Molecular characterization of 'Clover proliferation' phytoplasma subgroup-D (16SrVI-D) associated with vegetables...

Nine vegetable plants species exhibiting phytoplasma suspected symptoms of white/purple leaf, little leaf, flat stem, witches' broom, phyllody and leaf yellowing were observed in experimental fields at Indian Agricultural Research Institute, New Delhi from December 2015 to July 2016. Total DNA extracted from the three healthy and three symptomatic leaves of all the nine vegetables were subjected to PCR assays using phytoplasma specific primers P1/P7 followed by R16F2n/R16R2 and 3Far/3Rev to amplify the 16S rDNA fragments. No amplifications of DNA were observed in first round PCR assays with primer pair P1/P7 from any of the symptomatic samples. However, phytoplasma DNA specific fragments of ~ 1.3 kb were amplified from Apium graveolens L. (two isolates), Brassica oleracea vr. capitata L. (one isolate) and Solanum melongena L. (one isolate) by using 3Far/3Rev primer pair and 1.2 kb fragment was amplified from Lactuca sativa L. (one isolate) by using R16F2n/R16R2 primer pair. No DNA amplification was seen in other symptomatic vegetable samples of tomato, carrot, cucurbit, bitter gourd and Amaranthus species utilizing either P1/P7 primer pair followed by 3Far/3Rev or R16F2n/R16R2 primer pairs. Out of three leafhopper species collected from the symptomatic vegetable fields, only Hishimonus phycitis was found positive for association of phytoplasma. No DNA amplifications were observed in healthy plant samples and insects collected from non-symptomatic fields. Comparative sequence comparison analyses of 16S rDNA of positive found vegetable phytoplasma strains revealed 100% sequence identities among each other and with phytoplasma strains of 'clover proliferation' (16SrVI) group. Phytoplasma sequences, virtual RFLPs and phylogenetic analyses of 16S rDNA sequence comparison confirmed the identification of 16SrVI subgroup D strain of phytoplasmas in four vegetables and one leafhopper (HP) species. Further virtual RFLP analysis of 16S rDNA sequence of the vegetables phytoplasma strains confirmed their taxonomic classification with strains of 'clover proliferation' subgroup D. Since, H. phycitis feeding on symptomatic vegetable species in the study was also tested positive for the 16SrVI phytoplasma subgroup-D as of vegetables; it may act as potent natural reservoir of 16SrVI-D subgroup of phytoplasmas infecting vegetable and other important agricultural crops.