Our understanding about phytoplasma research scenario in India

Abstract

Since the first report of existence of phytoplasma disease in 1874 in India, now more than 172 plant species are reported to be the host of ten different groups of phytoplasmas. The phytoplasma diseases are well documented from all parts of the country and aster yellows group (16SrI) is the most prevalent and widespread group followed by 16SrII, 16SrVI, 16SrXI and 16SrXIV groups. Sesame phyllody, brinjal little leaf, sugarcane grassy shoot, sandal spike, coconut root wilt, areca nut yellow leaf and many ornamentals and vegetables maladies are the major phytoplasma threats in India causing serious economic losses every year. Nearly a dozen of leafhopper and planthopper species have been identified as natural vectors and more than 40 weed species have been identified as natural reservoirs of different phytoplasma strains. A large number of new phytoplasma strains on several crops have been characterized in India during last decade, which revealed that phytoplasmas are more diverse than the earlier thought. Very recently two draft genome sequences of 16SrXI (sugarcane grassy shoot) and 16SrXIV (Bermudagrass white leaf disease) have been submitted from India which open-ups the possibilities of studies on functional genomics. Mixed infections of phytoplasmas and viruses have also emerged as serious problems in several crops and are responsible for more synergistic losses. Nested PCR assays have been routinely employed to detect phytoplasma strains in symptomatic plants and insect vectors by utilizing 16S rRNA, 16Sr-23S rRNA spacer region and several multilocus gene specific primers. However, new developed isothermal amplifications methods (LAMP and RPAs) for sensitive and quick detection of phytoplasmas need to be explored for easier identification of phytoplasma strains. Multiplex PCR system has also been developed to detect the phytoplasma utilizing nested PCR primers and multilocus gene specific primers in one PCR cycle. Very little attempt has been made to study metabolic interaction in phytoplasma hosts and insects in India. However, some efforts in last decade has been made to elucidate the host responses at biochemical, genomics, proteomics and metabolomic levels for understanding the host–pathogen interactions but still more concentrated efforts need to be explored that will help to formulate strategies for controlling phytoplasma diseases. Presently the suggested effective management practices in India are: pruning infected portions, growing resistant varieties, heat therapy, use of tertracycline, control of insect vectors and natural host reservoirs. However, none of them have completely proved successful. Future approach on epidemiology, resistance, host-pathogen-insect interaction, full genome sequencing of major phytoplasma strains and RNA interference needs to be focussed in India for a better understanding and management of these peculiar microbial pathogens.