Differential involvement of indole-3-acetic acid biosynthetic pathways in pathogenicity and epiphytic fitness of Erwinia herbicola pv. gypsophilae

Auxin and plant-microbe interactions

Microbial synthesis of the phytohormone auxin has been known for a long time. This property is best documented for bacteria that interact with plants because bacterial auxin can cause interference with the many plant developmental processes regulated by auxin. Auxin biosynthesis in bacteria can occur via multiple pathways as has been observed in plants. There is also increasing evidence that indole-3-acetic acid (IAA), the major naturally occurring auxin, is a signaling molecule in microorganisms because IAA affects gene expression in some microorganisms. Therefore, IAA can act as a reciprocal signaling molecule in microbe-plant interactions. Interest in microbial synthesis of auxin is also increasing in yet another recently discovered property of auxin in Arabidopsis. Down-regulation of auxin signaling is part of the plant defense system against phytopathogenic bacteria. Exogenous application of auxin, e.g., produced by the pathogen, enhances susceptibility to the bacterial pathogen.

The multifactorial basis for plant health promotion by plant-associated bacteria

On plants, microbial populations interact with each other and their host through the actions of secreted metabolites. However, the combined action of diverse organisms and their different metabolites on plant health has yet to be fully appreciated. Here, the multifactorial nature of these interactions, at the organismal and molecular level, leading to the biological control of plant diseases is reviewed. To do so, we describe in detail the ecological significance of three different classes of secondary metabolites and discuss how they might contribute to biological control. Specifically, the roles of auxin, acetoin, and phenazines are considered, because they represent very different but important types of secondary metabolites. We also describe how studies of the global regulation of bacterial secondary metabolism have led to the discovery of new genes and phenotypes related to plant health promotion. In conclusion, we describe three avenues for future research that will help to integrate these complex and diverse observations into a more coherent synthesis of bacterially mediated biocontrol of plant diseases.

Hormone crosstalk in plant disease and defense: more than just jasmonate-salicylate antagonism

Until recently, most studies on the role of hormones in plant-pathogen interactions focused on salicylic acid (SA), jasmonic acid (JA), and ethylene (ET). It is now clear that pathogen-induced modulation of signaling via other hormones contributes to virulence. A picture is emerging of complex crosstalk and induced hormonal changes that modulate disease and resistance, with outcomes dependent on pathogen lifestyles and the genetic constitution of the host. Recent progress has revealed intriguing similarities between hormone signaling mechanisms, with gene induction responses often achieved by derepression. Here, we report on recent advances, updating current knowledge on classical defense hormones SA, JA, and ET, and the roles of auxin, abscisic acid (ABA), cytokinins (CKs), and brassinosteroids in molding plant-pathogen interactions. We highlight an emerging theme that positive and negative regulators of these disparate hormone signaling pathways are crucial regulatory targets of hormonal crosstalk in disease and defense.

Water relations in the interaction of foliar bacterial pathogens with plants

This review examines the many ways in which water influences the relations between foliar bacterial pathogens and plants. As a limited resource in aerial plant tissues, water is subject to manipulation by both plants and pathogens. A model is emerging that suggests that plants actively promote localized desiccation at the infection site and thus restrict pathogen growth as one component of defense. Similarly, many foliar pathogens manipulate water relations as one component of pathogenesis. Nonvascular pathogens do this using effectors and other molecules to alter hormonal responses and enhance intercellular watersoaking, whereas vascular pathogens use many mechanisms to cause wilt. Because of water limitations on phyllosphere surfaces, bacterial colonists, including pathogens, benefit from the protective effects of cellular aggregation, synthesis of hygroscopic polymers, and uptake and production of osmoprotective compounds. Moreover, these bacteria employ tactics for scavenging and distributing water to overcome water-driven barriers to nutrient acquisition, movement, and signal exchange on plant surfaces.

Aberrant synthesis of indole-3-acetic acid in Saccharomyces cerevisiae triggers morphogenic transition, a virulence trait of pathogenic fungi

Many plant-associated microbes synthesize the auxin indole-3-acetic acid (IAA), and several IAA biosynthetic pathways have been identified in microbes and plants. Saccharomyces cerevisiae has previously been shown to respond to IAA by inducing pseudohyphal growth. We observed that IAA also induced hyphal growth in the human pathogen Candida albicans and thus may function as a secondary metabolite signal that regulates virulence traits such as hyphal transition in pathogenic fungi. Aldehyde dehydrogenase (Ald) is required for IAA synthesis from a tryptophan (Trp) precursor in Ustilago maydis. Mutant S. cerevisiae with deletions in two ALD genes are unable to convert radiolabeled Trp to IAA, yet produce IAA in the absence of exogenous Trp and at levels higher than wild type. These data suggest that yeast may have multiple pathways for IAA synthesis, one of which is not dependent on Trp.

Assessing the impact of fungicide enostroburin application on bacterial community in wheat phyllosphere

Fungicides have been used extensively for controlling fungal pathogens of plants. However, little is known regarding the effects that fungicides upon the indigenous bacterial communities within the plant phyllosphere. The aims of this study were to assess the impact of fungicide enostroburin upon bacterial communities in wheat phyllosphere. Culture-independent methodologies of 16S rDNA clone library and 16S rDNA directed polymerase chain reaction with denaturing gradient gel electrophoresis (PCR-DGGE) were used for monitoring the change of bacterial community. The 16S rDNA clone library and PCR-DGGE analysis both confirmed the microbial community of wheat plant phyllosphere were predominantly of the gamma-Proteobacteria phyla. Results from PCR-DGGE analysis indicated a significant change in bacterial community structure within the phyllosphere following fungicide enostroburin application. Bands sequenced within control cultures were predominantly of Pseudomonas genus, but those bands sequenced in the treated samples were predominantly strains of Pantoea genus and Pseudomonas genus. Of interest was the appearance of two DGGE bands following fungicide treatment, one of which had sequence similarities (98%) to Pantoea sp. which might be a competitor of plant pathogens. This study revealed the wheat phyllosphere bacterial community composition and a shift in the bacterial community following fungicide enostroburin application.

Pseudomonas syringae pv. syringae B728a hydrolyses indole-3-acetonitrile to the plant hormone indole-3-acetic acid

Nitrilase enzymes catalyse the hydrolysis of nitrile compounds to the corresponding carboxylic acid and ammonia, and have been identified in plants, bacteria and fungi. There is mounting evidence to support a role for nitrilases in plant-microbe interactions, but the activity of these enzymes in plant pathogenic bacteria remains unexplored. The genomes of the plant pathogenic bacteria Pseudomonas syringae pv. syringae B728a and Pseudomonas syringae pv. tomato DC3000 contain nitrilase genes with high similarity to characterized bacterial arylacetonitrilases. In this study, we show that the nitrilase of P. syringae pv. syringae B728a is an arylacetonitrilase, which is capable of hydrolysing indole-3-acetonitrile to the plant hormone indole-3-acetic acid, and allows P. syringae pv. syringae B728a to use indole-3-acetonitrile as a nitrogen source. This enzyme may represent an additional mechanism for indole-3-acetic acid biosynthesis by P. syringae pv. syringae B728a, or may be used to degrade and assimilate aldoximes and nitriles produced during plant secondary metabolism. Nitrilase activity was not detected in P. syringae pv. tomato DC3000, despite the presence of a homologous nitrilase gene. This raises the interesting question of why nitrilase activity has been retained in P. syringae pv. syringae B728a and not in P. syringae pv. tomato DC3000.

Regulatory interactions between quorum-sensing, auxin, cytokinin, and the Hrp regulon in relation to gall formation and epiphytic fitness of Pantoea agglomerans pv. gypsophilae

Gall formation by Pantoea agglomerans pv. gypsophilae is controlled by hrp/hrc genes, phytohormones, and the quorum-sensing (QS) regulatory system. The interactions between these three components were investigated. Disruption of the QS genes pagI and pagR and deletion of both substantially reduced the transcription levels of the hrp regulatory genes hrpXY, hrpS, and hrpL, as determined by quantitative reverse-transcriptase polymerase chain reaction. Expression of hrpL in planta was inhibited by addition of 20 microM or higher concentrations of the QS signal C(4)-HSL. The pagR and hrpL mutants caused an equivalent reduction of 1.3 orders in bacterial multiplication on bean leaves, suggesting possible mediation of the QS effect on epiphytic fitness of P. agglomerans pv. gypsophilae by the hrp regulatory system. indole-3-acetic acid (IAA) and cytokinin significantly affected the expression of the QS and hrp regulatory genes. Transcription of pagI, pagR, hrpL, and hrpS in planta was substantially reduced in iaaH mutant (disrupted in IAA biosynthesis via the indole-3-acetamide pathway) and etz mutant (disrupted in cytokinin biosynthesis). In contrast, the ipdC mutant (disrupted in IAA biosynthesis via the indole-3-pyruvate pathway) substantially increased expression of pagI, pagR, hrpL, and hrpS. Results presented suggest the involvement of IAA and cytokinins in regulation of the QS system and hrp regulatory genes.

Recent evolution of bacterial pathogens: the gall-forming Pantoea agglomerans case

Pantoea agglomerans, a widespread epiphyte and commensal bacterium, has evolved into an Hrp-dependent and host-specific tumorigenic pathogen by acquiring a plasmid containing a pathogenicity island (PAI). The PAI was evolved on an iteron plasmid of the IncN family, which is distributed among genetically diverse populations of P. agglomerans. The structure of the PAI supports the premise of a recently evolved pathogen. This review offers insight into a unique model for emergence of new bacterial pathogens. It illustrates how horizontal gene transfer was the major driving force in the creation of the PAI, although a pathoadaptive mechanism might also be involved. It describes the crucial function of plant-produced indole-3-acetic acid (IAA) and cytokinines (CK) in gall initiation as opposed to the significant but secondary role of pathogen-secreted phytohormones. It also unveils the role of type III effectors in determination of host specificity and evolution of the pathogen into pathovars. Finally, it describes how interactions between the quorum sensing system, hrp regulatory genes, and bacterially secreted IAA or CKs affect gall formation and epiphytic fitness.

Comparative transcriptome analysis of Agrobacterium tumefaciens in response to plant signal salicylic acid, indole-3-acetic acid and gamma-amino butyric acid reveals signalling cross-talk and Agrobacterium--plant co-evolution

Agrobacterium has evolved sophisticated strategies to perceive and transduce plant-derived cues. Recent studies have found that numerous plant signals, including salicylic acid (SA), indole-3-acetic acid (IAA) and gamma-amino butyric acid (GABA), profoundly affect Agrobacterium-plant interactions. Here we determine and compare the transcriptome profiles of Agrobacterium in response to these three plant signals. Collectively, the transcription of 103, 115 and 95 genes was significantly altered by SA, IAA and GABA respectively. Both distinct cellular responses and overlapping signalling pathways were elicited by these three plant signals. Interestingly, these three plant compounds function additively to shut off the Agrobacterium virulence programme and activate the quorum-quenching machinery. Moreover, the repression of the virulence programme by SA and IAA and the inactivation of quorum-sensing signals by SA and GABA are regulated through independent pathways. Our data indicate that these plant signals, while cross-talk in plant signalling networks, also act as cross-kingdom signals and play redundant roles in tailoring Agrobacterium regulatory pathways, resulting in intensive signalling cross-talk in Agrobacterium. Our results support the notion that Agrobacterium has evolved the ability to hijack plant signals for its own benefit. The complex signalling interplay between Agrobacterium and its plant hosts reflects an exquisite co-evolutionary balance.

Quorum-sensing system affects gall development incited by Pantoea agglomerans pv. gypsophilae

The quorum-sensing (QS) regulatory system of the gall-forming Pantoea agglomerans pv. gypsophilae was identified. Mass spectral analysis, together with signal-specific biosensors, demonstrated that P. agglomerans pv. gypsophilae produced N-butanoyl-l-homoserine lactone (C4-HSL) as a major and N-hexanoyl-l-homoserine lactone (C6-HSL) as a minor QS signal. Homologs of luxI and luxR regulatory genes, pagI and pagR, were characterized in strain P. agglomerans pv. gypsophilae Pag824-1 and shown to be convergently transcribed and separated by 14 bp. The deduced PagI (23.8 kDa) and PagR (26.9 kDa) show high similarity with SmaI (41% identity) and SmaR (43% identity), respectively, of Serratia sp. American Type Culture Collection 39006. PagR possesses characteristic autoinducer binding and a helix-turn-helix DNA-binding domain. Gall formation by P. agglomerans pv. gypsophilae depends on a plasmid-borne hrp/hrc gene cluster, type III effectors, and phytohormones. Disruption of pagI, pagR, or both genes simultaneously in Pag824-1 reduced gall size in gypsophila cuttings by 50 to 55% when plants were inoculated with 10(6) CFU/ml. Higher reductions in gall size (70 to 90%) were achieved by overexpression of pagI or addition of exogenous C4-HSL. Expression of the hrp/hrc regulatory gene hrpL and the type III effector pthG in the pagI mutant, as measured with quantitative reverse-transcriptase polymerase chain reaction, was reduced by 5.8 and 6.6, respectively, compared with the wild type, suggesting an effect of the QS system on the Hrp regulon.

Repression of the auxin response pathway increases Arabidopsis susceptibility to necrotrophic fungi

In plants, resistance to necrotrophic pathogens depends on the interplay between different hormone systems, such as those regulated by salicylic acid (SA), jasmonic acid (JA), ethylene, and abscisic acid. Repression of auxin signaling by the SA pathway was recently shown to contribute to antibacterial resistance. Here, we demonstrate that Arabidopsis auxin signaling mutants axr1, axr2, and axr6 that have defects in the auxin-stimulated SCF (Skp1-Cullin-F-box) ubiquitination pathway exhibit increased susceptibility to the necrotrophic fungi Plectosphaerella cucumerina and Botrytis cinerea. Also, stabilization of the auxin transcriptional repressor AXR3 that is normally targeted for removal by the SCF-ubiquitin/proteasome machinery occurs upon P. cucumerina infection. Pharmacological inhibition of auxin transport or proteasome function each compromise necrotroph resistance of wild-type plants to a similar extent as in non-treated auxin response mutants. These results suggest that auxin signaling is important for resistance to the necrotrophic fungi P. cucumerina and B. cinerea. SGT1b (one of two Arabidopsis SGT1 genes encoding HSP90/HSC70 co-chaperones) promotes the functions of SCF E3-ubiquitin ligase complexes in auxin and JA responses and resistance conditioned by certain Resistance (R) genes to biotrophic pathogens. We find that sgt1b mutants are as resistant to P. cucumerina as wild-type plants. Conversely, auxin/SCF signaling mutants are uncompromised in RPP4-triggered resistance to the obligate biotrophic oomycete, Hyaloperonospora parasitica. Thus, the predominant action of SGT1b in R gene-conditioned resistance to oomycetes appears to be at a site other than assisting SCF E3-ubiquitin ligases. However, genetic additivity of sgt1b axr1 double mutants in susceptibility to H. parasitica suggests that SCF-mediated ubiquitination contributes to limiting biotrophic pathogen colonization once plant-pathogen compatibility is established.

Distribution and replication of the pathogenicity plasmid pPATH in diverse populations of the gall-forming bacterium Pantoea agglomerans

Pantoea agglomerans has been transformed from a commensal bacterium into two related gall-forming pathovars by acquisition of pPATH plasmids containing a pathogenicity island (PAI). This PAI harbors an hrp/hrc gene cluster, type III effectors, and phytohormone biosynthetic genes. DNA typing by pulsed-field gel electrophoresis revealed two major groups of P. agglomerans pv. gypsophilae and one group of P. agglomerans pv. betae. The pPATH plasmids of the different groups had nearly identical replicons (98% identity), and the RepA protein showed the highest level of similarity with IncN plasmid proteins. A series of plasmids, designated pRAs, in which the whole replicon region (2,170 bp) or deleted derivatives of it were ligated with nptI were generated for replicon analysis. A basic 929-bp replicon (pRA6) was sufficient for replication in Escherichia coli and in nonpathogenic P. agglomerans. However, the whole replicon region (pRA1) was necessary for expulsion of the pPATH plasmid, which resulted in the loss of pathogenicity. The presence of direct repeats in the replicon region suggests that the pPATH plasmid is an iteron plasmid and that the repeats may regulate its replication. The pPATH plasmids are nonconjugative but exhibit a broad host range, as shown by replication of pRA1 in Erwinia, Pseudomonas, and Xanthomonas. Restriction fragment length polymorphism analyses indicated that the PAIs in the two groups of P. agglomerans pv. gypsophilae are similar but different from those in P. agglomerans pv. betae. The results could indicate that the pPATH plasmids evolved from a common ancestral mobilizable plasmid that was transferred into different strains of P. agglomerans.

Salicylic acid inhibits pathogen growth in plants through repression of the auxin signaling pathway

The phytohormone auxin regulates almost every aspect of plant development. At the molecular level, auxin induces gene expression through direct physical interaction with the TIR1-like F box proteins, which in turn remove the Aux/IAA family of transcriptional repressors [1-4]. A growing body of evidence indicates that many plant pathogens can either produce auxin themselves or manipulate host auxin biosynthesis to interfere with the host's normal developmental processes [5-11]. In response, plants probably evolved mechanisms to repress auxin signaling during infection as a defense strategy. Plants overaccumulating the defense signal molecule salicylic acid (SA) frequently display morphological phenotypes that are reminiscent of auxin-deficient or auxin-insensitive mutants, indicating that SA might interfere with auxin responses. By using the Affymetrix ATH1 GeneChip for Arabidopsis thaliana, we performed a comprehensive study of the effects of SA on auxin signaling [12]. We found that SA causes global repression of auxin-related genes, including the TIR1 receptor gene, resulting in stabilization of the Aux/IAA repressor proteins and inhibition of auxin responses. We demonstrate that this inhibitory effect on auxin signaling is a part of the SA-mediated disease-resistance mechanism.

Orchid-associated bacteria produce indole-3-acetic acid, promote seed germination, and increase their microbial yield in response to exogenous auxin

Germination of orchid seeds is a complex process. In this paper we focus on interactions between the host-plant and its bacterial partners via indole-3-acetic acid (IAA). Originally isolated from the roots of the epiphytic orchid Dendrobium moschatum, the strains of Rhizobium, Microbacterium, Sphingomonas, and Mycobacterium genera were among the most active IAA producers. Addition of exogenous tryptophan significantly enhanced auxin formation both in mineral and complex media. The presence of IAA and indole-3-acetaldehyde was confirmed by HPLC. Indole-3-pyruvic and indole-3-lactic acids were also detected in supernatants of culture filtrates of Sphingomonas sp., Rhizobium sp., and Microbacterium sp., while indole-3-acetamide was identified only in Mycobacterium sp. Some concentration- and strain-dependent effects of exogenous IAA on bacterial development were also established. Treatment of the cultures with 10 and 100 microg/ml of auxin resulted in an increase in microbial yield. None of the investigated strains was able to utilize IAA as a source of carbon and energy. Furthermore, inoculation of D. moschatum seeds with Sphingomonas sp. and Mycobacterium sp. resulted in considerable enhancement of orchid seeds germination. This growth-promoting activity was observed in the absence of any plant growth stimulators or mycorrhizal fungi, usually required for orchid germination.

Indole-3-acetic acid in microbial and microorganism-plant signaling

Diverse bacterial species possess the ability to produce the auxin phytohormone indole-3-acetic acid (IAA). Different biosynthesis pathways have been identified and redundancy for IAA biosynthesis is widespread among plant-associated bacteria. Interactions between IAA-producing bacteria and plants lead to diverse outcomes on the plant side, varying from pathogenesis to phyto-stimulation. Reviewing the role of bacterial IAA in different microorganism-plant interactions highlights the fact that bacteria use this phytohormone to interact with plants as part of their colonization strategy, including phyto-stimulation and circumvention of basal plant defense mechanisms. Moreover, several recent reports indicate that IAA can also be a signaling molecule in bacteria and therefore can have a direct effect on bacterial physiology. This review discusses past and recent data, and emerging views on IAA, a well-known phytohormone, as a microbial metabolic and signaling molecule.

Global effect of indole-3-acetic acid biosynthesis on multiple virulence factors of Erwinia chrysanthemi 3937

Production of the plant hormone indole-3-acetic acid (IAA) is widespread among plant-associated microorganisms. The non-gall-forming phytopathogen Erwinia chrysanthemi 3937 (strain Ech3937) possesses iaaM (ASAP16562) and iaaH (ASAP16563) gene homologues. In this work, the null knockout iaaM mutant strain Ech138 was constructed. The IAA production by Ech138 was reduced in M9 minimal medium supplemented with l-tryptophan. Compared with wild-type Ech3937, Ech138 exhibited reduced ability to produce local maceration, but its multiplication in Saintpaulia ionantha was unaffected. The pectate lyase production of Ech138 was diminished. Compared with wild-type Ech3937, the expression levels of an oligogalacturonate lyase gene, ogl, and three endopectate lyase genes, pelD, pelI, and pelL, were reduced in Ech138 as determined by a green fluorescent protein-based fluorescence-activated cell sorting promoter activity assay. In addition, the transcription of type III secretion system (T3SS) genes, dspE (a putative T3SS effector) and hrpN (T3SS harpin), was found to be diminished in the iaaM mutant Ech138. Compared with Ech3937, reduced expression of hrpL (a T3SS alternative sigma factor) and gacA but increased expression of rsmA in Ech138 was also observed, suggesting that the regulation of T3SS and pectate lyase genes by IAA biosynthesis might be partially due to the posttranscriptional regulation of the Gac-Rsm regulatory pathway.

Domain switching and host recognition

The collective function of secreted pathogen effector molecules is to enhance the virulence and avirulence activity of the pathogen during the infection of its host. While the activity of a majority of pathogen effectors is unknown, several classes of effector molecules have been well characterized. Among these include proteins which function to modulate host defences either through proteolysis, post-translational modifications, or by directly manipulating the host transcriptional machinery that regulates the induction of defence responses. In recent years, several key advances have been made in the characterization of the latter class of effector molecules. Among these include research characterizing the processes associated with host nuclear import and the targeting of host transcriptional defences. While current research is beginning to reveal the biochemical and genetic mechanisms controlling the induction of host resistance, the signalling events that control host specificity remain largely unknown. In this issue of Molecular Microbiology, work by Nissan et al. sheds light onto the molecular-genetic patterns involved in determining host specificity and pathogen virulence in the Pantoea-gypsophila interaction.

The type III effectors HsvG and HsvB of gall-forming Pantoea agglomerans determine host specificity and function as transcriptional activators

Pantoea agglomerans pv. gypsophilae (Pag) elicits galls on gypsophila and a hypersensitive response on beet, whereas P. agglomerans pv. betae (Pab) induces galls on both beet and gypsophila. The pathogenicity of both pathovars is dependent on the presence of a plasmid harbouring type III secretion system (TTSS) components and effectors. The HsvG TTSS effectors of Pag (HsvG-Pag) and Pab (HsvG-Pab) determine the host specificity of both pathovars on gypsophila. Here we describe a novel HsvG homologue, HsvB, which determines the host specificity of Pag and Pab on beet. HsvG requires two direct amino acid repeats for pathogenicity on gypsophila, whereas one repeat in HsvB is sufficient for pathogenicity on beet. Exchanging repeats between HsvG-Pag and HsvB-Pab resulted in a switch of host specificities. Transient expression of GFP-HsvG or GFP-HsvB fusions in gypsophila, beet or melon leaves showed that HsvG and HsvB were localized to the nuclei of host and non-host plants. A yeast one-hybrid assay revealed that a single repeat of HsvG or HsvB was sufficient to activate transcription. By employing random binding-site selection and gel-shift assay HsvG was demonstrated to be a double-stranded DNA-binding protein with an ACACC/aAA consensus binding site. These results suggest that HsvG and HsvB are host-specificity determinants and bear the potential to affect the host transcriptional machinery.

Comparative anatomy of gall development on Gypsophila paniculata induced by bacteria with different mechanisms of pathogenicity

Galls induced on Gypsophila paniculata by Pantoea agglomerans pv. gypsophilae (Pag) and Agrobacterium tumefaciens (At), bacteria with different mechanisms of pathogenicity, were compared morphologically and anatomically. The pathogenicity of Pag is dependent on the presence of an indigenous plasmid that harbors hrp gene cluster, genes encoding Hop virulence proteins and biosynthetic genes for auxin (IAA) and cytokinins (CKs), whereas that of At involves host transformation. The Pag-induced gall was rough, brittle and exhibited limited growth, in contrast to the smooth, firm appearance and continuous growth of the At-induced gall. Anatomical analysis revealed the presence of cells with enlarged nuclei and multiple nucleoli, giant cells and suberin deposition in Pag that were absent from At-induced galls. Although circular vessels were observed in both gall types, they were more numerous and the vascular system was more organized in At. An aerenchymal tissue was observed in the upper part of the galls. Ethylene emission from Pag galls, recorded 6 days after inoculation, was eight times as great as that from non-infected controls. In contrast, a significant decrease in ethylene production was observed in Gypsophila cuttings infected with Pag mutants deficient in IAA and CK production. The results presented are best accounted for by the two pathogens having distinct pathogenicity mechanisms that lead to their differential recognition by the host as non-self (Pag) and self (At).

The arms race continues: battle strategies between plants and fungal pathogens

Plants are under constant attack by a vast array of pathogens. To impede their attackers they use both broad-spectrum and pathogen-specific defence mechanisms. The arms race between plants and fungal pathogens is fascinatingly varied, and what might be elicited as a plant defence mechanism against a pathogen could promote or enhance the virulence of other pathogens. Fungi use countermeasures to detoxify plant antimicrobial compounds and to evade host resistance mechanisms. Certain fungal species also manipulate the host hormone balance to create an environment that is beneficial to their survival. Several lines of evidence indicate a co-evolutionary arms race in which both plants and fungi can respond to changes that occur in their opponents.

Pseudomonas syringae genes induced during colonization of leaf surfaces

The foliar pathogen and ice nucleator, Pseudomonas syringae pv. syringae B728a, demonstrates a high level of epiphytic fitness on plants. Using a promoter-trapping strategy termed habitat-inducible rescue of survival (HIRS), we identified genes of this organism that are induced during colonization of healthy bean leaf surfaces. These plant-inducible genes (pigs) encode diverse cellular functions including virulence, transcription regulation, transport, nutrient acquisition and other known and unknown loci, some of which may result in antisense transcripts to annotated P. syringae genes. Prominent among the pigs was ssuE, a gene in the sulfate-starvation regulon, indicating that sulfate is not abundant on leaf surfaces. inaZ reporter gene fusion assays of the plant-inducible loci revealed up to 300-fold higher levels of pig transcriptional activity on plant leaves compared with minimal medium. However, the maximum levels of pig transcriptional activity were typically too weak to be measured using a gfp reporter gene. One exception was orf6 in the hrp/hrc pathogenicity island which was highly induced in epiphytic P. syringae cells. Four pigs were disrupted by insertional mutagenesis. While growth of the ssuE mutant was impaired under certain conditions in laboratory medium, the epiphytic and virulence properties of the mutants on bean plants were identical to wild-type P. syringae. Our results demonstrate the utility of HIRS to identify genes expressed on leaves and provide new insight into the leaf surface environment.

Analysis of promoters recognized by HrpL, an alternative sigma-factor protein from Pantoea agglomerans pv. gypsophilae

HrpL, an alternative sigma factor, activates the transcription of the Hrp regulon by its binding to a common "hrp box" promoter. Based on computational techniques, the hrp box previously was defined as a consensus bipartite cis element, 5'-GGAACC-N(15-16)-CCACNNA-3'. The present report combines a quantitative in vivo assay for measuring Hrp promoter activity with site-specific mutagenesis to analyze the effect of consensus and nonconsensus nucleotides on promoter activity. The analysis was carried out with Hop effectors of the tumorigenic bacterium Pantoea agglomerans pv. gypsophilae, in which HrpL is indispensable for gall formation. Mutational analysis indicates that the hrp box consensus can be divided into crucial and noncrucial nucleotides. The first 5 nucleotides (nt) of the--35 consensus motif (GGAAC) and the 3 nt of the--10 motif (ACNNA) are crucial, whereas other consensus and adjacent nonconsensus nucleotides exert a significant effect on the promoter's strength. With spacing of 13 or 17 nt between the two motifs, significant activity was still retained. Gel shift assays indicated that deletion of GG from the--35 consensus motif eliminated HrpL binding, whereas mutations in the--10 consensus motif or modification of the spacing, which eliminates promoter activity, did not elicit any effect. The degeneracy in Hrp promoters of four hrp and type III effector genes of P agglomerans pv. gypsophilae indicated significant differences in promoter activity, whereas increasing the promoter strength of the Hop effector, HsvG, resulted in overexpression of gall formation.

Biosynthesis of auxin by the gram-positive phytopathogen Rhodococcus fascians is controlled by compounds specific to infected plant tissues

The role and metabolism of indole-3-acetic acid in gram-negative bacteria is well documented, but little is known about indole-3-acetic acid biosynthesis and regulation in gram-positive bacteria. The phytopathogen Rhodococcus fascians, a gram-positive organism, incites diverse developmental alterations, such as leafy galls, on a wide range of plants. Phenotypic analysis of a leafy gall suggests that auxin may play an important role in the development of the symptoms. We show here for the first time that R. fascians produces and secretes the auxin indole-3-acetic acid. Interestingly, whereas noninfected-tobacco extracts have no effect, indole-3-acetic acid synthesis is highly induced in the presence of infected-tobacco extracts when tryptophan is not limiting. Indole-3-acetic acid production by a plasmid-free strain shows that the biosynthetic genes are located on the bacterial chromosome, although plasmid-encoded genes contribute to the kinetics and regulation of indole-3-acetic acid biosynthesis. The indole-3-acetic acid intermediates present in bacterial cells and secreted into the growth media show that the main biosynthetic route used by R. fascians is the indole-3-pyruvic acid pathway with a possible rate-limiting role for indole-3-ethanol. The relationship between indole-3-acetic acid production and the symptoms induced by R. fascians is discussed.

Nitric Oxide Signaling in Plants

Plants have four nitric oxide synthase (NOS) enzymes. NOS1 appears mitochondrial, and inducible nitric oxide synthase (iNOS) chloroplastic. Distinct peroxisomal and apoplastic NOS enzymes are predicted. Nitrite-dependent NO synthesis is catalyzed by cytoplasmic nitrate reductase or a root plasma membrane enzyme, or occurs nonenzymatically. Nitric oxide undergoes both catalyzed and uncatalyzed oxidation. However, there is no evidence of reaction with superoxide, and S-nitrosylation reactions are unlikely except during hypoxia. The only proven direct targets of NO in plants are metalloenzymes and one metal complex. Nitric oxide inhibits apoplastic catalases/ascorbate peroxidases in some species but may stimulate these enzymes in others. Plants also have the NO response pathway involving cGMP, cADPR, and release of calcium from internal stores. Other known targets include chloroplast and mitochondrial electron transport. Nitric oxide suppresses Fenton chemistry by interacting with ferryl ion, preventing generation of hydroxyl radicals. Functions of NO in plant development, response to biotic and abiotic stressors, iron homeostasis, and regulation of respiration and photosynthesis may all be ascribed to interaction with one of these targets. Nitric oxide function in drought/abscisic acid (ABA)-induction of stomatal closure requires nitrate reductase and NOS1. Nitric oxide synthasel likely functions to produce sufficient NO to inhibit photosynthetic electron transport, allowing nitrite accumulation. Nitric oxide is produced during the hypersensitive response outside cells undergoing programmed cell death immediately prior to loss of plasma membrane integrity. A plasma membrane lipid-derived signal likely activates apoplastic NOS. Nitric oxide diffuses within the apoplast and signals neighboring cells via hydrogen peroxide (H2O2)-dependent induction of salicylic acid biosynthesis. Response to wounding appears to involve the same NOS and direct targets.

Isolation and characterization of soybean-associated bacteria and their potential for plant growth promotion

Endophytic and epiphytic bacteria were isolated from two soybean cultivars (Foscarin and Cristalina). Significant differences were observed in bacterial population densities in relation to season of isolation, soybean growth phase and the tissues from which the isolates were obtained. The isolates were identified by partial 16S rDNA sequence analysis, with most of the isolates belonging to the Pseudomonaceae, Burkholderiacea and Enterobacteriaceae groups. The potential of the isolates for plant growth promotion was evaluated by screening for indoleacetic acid (IAA) production and mineral phosphate solubilization; 34% of endophytic bacteria produced IAA and 49% were able to solubilize mineral phosphate whereas only 21% of epiphytic bacteria produced IAA although 52% were able to solubilize mineral phosphate. A high frequency of IAA producing isolates occurred in the early ripening Foscarin cultivar whereas a high percentage of phosphate solubilizing isolates were obtained from plants in the initial development stage (V6). We also found that 60% of endophytic and 69% of epiphytic isolates that produced IAA and solubilized mineral phosphate were also able to fix nitrogen in vitro. The soybean-associated bacteria showing characteristics related to plant growth promotion were identified as belonging to the genera Pseudomonas, Ralstonia, Enterobacter, Pantoea and Acinetobacter.

Indole-3-acetic Acid-producing bacteria are associated with cranberry stem gall

ABSTRACT Cranberry stem gall is characterized by tumors that girdle stems, thereby killing all distal leaves, flowers, and fruit. Among bacteria isolated from galls, all 11 isolates that were identified as members of the family Enterobacteriaceae caused galls on 50 to 100% of micropropagated cranberry plants that were inoculated. Four of fifteen isolates identified as Pseudomonas spp. caused galls on 10 to 83% of plants inoculated. Twelve of fifteen isolates identified as either Agrobacterium spp. or Rhizobium spp. caused galls on 10 to 50% of plants inoculated, but the galls were smaller than those caused by members of the family Enterobacteriaceae or Pseudomonas spp. There was a positive correlation between the ability of bacteria to produce IAA in vitro and cause galls. In 2002 and 2003, bacteria were isolated from plant and soil samples collected from beds where stem gall had been observed in the past 2 years and beds where stem gall had never been observed. IAA-producing bacteria were common in all samples, although trends were different across years. The results of this study support the hypothesis that IAA-producing bacteria cause cranberry stem gall and suggest that rather than one bacterial species being the cause, multiple strains of bacteria that produce IAA may be responsible for gall formation.

In planta production of indole-3-acetic acid by Colletotrichum gloeosporioides f. sp. aeschynomene

The plant pathogenic fungus Colletotrichum gloeosporioides f. sp. aeschynomene utilizes external tryptophan to produce indole-3-acetic acid (IAA) through the intermediate indole-3-acetamide (IAM). We studied the effects of tryptophan, IAA, and IAM on IAA biosynthesis in fungal axenic cultures and on in planta IAA production by the fungus. IAA biosynthesis was strictly dependent on external tryptophan and was enhanced by tryptophan and IAM. The fungus produced IAM and IAA in planta during the biotrophic and necrotrophic phases of infection. The amounts of IAA produced per fungal biomass were highest during the biotrophic phase. IAA production by this plant pathogen might be important during early stages of plant colonization.

Knot Formation Caused by Pseudomonas syringae subsp. savastanoi on Olive Plants Is hrp-Dependent

ABSTRACT The virulence of Pseudomonas syringae subsp. savastanoi, which causes hyperplastic symptoms (knots) on olive plants, is associated with secreted phytohormones. We identified a Tn5-induced mutant of P. syringae subsp. savastanoi that did not cause disease symptoms on olive plants although it was still able to produce phytohormones. In addition, the mutant failed to elicit a hypersensitive response in a nonhost plant. Molecular characterization of the mutant revealed that a single Tn5 insertion occurred within an open reading frame encoding a protein 92% identical to the HrcC protein of P. syringae pv. syringae. Moreover, sequence analysis revealed that the gene encoding the HrcC protein in P. syringae subsp. savastanoi was part of an operon that included five genes arranged as in other phytopathogenic bacteria. These results imply that hrp/hrc genes are functional in P. syringae subsp. savastanoi and that they play a key role in the pathogenicity of this plant pathogen.

Pantoea agglomerans pvs. gypsophilae and betae, recently evolved pathogens?

SUMMARY Pantoea agglomerans pvs. gypsophilae and betae

TAXONOMY

Bacteria; Proteobacteria; gamma subdivision; order Enterobacteriales; family Enterobacteriaceae; species Pantoea agglomerans. Microbiological properties: Gram-negative, non-capsulated, non-spore-forming, predominately motile rode. Disease symptoms: Gall formation at wound sites, mainly in the crown region of the stem. The host range of P. agglomerans pv. gypsophilae is restricted to Gypsophila paniculata, whereas P. agglomerans pv. betae is pathogenic on Beta vulgaris and gypsophila. Disease control: Pathogenic-free transplants and sanitation. No resistant cultivars are available. Major virulence determinants: Pathogenicity plasmid (pPATH), hrp cluster, type III virulence effectors, phytohormones.

Engineering hypervirulence in a mycoherbicidal fungus for efficient weed control

Agents proposed for biocontrol of major weeds in arable row-crop agriculture have not met expectations because an evolutionary balance has developed between microorganism and weed, even when the mycoherbicide is used inundatively at very high levels (>10(4)spores/cm<(2)). Sufficient virulence can be achieved by transferring genes to the microorganism, tipping the evolutionary balance. Virulence was increased ninefold and was more rapidly effected; furthermore, the requirement for a long duration at high humidity was decreased by introducing NEP1 encoding a phytotoxic protein, to an Abutilon theophrasti-specific, weakly mycoherbicidal strain of Colletotrichum coccodes. The parent strain was at best infective on juvenile cotyledons of this intransigent weed. The transgenic strain was lethal through the three-leaf stage, a sufficient time window to control this asynchronously germinating weed. Strategies of coupling virulence genes with fail-safe mechanisms to prevent spread (due to broadened host range) and to mitigate transgene introgression into crop pathogens could be very useful in the biocontrol of major weeds in row crops.

Fitness of Salmonella enterica serovar Thompson in the cilantro phyllosphere

The epiphytic fitness of Salmonella enterica was assessed on cilantro plants by using a strain of S. enterica serovar Thompson that was linked to an outbreak resulting from cilantro. Salmonella serovar Thompson had the ability to colonize the surface of cilantro leaves, where it was detected by confocal laser scanning microscopy (CLSM) at high densities on the veins and in natural lesions. The population sizes of two common colonizers of plant surfaces, Pantoea agglomerans and Pseudomonas chlororaphis, were 10-fold higher than that of the human pathogen on cilantro incubated at 22 degrees C. However, Salmonella serovar Thompson achieved significantly higher population levels and accounted for a higher proportion of the total culturable bacterial flora on cilantro leaves when the plants were incubated at warm temperatures, such as 30 degrees C, after inoculation, indicating that the higher growth rates exhibited by Salmonella serovar Thompson at warm temperatures may increase the competitiveness of this organism in the phyllosphere. The tolerance of Salmonella serovar Thompson to dry conditions on plants at 60% relative humidity was at least equal to that of P. agglomerans and P. chlororaphis. Moreover, after exposure to low humidity on cilantro, Salmonella serovar Thompson recovered under high humidity to achieve its maximum population size in the cilantro phyllosphere. Visualization by CLSM of green fluorescent protein-tagged Salmonella serovar Thompson and dsRed-tagged P. agglomerans inoculated onto cilantro revealed that the human pathogen and the bacterial epiphyte formed large heterogeneous aggregates on the leaf surface. Our studies support the hypothesis that preharvest contamination of crops by S. enterica plays a role in outbreaks linked to fresh fruits and vegetables.

The presence of diverse IS elements and an avrPphD homologue that acts as a virulence factor on the pathogenicity plasmid of Erwinia herbicola pv. gypsophilae

The pathogenicity of Erwinia herbicola pv. gypsophilae (Ehg) and Erwinia herbicola pv. betae (Ehb) is dependent on a native plasmid (pPATH(Ehg) or pPATH(Ehb)) that harbors the hrp gene cluster, genes encoding type III effectors, phytohormones, biosynthetic genes, and several copies of IS1327. Sequence analysis of the hrp-flanking region in pPATH(Ehg) (cosmid pLA150) revealed a cluster of four additional IS elements designated as ISEhel, ISEhe2, ISEhe3, and ISEhe4. Two copies of another IS element (ISEhe5) were identified on the upstream region of the indole-3-acetic acid operon located on the same cosmid. Based on homology of amino acids and genetic organization, ISEhe1 belongs to the IS630 family, ISEhe2 to the IS5 family, ISEhe3 and ISEhe4 to different groups of the IS3 family, and ISEhe5 to the IS1 family. With the exception of ISEhe4, one to three copies of all the other IS elements were identified only in pathogenic strains of Erwinia herbicola pv. gypsophilae and Erwinia herbicola pv. betae whereas ISEhe4 was present in both pathogenic and nonpathogenic strains. An open reading frame that exhibited high identity (89% in amino acids) to AvrPphD of Pseudomonas syringae pv. phaseolicola was present within the cluster of IS elements. An insertional mutation in the AvrPphDEh, reduced gall size in gypsophila by approximately 85%. In addition, remnants of known genes from four different bacteria were detected on the same cosmid.

The operon for cytokinin biosynthesis of Erwinia herbicola pv. gypsophilae contains two promoters and is plant induced

The operon for cytokinin biosynthesis in the gall-forming bacterium Erwinia herbicola pv. gypsophilae (Ehg) has been previously shown to reside on an indigenous plasmid (pPATH(Ehg)) that is mandatory for pathogenicity. This operon consists of two genes: the first open reading frame (pre-etz) is of unknown function, whereas the second one (etz) encodes for isopentenyl transferase. Northern hybridization performed with the wild-type strain Ehg824-1 grown in Luria-Bertani broth demonstrated two transcripts of which an etz-specific transcript (1.0 kb) was predominant. Fusion of upstream DNA fragments of both pre-etz and etz to the ice nucleation reporter gene inaZ in pVSP61 showed high ice nucleation activity in both cultures, confirming the presence of two independent promoters. An increase of 1-1.5 orders in transcriptional activity of these promoters was observed following inoculation of gypsophila cuttings. Mutants of Ehg824-1 were generated by insertion of inaZ into pre-etz and etz using the transposon reporter Tn3-Spice. An increase of about two orders in transcriptional activity was recorded with both mutants following inoculation of gypsophila or bean cuttings. A similar induction was also observed when the bacteria were applied to the leaf surface of these plants. Unlike other virulence genes present on the pPATH(Ehg), neither pre-etz nor etz was regulated by the adjacent hrp gene cluster.

Heterogeneous transcription of an indoleacetic acid biosynthetic gene in Erwinia herbicola on plant surfaces

We investigated the spatial pattern of expression of ipdC, a plant inducible gene involved in indoleacetic acid biosynthesis in Erwinia herbicola, among individual cells on plants to gain a better understanding of the role of this phenotype in the epiphytic ecology of bacteria and the factors involved in the regulation of ipdC. Nonpathogenic E. herbicola strain 299R harboring a transcriptional fusion of ipdC to gfp was inoculated onto bean plants, recovered from individual leaves 48 h after inoculation, and subjected to fluorescence in situ hybridization using a 16S rRNA oligonucleotide probe specific to strain 299R. Epifluorescence images captured through a rhodamine filter were used to distinguish the 5carboxytetramethylrhodamine-labeled cells of strain 299R from other leaf microflora. Quantification of the green fluorescence intensity of individual cells by analysis of digital images revealed that about 65% of the 299R cells recovered from bean leaves had higher ipdC expression than in culture. Additionally, 10% of the cells exhibited much higher levels of green fluorescence than the median fluorescence intensity, indicating that they are more heterogeneous with respect to ipdC expression on plants than in culture. Examination of 299R cells in situ on leaf surfaces by confocal laser scanning microscopy after fluorescence in situ hybridization of cells on leaf samples showed that even cells that were in close proximity exhibited dramatically different green fluorescence intensities, and thus, were in a physical or chemical microenvironment that induced differential expression of ipdC.

Molecular characterization of global regulatory RNA species that control pathogenicity factors in Erwinia amylovora and Erwinia herbicola pv. gypsophilae

rsmB(Ecc) specifies a nontranslatable RNA regulator that controls exoprotein production and pathogenicity in soft rot-causing Erwinia carotovora subsp. carotovora. This effect of rsmB(Ecc) RNA is mediated mostly by neutralizing the function of RsmA(Ecc), an RNA-binding protein of E. carotovora subsp. carotovora, which acts as a global negative regulator. To determine the occurrence of functional homologs of rsmB(Ecc) in non-soft-rot-causing Erwinia species, we cloned the rsmB genes of E. amylovora (rsmB(Ea)) and E. herbicola pv. gypsophilae (rsmB(Ehg)). We show that rsmB(Ea) in E. amylovora positively regulates extracellular polysaccharide (EPS) production, motility, and pathogenicity. In E. herbicola pv. gypsophilae, rsmB(Ehg) elevates the levels of transcripts of a cytokinin (etz) gene and stimulates the production of EPS and yellow pigment as well as motility. RsmA(Ea) and RsmA(Ehg) have more than 93% identity to RsmA(Ecc) and, like the latter, function as negative regulators by affecting the transcript stability of the target gene. The rsmB genes reverse the negative effects of RsmA(Ea), RsmA(Ehg), and RsmA(Ecc), but the extent of reversal is highest with homologous combinations of rsm genes. These observations and findings that rsmB(Ea) and rsmB(Ehg) RNA bind RsmA(Ecc) indicate that the rsmB effect is channeled via RsmA. Additional support for this conclusion comes from the observation that the rsmB genes are much more effective as positive regulators in a RsmA(+) strain of E. carotovora subsp. carotovora than in its RsmA(-) derivative. E. herbicola pv. gypsophilae produces a 290-base rsmB transcript that is not subject to processing. By contrast, E. amylovora produces 430- and 300-base rsmB transcripts, the latter presumably derived by processing of the primary transcript as previously noted with the transcripts of rsmB(Ecc). Southern blot hybridizations revealed the presence of rsmB homologs in E. carotovora, E. chrysanthemi, E. amylovora, E. herbicola, E. stewartii and E. rhapontici, as well as in other enterobacteria such as Escherichia coli, Salmonella enterica serovar Typhimurium, Serratia marcescens, Shigella flexneri, Enterobacter aerogenes, Klebsiella pneumoniae, Yersinia enterocolitica, and Y. pseudotuberculosis. A comparison of rsmB sequences from several of these enterobacterial species revealed a highly conserved 34-mer region which is predicted to play a role in positive regulation by rsmB RNA.

Leafy gall formation by Rhodococcus fascians

Rhodococcus fascians infects a wide range of plants, initiating the formation of leafy galls that consist of centers of shoot amplification and shoot growth inhibition. R. fascians is an epiphyte but it also can establish endophytic populations. Bacterial signals involved in symptom development initiate de novo cell division and shoot meristem formation in differentiated tissues. The R. fascians signals exert activities that are distinct from mere cytokinin effects, and the evidence points to a process that adopted cytokinin biosynthetic enzymes to form derivatives with unique activity. Genes implicated in leafy gall formation are located on a linear plasmid and are subject to a highly controlling, complex regulatory network, integrating autoregulatory compounds and environmental signals. Leafy galls are considered as centers with specific metabolic features, a niche where populations of R. fascians experience a selective advantage. Such "metabolic habitat modification" might be universal for gall-inducing bacteria.

Vascularization is a general requirement for growth of plant and animal tumours

Solid-tumour growth in animals as in humans depends on angiogenesis. Tumours that fail to induce the formation of new blood vessels do not enlarge beyond a few millimetres in diameter. Plant tumours induced by Agrobacterium tumefaciens can reach diameters of more than 100 mm, thus raising the question of how they are sufficiently supplied with nutrients and water. Until recently, these rapidly growing tumours were considered unorganized or partly organized masses. However, in analogy to animal and human tumours, growth of leaf and stem tumours depends on neovascularization. Plant tumour cells induce the formation of a sophisticated vascular network consisting of water-conducting vessels and assimilate-transporting sieve elements. Similar to animal and human tumours that overexpress angiogenic growth factors, plant tumours overexpress the T-DNA-encoded vascularization-promoting growth factors auxin and cytokinin upon AGROBACTERIUM: infection. High auxin levels induce ethylene emission from the tumours, which has a strong impact on tumour and host stem, as well as on root structure and function. Ethylene apparently stimulates abscisic acid synthesis in the leaves above the tumour, which reduces transpiration and thus protects the host plant from rapid wilting. Hence, for the elucidation of phytohormone-dependent vascular development in plants, such tumours are regarded as an excellent model system. The comparison of analogous requirement of neovascularization for tumour growth in plants, as in animals and humans, is discussed in terms of interdisciplinary strategies of possible prevention and therapy.

The dual function in virulence and host range restriction of a gene isolated from the pPATH (Ehg) plasmid of Erwinia herbicola pv. gypsophilae

The host range of the gall-forming bacterium Erwinia herbicola pv. gypsophilae (Ehg) is restricted to gypsophila whereas Erwinia herbicola pv. betae (Ehb) attacks beet as well as gypsophila. Both pathovars contain an indigenous plasmid (pPATH(Ehg or pPATH(Ehb)) that harbors pathogenicity genes, including the hrp gene cluster. A cosmid library of Ehg824-1 plasmid DNA was mobilized into Ehb4188 and the transconjugants were screened for pathogenicity on beet. One Ehb transconjugant harboring the cosmid pLA173 of pPATHEb induced a hypersensitive-like response and abolished pathogenicity on beet. Transposon mutagenesis of an open reading frame (ORF) located on this cosmid eliminated its affect on pathogenicity. Marker exchange of this mutation into Ehg824-1 caused a substantial reduction in gall size on gypsophila and caused Ehg824-1 to extend its host range and incite galls on beet. The ORF (1.5 kb) was designated as pthG (pathogenicity gene on gypsophila). DNA sequence analysis of pthG revealed no significant homology to known genes in the data bank. Only remnants of the pthG sequences were identified on the pPATH of Ehb4188. The deduced protein lacked an N-terminal signal peptide but contained a short trans-membrane helix in its C terminus. The gene product, as determined by expression in Escherichia coli and Western blots (immunoblots), was a 56-kDa protein.

Redundancy as a way of life - IAA metabolism

Plants have evolved elaborate systems for regulating cellular levels of indole-3-acetic acid (IAA). The redundancy of this network has complicated the elucidation of IAA metabolism, but molecular genetic studies and precise analytical methods have begun to expose the circuitry. It is now clear that plants synthesize, inactivate and catabolize IAA by multiple pathways, and multiple genes can encode a particular enzyme within a pathway. A number of these genes are now cloned, which greatly facilitates the future dissection of IAA metabolism.

Bacterial colonization of leaves: a spectrum of strategies

ABSTRACT Bacteria associated with plant leaves, or phyllobacteria, probably employ a range of colonization strategies. Steps in these colonization strategies include modification of the leaf habitat, aggregation, ingression, and egression. Considerable evidence indicates that bacteria can modify their environment to enhance their colonization of plants, such as by increasing local nutrient concentrations or by producing a layer of extracellular polysaccharides. This local habitat modification may occur on the surface of leaves, as well as in the leaf interior, and may be enhanced by the formation of bacterial aggregates. The conspicuous presence of bacterial aggregates on leaves and the finding that the behavior of bacteria on plants varies in a density-dependent manner indicate the potential importance of cooperative interactions among phyllobacteria. Such cooperative interactions may occur among both homogeneous and heterogeneous populations, thus influencing the development of microbial communities. While the sites commonly colonized by most phyllobacteria have not been unambiguously identified, there is strong circumstantial evidence that a sizable proportion of cells, particularly of phytopathogenic strains, are localized within "protected sites" on plants. The likelihood that these protected sites are located in the interior of leaves indicates that phytopathogenic bacteria have access to more resources and greater protection from stresses associated with the leaf surface than bacteria that are restricted to the leaf surface. The internal and external leaf-associated populations probably form a continuum due to the processes of ingression and egression. For a specific pathogen, however, the extent of egression that occurs prior to disease induction is likely to influence the success of disease predictions based on external population size, i.e., the number of bacteria in leaf washings. In this review, we illustrate the complexity of the ecology of leaf-associated bacteria and propose a model of leaf colonization that emphasizes the common elements in bacterial colonization strategies, as well as allows for distinct behavior of different phyllobacterial species.

Indole-3-acetic acid biosynthesis in colletotrichum gloeosporioides f. sp. aeschynomene

We characterized the biosynthesis of indole-3-acetic acid by the mycoherbicide Colletotrichum gloeosporioides f. sp. aeschynomene. Auxin production was tryptophan dependent. Compounds from the indole-3-acetamide and indole-3-pyruvic acid pathways were detected in culture filtrates. Feeding experiments and in vitro assay confirmed the presence of both pathways. Indole-3-acetamide was the major pathway utilized by the fungus to produce indole-3-acetic acid in culture.