Differential expression of genes of Pseudomonas syringae on leaves and in culture evaluated with random genomic lux fusions

Colonization of tomato seedlings by bioluminescent Clavibacter michiganensis subsp. michiganensis under different humidity regimes

Tomato bacterial canker, caused by Clavibacter michiganensis subsp. michiganensis, is transmitted by infected or infested seed and mechanically from plant to plant. Wounds occurring during seedling production and crop maintenance facilitate the dissemination of the pathogen. However, the effects of environmental factors on C. michiganensis subsp. michiganensis translocation and growth as an endophyte have not been fully elucidated. A virulent, stable, constitutively bioluminescent C. michiganensis subsp. michiganensis strain BL-Cmm 17 coupled with an in vivo imaging system allowed visualization of the C. michiganensis subsp. michiganensis colonization process in tomato seedlings in real time. The dynamics of bacterial infection in seedlings through wounds were compared under low (45%) and high (83%) relative humidity. Bacteria multiplied rapidly in cotyledon petioles remaining after clip inoculation and moved in the stem toward both root and shoot. Luminescent signals were also observed in tomato seedling roots over time, and root development was reduced in inoculated plants maintained under both humidity regimes. Wilting was more severe in seedlings under high-humidity regimes. A strong positive correlation between light intensity and bacterial population in planta suggests that bioluminescent C. michiganensis subsp. michiganensis strains will be useful in evaluating the efficacy of bactericides and host resistance.

Bioluminescence imaging of Clavibacter michiganensis subsp. michiganensis infection of tomato seeds and plants

Clavibacter michiganensis subsp. michiganensis is a Gram-positive bacterium that causes wilting and cankers, leading to severe economic losses in commercial tomato production worldwide. The disease is transmitted from infected seeds to seedlings and mechanically from plant to plant during seedling production, grafting, pruning, and harvesting. Because of the lack of tools for genetic manipulation, very little is known regarding the mechanisms of seed and seedling infection and movement of C. michiganensis subsp. michiganensis in grafted plants, two focal points for application of bacterial canker control measures in tomato. To facilitate studies on the C. michiganensis subsp. michiganensis movement in tomato seed and grafted plants, we isolated a bioluminescent C. michiganensis subsp. michiganensis strain using the modified Tn1409 containing a promoterless lux reporter. A total of 19 bioluminescent C. michiganensis subsp. michiganensis mutants were obtained. All mutants tested induced a hypersensitive response in Mirabilis jalapa and caused wilting of tomato plants. Real-time colonization studies of germinating seeds using a virulent, stable, constitutively bioluminescent strain, BL-Cmm17, showed that C. michiganensis subsp. michiganensis aggregated on hypocotyls and cotyledons at an early stage of germination. In grafted seedlings in which either the rootstock or scion was exposed to BL-Cmm17 via a contaminated grafting knife, bacteria were translocated in both directions from the graft union at higher inoculum doses. These results emphasize the use of bioluminescent C. michiganensis subsp. michiganensis to help better elucidate the C. michiganensis subsp. michiganensis-tomato plant interactions. Further, we demonstrated the broader applicability of this tool by successful transformation of C. michiganensis subsp. nebraskensis with Tn1409::lux. Thus, our approach would be highly useful to understand the pathogenesis of diseases caused by other subspecies of the agriculturally important C. michiganensis.

Extraction of high-quality bacterial RNA from infected leaf tissue for bacterial in planta gene expression analysis by multiplexed fluorescent Northern hybridization

Plant pathogenic bacteria possess a large number of genes that allow them to grow and cause disease on plants. In planta gene expression analysis is important to understand the impact of these genes on bacterial virulence. A new mRNA-based approach using multiplexed Northern hybridization was developed. High-quality bacterial and plant total RNA was successfully isolated from leaf tissue infiltrated with Pseudomonas syringae. The procedure employs a new extraction buffer formulation containing glycine, sodium dodecylsulphate, cetyltrimethylammonium bromide, high-molecular-weight polyethylene glycol and beta-mercaptoethanol. Cell lysis and classical acid-phenol extraction steps followed by LiCl precipitation yielded large amounts of total RNA of high purity and integrity. Multiplexing of DIG and chemically fluorescently labelled RNA probes was developed and expression data were normalized using the 23S rRNA gene as reference. The method was validated by studying in planta expression of the P. syringae genes mucD, cmaA, cfl, corR, corS and corP comprising a selection of highly expressed biosynthetic and low-expressed regulatory genes. The method was assessed regarding its sensitivity and might by useful for studying a variety of plant-microbe interactions.

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.

Conditional survival as a selection strategy to identify plant-inducible genes of Pseudomonas syringae

A novel strategy termed habitat-inducible rescue of survival (HIRS) was developed to identify genes of Pseudomonas syringae that are induced during growth on bean leaves. This strategy is based on the complementation of metXW, two cotranscribed genes that are necessary for methionine biosynthesis and required for survival of P. syringae on bean leaves exposed to conditions of low humidity. We constructed a promoter trap vector, pTrap, containing a promoterless version of the wild-type P. syringae metXW genes. Only with an active promoter fused to metXW on pTrap did this plasmid restore methionine prototrophy to the P. syringae metXW mutant B7MX89 and survival of this strain on bean leaves. To test this method, a partial library of P. syringae genomic DNA was constructed in pTrap and a total of 1,400 B7MX89 pTrap clones were subjected to HIRS selection on bean leaves. This resulted in the enrichment of five clones, each with a unique RsaI restriction pattern of their DNA insert. Sequence analysis of these clones revealed those P. syringae genes for which putative plant-inducible activity could be assigned. Promoter activity experiments with a gfp reporter gene revealed that these plant-inducible gene promoters had very low levels of expression in minimal medium. Based on green fluorescent protein fluorescence levels, it appears that many P. syringae genes have relatively low expression levels and that the metXW HIRS strategy is a sensitive method to detect weakly expressed P. syringae genes that are active on plants. Furthermore, we found that protected sites on the leaf surface provided a higher level of enrichment for P. syringae expressing metXW than exposed sites. Thus, the metXW HIRS strategy should lead to the identification of P. syringae genes that are expressed primarily in these areas on the leaf.