–By Kranti Karande
Phytoplasma is a group of extremely small bacteria (mollicutes). They don’t have a cell wall and any particular shape (pleomorphic). Phytoplasma was first identified by a Japanese scientist Yoji Doi as ‘mycoplasma-like-organisms’ in 1967. They are bacterial parasites of plants and insects. Phytoplasmas reside in plant’s phloem tissue while insects serve as vectors for the transmission of infection from plant to plant. Once disease caused by phytoplasma is established, entire fields of crops might be wiped out. Indian sandalwood production has been drastically reduced due to phytoplasma infection during the last few decades.
The major characteristic symptom of phytoplasma-caused disease is abnormal growth of the floral organ,green flowers development (phyllody) and numerous growth of tree branches which appears like ‘Witches’ boom’ and reduction in leaf size called ‘little leaf’. Phytoplasma infection also causes curling and yellowing of the leaves (virescence). Disease caused by phytoplasma is called Sandalwood Spike Disease (SSD) in sandalwood plants, as leaves show growth in ‘spike-like’ structure. In few species of plants, such as onion and bamboo, the symptoms of witches’ broom and shortening of plant height (stunting) are observed due to phytoplasma infection.
Some plant pathogens produce virulence factors (effectors) that modulate or interfere with normal host processes for the pathogen’s benefit. Phytoplasma is reported to express secretory proteins including SAP proteins. The major obstacle limiting research in phytoplasma disease is that the organism is difficult to cultivate in vitro (laboratory cultivation).
Dr. Amit Yadav’s research group at NCMR, NCCS Pune is interested in understanding these pathogenic microbes which cause disease in over a thousand species of plants and affect agricultural productivity around the world. They’re interested in understanding genetic factors related to pathogenicity and phytoplasma transmission. They study phytoplasma taxonomy and the role of insect gut micro biome in vectoring ability of the insects.
It’s important to study the genome of this organism to understand how this tiny microbe causes infection in plants and gets transmitted through insect vectors. Phytoplasma DNA is difficult to isolate and then sequence it further, as researchers have not yet been active in this organism’s laboratory cultivation. Dr. Yadav’s research group is currently working on 40-45 symptomatic plants associated with phytoplasma infection. Recently, the group was successful in isolating and sequencing sugarcane phytoplasma. Dr. Yadav’s group is interested in studying other phytoplasma species that are prevalent in India; mainly associated with diseases in economically important pulse crops, sandalwood and weed species; and will be the first to publish phytoplasma genome sequence from India.
There is no cure yet available for this infection. Phytoplasmas are transmitted from plants to plants via the insect vectors. Weeds are also known to be secondary phytoplasma carriers. The weeds around the field are not eradicated by farmers, insects introduce phytoplasma to these weeds and then again to the new crop transmitting phytoplasma infection. Dr. Yadav’s group is also interested in studying the insects’ vector families and mechanism involved in the transmission of infection with Phytoplasma.
Some researchers are trying to understand if phytoplasma are beneficial for the insect vectors transmitting phytoplasma, and why some insects act as transmitters and some not.
Phytoplasmas have one of the smallest genomes as compared to other common bacteria. They completely rely on their host for nutrient supply, growth, spread and reproduction. “How organism with such a small genome affects over 1000 plant species is a curious question that drives me in this study,” said Kiran Kirdat, researcher from Dr. Yadav’s group. She also commented that “Dr. Yadav has been a great motivator for me”.
Dr. Yadav’s group has reported novel species of phytoplasma from various economically important plants like from Mango, Sandalwood, Sugarcane, Tamarind etc. Along with taxonomy study their research focus is also on understanding role of insect gut microbiome in the vectoring ability of these insects. They are interested in understanding the diversity of microbiota in the gut of insect vectors. The Wikipedia page of ‘Sugarcane Grassy Shoot Disease’ has been created by Dr Yadav and he has contributed significantly in other phytoplasma related article on Wikipedia.
Dr. Yadav said that, “People are more aware of phytoplasma associated diseases than a decade ago, when I started working on this notorious bacterium. However, much needs to be done further. Many crops like Sugarcane, Sesame, Sandalwood, coconut in India are facing great challenge to sustain due to phytoplasma infection. We are working on two different aspect to study phytoplasma; one is to understand how this pathogen spread the disease from one plant to other through insect vector and other is to study the genome of this organism to find out disease biology. We are also working on development of reliable and robust detection of phytoplasmas to help on field workers.”
Phytoplasma affects many economically important plant species and thus is a major threat to agriculture economy. Understanding the epidemiology of the phytoplasma diseases, pathogen and genomics of this tiny microorganism will help to cure the infection in future.