Molecular characterisation of the causal agent of bacterial leaf streak of maize
Niemann, Nicolaas Johannes Jacobus
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All members of the genus Xanthomonas are considered to be plant pathogenic, with specific pathovars infecting several high value agricultural crops. One of these pathovars, X. campestris pv. zeae (as this is only a proposed name it will further on be referred to as Xanthomonas BLSD) the causal agent of bacterial leaf steak of maize, has established itself as a widespread significant maize pathogen within South Africa. Insufficient information about the present distribution of the pathogen is available. The main aim of the study was thus to isolate and characterise the pathogen using molecular methods. Results demonstrated that the causal agent of bacterial leaf streak disease (Xanthomonas BLSD: potentially X. campestris pv. zeae) was widely distributed within the major maize cultivation regions of South Africa. Most of the isolates collected originated from the Highveld maize production provinces (North West, Free State, Gauteng and Mpumalanga provinces) as well as from irrigated maize fields in the Northern Cape province. The XgumD gene marker was used to determine if the isolates belonged to the genus Xanthomonas. The gumD gene fragment is located within the gumB-gumM region of the operon and is conserved among Xanthomonas species. This gene fragment is partially responsible for xanthan production. This marker was amplified from all isolates and a selected number were sequenced. The marker was only able to confirm that the causal agent was a member of the genus Xanthomonas. PCR methods were used for the characterisation of the isolates. This included PCR and sequencing of ribosomal RNA- gyraseB and gumD genes. A fingerprinting method BOX-PCR was also employed. Good quality DNA of sufficient quantities was obtained from the various isolates. Amplification produced no non-specific amplification products. This resulted in good quality sequences that could be analysed using bioinformatics tools. Phylogenetic analyses of the ribosomal RNA and gyraseB genes could not detect differences amongst the 47 Xanthomonas BLSD isolates. However, these genes were able to distinguish between the type strain of these isolates and various Xanthomonas species and pathovars. From all three neighbour joining trees the Xanthomonas BLSD isolates had close association with X. axonopodis pv. vasculorum strain ATCC 35938. For the 16S rRNA gene there exists no sequence differences between Xanthomonas BLSD and X. axonopodis pv. vasculorum strain ATCC 35938. A single nucleotide difference was observed between Xanthomonas BLSD and X. axonopodis pv. vasculorum strain ATCC 35938 for the 23S rRNA gene. The gyraseB gene detected a total of six nucleotide variations between these two Xanthomonas species. For all of the phylogenetic trees there was no clustering of Xanthomonas BLSD with X. campestris pathovars. Genetic profiling (via BOX-PCR) based on present/absent analysis revealed no variations amongst the Xanthomonas BLSD isolates. All isolates shared an identical pattern produced by 12 distinct PCR products. This profiling technique did differentiate between the isolates of Xanthomonas BLSD and X. axonopodis pv. vasculorum strain ATCC 35938. Their profiles shared common bands, but differed in the number and overall pattern of the bands. These results suggest two main conclusions: (i) Xanthomonas BLSD has a clonal origin with geographical separation not impacting genetic variation. The fact that all the isolates appear to be clonal may imply that when resistant maize cultivars are developed these should be resistant to all isolates of the pathovar irrespective of their geographical origin. This is a suggestion that will have to be corroborated using more isolates and additional genetic fingerprinting techniques (ii) the Xanthomonas BLSD isolates from this study may not belong to X. campestris. Further studies using other markers should be conducted to determine the real identity of Xanthomonas BLSD.