the leaves of soybean and Arabidopsis carrying the corresponding resistance genes, cannot obviate the requirement for a functional Hrp system in the elicitation of a genotype-specific HR. July 1996,Vol. 1, No. 7
209