Simplified DNA Extraction and Improved PCR Based Detection of Greening Bacterium in Citrus

Citrus greening disease caused by a fastidious bacterium is an important graft transmissible disease in commercial citrus in India and other parts of the world. Polymerase chain reaction (PCR) is a sensitive and convenient method for detection of greening bacterium. A non-phenol chloroform method of DNA extraction was evaluated for DNA quality and PCR based detection of greening bacterium. The method was comparable with a commercial DNA extraction kit (Qiagen) and better than a CTAB based DNA extraction method. To improve the reliability, three primer sets (primers A, B, and C yielding amplicons of 1160 bp, 703 bp and 451 bp, respectively) and two polymerase enzymes (Taq polymerase and Klen Taq polymerase) were evaluated. The primer set C provided better amplification when compared to primer sets A and B. Primer C in combination with Taq polymerase provided amplification band at a DNA template concentration of 100 pg but good amplification band was obtained at still lower DNA template concentration of 0.1 pg when Klen Taq polymerase was used. The standardized PCR protocol combining non-phenol chloroform method of DNA isolation, primer set C and Klen Taq polymerase enzyme was found very effective in detecting greening bacterium in citrus trees. The sequence of cloned amplicon from 16S ribosomal RNA gene had 89–100 % sequence identity with corresponding sequence of Candidatus Liberibacter asiaticus from China, Brazil, Japan and Pune isolate of India, C. Liberibacter americnus from Brazil and C. Liberibacter africanus from Africa.