The Infiltration-centrifugation Technique for Extraction of Apoplastic Fluid from Plant Leaves Using <em>Phaseolus vulgaris</em> as an Example

The apoplast is a distinct extracellular compartment in plant tissues that lies outside the plasma membrane and includes the cell wall. The apoplastic ...

University of the Basque Country (UPV/EHU)

The bean (Phaseolus spp.) plant pathogen Pseudomonas syringae pv. phaseolicola is characterized by the ability to produce phaseolotoxin (Tox(+)). We recently reported that the majority of the Spanish P. syringae pv. phaseolicola population is unable to synthesize this toxin (Tox(-)). These Tox(-) isolates appear to lack the entire DNA region for the biosynthesis of phaseolotoxin (argK-tox gene cluster), as shown by PCR amplification and DNA hybridization using DNA sequences specific for separated genes of this cluster. Tox(+) and Tox(-) isolates also showed genomic divergence that included differences in ERIC-PCR and arbitrarily primed-PCR profiles. Tox(+) isolates showed distinct patterns of IS801 genomic insertions and contained a chromosomal IS801 insertion that was absent from Tox(-) isolates. Using a heteroduplex mobility assay, sequence differences were observed only among the intergenic transcribed spacer of the five rDNA operons of the Tox(-) isolates. The techniques used allowed the unequivocal differentiation of isolates of P. syringae pv. phaseolicola from the closely related soybean (Glycine max) pathogen, P. syringae pv. glycinea. Finally, a pathogenicity island that is essential for the pathogenicity of P. syringae pv. phaseolicola on beans appears to be conserved among Tox(+), but not among Tox(-) isolates, which also lacked the characteristic large plasmid that carries this pathogenicity island. It is proposed that the results presented here justify the separation of the Tox(+) and Tox(-) P. syringae pv. phaseolicola isolates into two distinct genetic lineages, designated Pph1 and Pph2, respectively, that show relevant genomic differences that include the pathogenicity gene complement.

Pseudomonas syringae pv. phaseolicola can be separated into two genetic lineages distinguished by the possession of the phaseolotoxin biosynthetic cluster.

The plant apoplast is the intercellular space that surrounds plant cells, in which metabolic and physiological processes relating to cell wall biosynthesis, nutrient transport, and stress responses occur. The apoplast is also the primary site of infection for hemibiotrophic pathogens such as P. syringae, which obtain nutrients directly from apoplastic fluid. We have used apoplastic fluid extracted from healthy tomato leaves as a growth medium for Pseudomonas spp. in order to investigate the role of apoplastic nutrients in plant colonization by Pseudomonas syringae. We have confirmed that apoplast extracts mimic some of the environmental and nutritional conditions that bacteria encounter during apoplast colonization by demonstrating that expression of the plant-induced type III protein secretion pathway is upregulated during bacterial growth in apoplast extracts. We used a modified phenoarray technique to show that apoplast-adapted P. syringae pv. tomato DC3000 expresses nutrient utilization pathways that allow it to use sugars, organic acids, and amino acids that are highly abundant in the tomato apoplast. Comparative analyses of the nutrient utilization profiles of the genome-sequenced strains P. syringae pv. tomato DC3000, P. syringae pv. syringae B728a, P. syringae pv. phaseolicola 1448A, and the unsequenced strain P. syringae pv. tabaci 11528 with nine other genome-sequenced strains of Pseudomonas provide further evidence that P. syringae strains are adapted to use nutrients that are abundant in the leaf apoplast. Interestingly, P. syringae pv. phaseolicola 1448A lacks many of the nutrient utilization abilities that are present in three other P. syringae strains tested, which can be directly linked to differences in the P. syringae pv. phaseolicola 1448A genome.

Pseudomonas syringae pv. tomato DC3000 uses constitutive and apoplast-induced nutrient assimilation pathways to catabolize nutrients that are abundant in the tomato apoplast.

ABSTRACT From a collection of 152 pseudomonads isolated from diseased beans in Spain, 138 (91%) of the strains were identified as Pseudomonas syringae pv. phaseolicola and the rest as P. syringae pv. syringae. The P. syringae pv. phaseolicola strains produced typical water-soaked lesions on bean pods, although 95 of them did not produce phaseolotoxin in vitro. Ninety-four of these isolates did not produce the expected 0.5-kb product after polymerase chain reaction (PCR) amplification using primers specific for open reading frame (ORF) 6 of the phaseolotoxin (tox) gene cluster and did not contain DNA homologous to ORF 6 in Southern hybridization experiments. To our knowledge, this is the first report of the widespread occurrence in the field of strains of P. syringae pv. phaseolicola lacking the tox cluster, which contrasts sharply with the general belief that Tox(+) isolates are the only ones with epidemiological importance. Additionally, the tox(-) isolates were not specifically detected by a commercial polyclonal antisera in an enzyme-linked immunosorbent assay. Accordingly, it is possible that the certification of seed lots as free of the pathogen cannot be reliably done in Spain, or in any other country where tox(-) strains might occur frequently, using current PCR or serological protocols. The amplification of three avirulence genes by PCR allowed us to make predictions of the P. syringae pv. phaseolicola race structure, as confirmed by plant assays. Six races (races 1, 2, 5, 6, 7, and 9) were identified, with race 7 being the most prevalent (46.1%) followed by races 6 (21.3%) and 1 (9.0%). All the tox(-) isolates contained gene avrPphF, typical of races 1, 5, 7, and 9.

Nontoxigenic Strains of Pseudomonas syringae pv. phaseolicola Are a Main Cause of Halo Blight of Beans in Spain and Escape Current Detection Methods.

Erwinia amylovora, the causal agent of fire blight, is genetically very homogeneous, and current methodologies provide insufficient or contradictory information about the probable dispersal routes of the pathogen. With the final aim to obtain specific and reliable molecular markers for different lineages of the pathogen, we studied the molecular basis of rep-polymerase chain reaction (PCR) polymorphism using seven different arbitrary primers to fingerprint 93 E. amylovora strains from different countries, including Spain. Polymorphism was very low, and was displayed by only 11 E. amylovora strains, which produced 22 polymorphic bands. Five of 11 polymorphic bands cloned contained DNA that was present in more than 85% of the strains, whereas six bands were due to DNA present exclusively in the strains producing the rep-PCR polymorphism. Also, five of the polymorphic bands were due to the possession of either the ubiquitous plasmid pEA29, of plasmid pEU30, which was exclusively found in strains from North America, or of a 35-kb cryptic plasmid, present only in 28 strains from Northern Spain. We designed primer pairs from several cloned polymorphic bands that allowed the specific identification of the strains producing the polymorphism. Our results indicate that rep-PCR is not adequate for constructing genealogies of E. amylovora, although the strategy illustrated here, as well as the designed primers, can be used effectively in epidemiological studies with this pathogen.

Polymerase chain reaction fingerprinting of Erwinia amylovora has a limited phylogenetic value but allows the design of highly specific molecular markers.

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.

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

Agrobacterium tumefaciens strain GV3101 (pMP90) is widely used in transient gene expression assays, including assays to study pathogen effectors and plant disease resistance mechanisms. However, inoculation of A. tumefaciens GV3101 into Nicotiana tabacum (tobacco) leaves prior to infiltration with pathogenic and non-host strains of Pseudomonas syringae results in suppression of macroscopic symptoms when compared with leaves pre-treated with a buffer control.

Agroinfiltration reduces ABA levels and suppresses Pseudomonas syringae-elicited salicylic acid production in Nicotiana tabacum.

Metal hyperaccumulation, in which plants store exceptional concentrations of metals in their shoots, is an unusual trait whose evolutionary and ecological significance has prompted extensive debate. Hyperaccumulator plants are usually found on metalliferous soils, and it has been proposed that hyperaccumulation provides a defense against herbivores and pathogens, an idea termed the 'elemental defense' hypothesis. We have investigated this hypothesis using the crucifer Thlaspi caerulescens, a hyperaccumulator of zinc, nickel, and cadmium, and the bacterial pathogen Pseudomonas syringae pv. maculicola (Psm). Using leaf inoculation assays, we have shown that hyperaccumulation of any of the three metals inhibits growth of Psm in planta. Metal concentrations in the bulk leaf and in the apoplast, through which the pathogen invades the leaf, were shown to be sufficient to account for the defensive effect by comparison with in vitro dose-response curves. Further, mutants of Psm with increased and decreased zinc tolerance created by transposon insertion had either enhanced or reduced ability, respectively, to grow in high-zinc plants, indicating that the metal affects the pathogen directly. Finally, we have shown that bacteria naturally colonizing T. caerulescens leaves at the site of a former lead-zinc mine have high zinc tolerance compared with bacteria isolated from non-accumulating plants, suggesting local adaptation to high metal. These results demonstrate that the disease resistance observed in metal-exposed T. caerulescens can be attributed to a direct effect of metal hyperaccumulation, which may thus be functionally analogous to the resistance conferred by antimicrobial metabolites in non-accumulating plants.

Metal hyperaccumulation armors plants against disease.

The bacterial plant pathogen Pseudomonas syringae causes economically important diseases of a wide variety of plant species and is used as a model organism to understand the molecular basis of plant disease. Much existing research into P. syringae-plant interactions has focused on the molecular basis of plant disease resistance and the role of secreted effector proteins in the suppression of plant defences. However, researchers have speculated that the diverse array of effectors, toxins and hormones produced by this pathogen also play an important role in manipulating plant metabolism to promote infection. Recent advances in metabolomics, genomics, transcriptomics and metabolic modelling offer new opportunities to address this question and generate a system-level understanding of metabolic interactions at the host-pathogen interface.

Arantza Rico

School of Education of Vitoria-Gasteiz

University of the Basque Country (UPV/EHU)

The Infiltration-centrifugation Technique for Extraction of Apoplastic Fluid from Plant Leaves Using Phaseolus vulgaris as an Example

Arantza Rico

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}School of Education of Vitoria-Gasteiz

University of the Basque Country (UPV/EHU)

The Infiltration-centrifugation Technique for Extraction of Apoplastic Fluid from Plant Leaves Using Phaseolus vulgaris as an Example

School of Education of Vitoria-Gasteiz