Development of Polymerase Chain Reaction Primers Highly Specific for Xanthomonas albilineans, the Causal Bacterium of Sugarcane Leaf Scald Disease

Molecular Detection and Quantification of Xanthomonas albilineans in Juice from Symptomless Sugarcane Stalks Using a Real-Time Quantitative PCR Assay

Leaf scald, a bacterial disease caused by Xanthomonas albilineans (Ashby) Dowson, is a major limiting factor for sugarcane production worldwide. Accurate identification and quantification of X. albilineans is a prerequisite for successful management of this disease. A sensitive and robust quantitative PCR (qPCR) assay was developed in this study for detection and quantification of X. albilineans using TaqMan probe and primers targeting a putative adenosine triphosphate-binding cassette (ABC) transporter gene (abc). The novel qPCR assay was highly specific to the 43 tested X. albilineans strains belonging to different pulsed-field gel electrophoresis groups. The detection thresholds were 100 copies/µl of plasmid DNA, 100 fg/µl of bacterial genomic DNA, and 100 CFU/ml of bacterial suspension prepared from pure culture. This qPCR assay was 100 times more sensitive than a conventional PCR assay. The pathogen was detected by qPCR in 75.1% (410/546) of symptomless stalk samples, whereas only 28.4% (155/546) of samples tested positive by conventional PCR. Based on qPCR data, population densities of X. albilineans in symptomless stalks of the same varieties differed between two sugarcane production areas in China, Beihai (Guangxi Province) and Zhanjiang (Guangdong Province), and no significant correlation between these populations was identified. Furthermore, no relationship was found between these populations of the pathogen in asymptomatic stalks and the resistance level of the sugarcane varieties to leaf scald. The newly developed qPCR assay proved to be highly sensitive and reliable for the detection and quantification of X. albilineans in sugarcane stalks.

Detection and Quantification of Xanthomonas albilineans by qPCR and Potential Characterization of Sugarcane Resistance to Leaf Scald

Leaf scald is an important disease of sugarcane with erratic symptom expression. Latency represents a threat to germplasm exchange, and erratic symptom development makes accurate evaluation of disease resistance during breeding and selection problematic. Real-time quantitative polymerase chain reaction (qPCR) assays for Xanthomonas albilineans, the causal agent of leaf scald, were developed and evaluated for the sensitive, specific detection and quantification of the pathogen. Assays with SYBR Green primers and TaqMan probe and primers derived from the albicidin toxin biosynthesis gene cluster efficiently and reproducibly amplified X. albilineans. Detection was more sensitive with qPCR compared with conventional PCR. Assays were specific for X. albilineans and sap extracts did not inhibit the qPCR reaction. Leaf-scald-resistant and -susceptible cultivars were distinguished by infection incidence, disease severity, and X. albilineans population determined by SYBR Green qPCR in both greenhouse and field experiments. Populations of X. albilineans varied in different tissues. Differences were the greatest within tissues in resistant cultivars, and bacterial populations in systemically infected, young, not yet fully emerged leaves exhibited the greatest differences between resistant and susceptible cultivars. The results demonstrate that qPCR is a highly sensitive method for the detection of X. albilineans that could provide a reliable method for leaf scald resistance screening.

Simultaneous detection and identification of the Xanthomonas species complex associated with tomato bacterial spot using species-specific primers and multiplex PCR

AIMS

To establish protocols for the simultaneous detection and identification of Xanthomonas species causing tomato bacterial spot.

METHODS AND RESULTS

We verified the specificity and sensitivity of the previously reported sets of primers designed for strains of the four species of Brazilian tomato bacterial spot xanthomonads, consisting of 30 of Xanthomonas euvesicatoria, 30 of X. vesicatoria, 50 of X. perforans and 50 of X. gardneri. Furthermore, we tested a multiplex PCR protocol for the purpose of concurrent species identification. The possibility of direct detection of the pathogens in diseased leaf samples was also verified. The primers were highly specific, amplifying only target DNA. The sensitivity of the primers in conventional PCR was 50 pg μl(-1) for purified DNA and ranged from 5 × 10(2) to 5 × 10(4) CFU ml(-1) when bacterial suspensions were analysed. The multiplex PCR was suitable for the detection of all four species and showed similar sensitivity to conventional PCR when tested on purified DNA. When using bacterial suspensions, its sensitivity was similar to conventional PCR only when a biological amplification step (Bio-PCR) was included. Both methods were able to detect the pathogens in symptomatic tomato leaves.

CONCLUSIONS

Brazilian Xanthomonas strains causing tomato bacterial spot can be differentiated and identified at species level by a PCR-based method and by a multiplex PCR.

SIGNIFICANCE AND IMPACT OF THE STUDY

This protocol may be a feasible alternative tool for the identification and detection of these pathogens in plant material and may be used for routine diagnostic purposes in plant pathology laboratories.

Molecular characterisation of Xanthomonas strains isolated from aroids in Mauritius

Mauritius is one of the largest world producers of Anthurium cut flowers but outbreaks of bacterial blight have never been reported on the island. This work was about the characterisation and identification of bacterial strains isolated from Anthurium andreanum, Dieffenbachia maculata and Aglaonema simplex in Mauritius. Fifteen strains, that showed the morphological properties of Xanthomonas on conventional media, were tested on two semi-selective media (Esculin-trehalose and cellobiose-starch). ELISA tests using a panel of monoclonal antibodies were carried out and three out of 15 strains reacted with a Xanthomonas-specific monoclonal antibody (MAb XII). Analysis using four sets of ribosomal primers revealed that the same three Mauritius strains shared conserved PCR products with reference xanthomonads including virulent strains of Xanthomonas axonopodis pv. dieffenbachiae (Xad). BIOLOG tests and the Sherlock Microbial Identification system (MIDI) identified these three new strains at the species level as X. axonopodis. The complementary tests that were carried out clearly confirmed that the three strains are xanthomonads and, moreover, a DNA probe which showed specificity to Xad strains suggested that the three Mauritius strains are non-virulent forms of the pathogen causing Anthurium blight.