Exploiting new sources of broad-spectrum resistance to Turnip mosaic virus in brassicas (ExBSR)

Lead Research Organisation: University of Warwick
Department Name: School of Life Sciences

Abstract

Turnip mosaic virus (TuMV) is an important pathogen that infects many crops worldwide (rated 2nd most important virus infecting field vegetables worldwide). It is most damaging in brassicas (cabbage, cauliflower, oilseed rape etc.) and spread by >89 different aphid species. It's very broad host range and the inability of pesticides to control it, emphasise the need for the development of plants with natural resistance to the virus.

Many different strains of TuMV exist and resistances to restricted ranges of strains have been described. We have identified Brassica rapa (the most important vegetable brassica worldwide) lines with natural resistance to a broad range of TuMV strains. We have been unable to find any TuMV strains able to overcome these resistances, indicating they may be durable. Having identified the genes responsible for one of the sources of resistance and their mechanism, we developed a simple test to identify lines possessing different sources of broad-spectrum resistance. By deploying additional/new sources of resistance to TuMV, this will reduce the selection pressure for resistance-breaking strains of the virus.

This project aims to exploit the best of these new sources of broad-spectrum resistance to TuMV. It will be necessary to breed the resistance into commercial plant varieties. This will be achieved in a collaborative venture between the University of Warwick (UoW), the student and the seed company, Sakata UK Ltd. To significantly speed up this process, the student will develop molecular markers for the plant genes responsible for TuMV resistance. The student and Sakata will use these markers in a process known as marker-assisted selection to get the resistance genes into plant varieties that have all the other plant traits needed for commercial production.

The student will be trained at UoW in a range of scientific skills related to plant pathology and molecular biology with generic applicability and in plant breeding during a 3 month placement at Sakata. There is a lack of students being trained in plant pathology and horticulture; this student will help to address these skills gaps.

The student will learn to handle and transmit plant viruses whilst testing the new sources of TuMV resistance against a broad range of TuMV isolates from Sakata and from the large Warwick TuMV diversity collection. The B. rapa line showing the most extreme resistance to the broadest range of TuMV isolates will be taken forward.

The student will then cross the best virus-resistant plant line to a rapid-cycling plant line and commercial plant lines. The offspring will be tested for resistance / susceptibility to TuMV with careful visual observation and ELISA testing of plants for TuMV infection. If the offspring from the rapid-cycling cross are resistant, they will be backcrossed to the susceptible rapid-cycling parent and if they are susceptible, they will be backcrossed to the resistant parent. This will be done to develop a large plant population for mapping. The offspring of the cross with the commercial plant lines will be back-crossed with the commercial parents to commence introgression of the resistance in to commercially acceptable plant lines.

The large plant population will be challenged with TuMV. Genotyping combined with the phenotyping will allow the resistance gene(s) to be mapped to regions of the chromosomes of the plant. This will acilitate the development of molecular markers that will make it possible for the student and subsequently Sakata, to track the resistance genes in later crosses between the offspring of crosses with the resistant plants and commercially acceptable plants. This will dramatically speed up the breeding of TuMV-resistant varieties. Using molecular markers to identify plants carrying the resistance genes for crossing will take hours, conventional means take months.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
BB/M016447/1 05/10/2015 04/10/2019
1644829 Studentship BB/M016447/1 05/10/2015 30/09/2019 Lawrence Bramham
 
Description Turnip mosaic virus (TuMV) detrimentally affects many cultivated plant species of high importance. It is particularly damaging to crops of the Brassica genus (cabbage, cauliflower, oilseed rape etc.) where sources of resistance to a restricted range of strains have been described. It also persists as a vast range of different strains with varying prevalence and severity worldwide. Within Brassica rapa (the vegetable brassica crop of highest worldwide importance), a source of resistance to a broad range of strains has been identified, however, due to the selection pressure for resistance-breaking mutants of TuMV that will result from its deployment, additional resistance sources remain highly desirable.

Within the primary scope of this work, three plant lines exhibiting broad-spectrum and potentially durable resistance to TuMV were developed. These resistances have also been demonstrated to resist TuMV via a different method to the previously identified source of broad-spectrum resistance. Phenotyping/challenging plant lines through mechanical inoculation with a representative isolate of TuMV for which limited resistance sources exist was used to assist in the generation of valuable populations with uniform resistance to TuMV. These uniform resistance sources were then used to produce populations which segregated for TuMV resistance through a backcrossing strategy to commercial plant lines, starting introgressing of TuMV resistance. A genotyping approach was subsequently developed for analysing these linkage mapping populations wherein parental material was first genotyped by a next generation sequencing approach, identifying genome-wide polymorphisms, followed by a highly cost-effective and reliable targeted genotyping by sequencing technique on mapping population samples.

Informative polymorphic molecular markers have been identified through analyses of genotypic data, associating TuMV resistance to genomic loci. These markers can be developed further to allow both selective breeding and additional research into the mode of action of resistance-associated gene(s). In addition to the primary TuMV resistance-associated objectives, comparable work has been undertaken to explore resistance to Turnip yellows virus (TuYV) using pre-established mapping populations. A region of one chromosome was identified as highly associated with TuYV resistance, whilst another region of a different chromosome was also significantly associated with the resistance and an additive effect between the two regions observed. A TuYV-resistant backcross population plant was crossed to the susceptible parent in order to produce a further population segregating for resistance and possessing additional recombination events. This will be analysed to narrow down the region of the plant's chromosomes associated with the resistance. The aim is to use the regions of the plant's chromosomes associated with the resistance to design molecular markers that plant breeders collaborating in the project can use to develop TuYV-resistant crops.
Exploitation Route The molecular markers developed for virus resistance-associated gene(s) will facilitate rapid breeding of resistant varieties by the commercial funders of my BBSRC iCASE studentship. If the resistances' mode(s) of action is/are new, further avenues of research may be highlighted and new potentials for developing virus-resistant cultivars generated. In any instance, the development of TuYV and broad-spectrum TuMV resistant varieties will complement those previously identified and contribute to limiting the emergence of resistance-breaking viral mutants.
Sectors Agriculture, Food and Drink,Environment