This is the report from a BSPP MSc/MRes Bursary.
Click here to read more/apply for one yourself.
As research into plant viruses has expanded from a focus on crop pathogens to viruses present throughout the plant Kingdom, many wild plants have been found to be infected with viruses, and in most of these cases infections are asymptomatic. A new terminology has been suggested by Professor Marilyn Roossinck to describe the differences in viral lifestyle and plant-virus interactions. Indeed, certain viruses appear to have evolved into non-pathogenic agents which are not infectious in a conventional sense but are exclusively transmitted vertically, i.e., transmitted solely via seed and/or pollen. Professor Roossinck has proposed the term “persistent virus” to describe such viruses. In contrast, she suggests the term “acute virus” to describe the more familiar infectious plant viruses which typically cause disease and are more commonly (but not exclusively) transmitted horizontally, i.e., by vectors or wounding. The Endornaviridae are one of five known families of persistent viruses and can be found in fungi, oomycetes and plants. Plant-infecting endornaviruses occur in many crop species including common bean (Phaseolus vulgaris), and these viruses are suspected to have beneficial rather than pathogenic effects.
Common bean is an essential crop in East and Central Africa where it provides, respectively, the second and third most important sources of dietary protein and calories to approximately 200 million citizens. The zinc and iron contents of beans offer a unique opportunity for alleviating anaemia, a condition which affects approximately 50% of children <5 years old globally. In East and Central Africa, beans are grown not only as a food crop but also as essential intercrops to enrich soils with nitrogen (due to the symbiosis of bean roots with nitrogen-fixing bacteria) so that other staple crops such as cassava, maize and banana can be grown successfully. There are three main acute viruses affecting common bean in the region: bean common mosaic virus (BCMV), bean common mosaic necrosis virus (BCMNV), and cucumber mosaic virus (CMV). All three acute viruses cause significant disease and decreased crop yield.
One of my aims was to find out if the presence of one or more endornaviruses in bean plants protected them from infection with BCMV, BCMNV or CMV. I investigated the interactions between Phaseolus vulgaris endornavirus (PvEV) 1, PvEV2 and PvEV3 and twelve cultivars of common bean grown in Kenya, Uganda and Rwanda. These cultivars carried various combinations of the three endornaviruses. I used reverse transcription-coupled quantitative polymerase chain reaction assays to measure levels of the endornaviruses and the accumulation of BCMV, BCMNV and CMV RNA. My results suggested that the genetic background of the cultivars rather than the presence or absence of endornaviruses influenced susceptibility to BCMV, BCMNV or CMV. I concluded that although endornaviruses do not increase the susceptibility of their hosts to attack by pathogenic viruses, they do not appear to provide additional protection.
I also analysed the interactions between PvEV1, PvEV2 and PvEV3. I found that in plants of cultivars that harboured combinations of two or three of the endornaviruses, PvEV1 RNA accumulated to a higher level than RNA of either PvEV2 or PvEV3. This suggested that PvEV1 is in some way dominant over PvEV2 and PvEV3. When plants were infected with CMV, BCMV or BCMNV, only BCMV infection caused changes in the accumulation of PvEV1 or PvEV2.
Although BCMV, BCMNV and CMV are transmitted predominantly by aphid vectors, they also can be transmitted through seeds of infected bean plants. Although it was hypothesised that the presence of endornaviruses would decrease the rate of seedborne transmission of acute viruses, I found using CMV and BCMV that this did not appear to be the case. There also did not seem to be a relationship between the presence of endornaviruses and the decrease in seed yield caused by infection by either CMV or BCMV. Overall, my work suggests that although endornaviruses are not deleterious to bean plants, they do not enhance protection against acute viruses.
I would like to thank my supervisors, Professor John Carr (Department of Plant Sciences, University of Cambridge) and Dr Francis Wamonje (NIAB EMR, Kent), as well as all my colleagues in the Carr lab and finally the BSPP whose generous funding made this fantastic opportunity a reality for me.
Tom Brine MPhil
University of Cambridge
This is the report from a BSPP MSc/MRes Bursary.
Click here to read more/apply for one yourself.
