BSPP News Spring 2001 - Online Edition

The Newsletter of the British Society for Plant Pathology
Number 38, Spring 2001 

BSPP Fellowship Report: Dr Lesley Boyd

Wheat  Resistance to South African Puccinia striiformis f.sp. tritici Pathotypes

In 1996 South Africa experienced an outbreak of wheat yellow rust, a disease hitherto unknown in this country1. Wheat yellow rust is caused by the biotrophic fungus Puccinia striiformis f.sp. tritici and a single pathotype, designated 6E16 was identified. Wheat farmers suffered heavy losses, the majority of South African wheat cultivars at the time having little or no resistance to this pathogen. A collaboration was initiated between me, at the John Innes Centre, Norwich, UK, Dr Renee Prins and Mr Willem Boshoff of the Small Grain Institute (SGI), Bethlehem, and Prof. Sakkie Pretorius, University of the Free State, Bloemfontein, South Africa, to characterise the genetics of yellow rust resistance in South African wheat germplasms.

Here at the John Innes Centre we assessed South African winter and spring type wheats for known seedling resistances and also identified a number of unknown resistances. Field trials were conducted in Norwich and identified a number of cultivars showing adult plant resistance (APR) to UK pathotypes. In South Africa, field trials similarly identified an APR to pathotype 6E16 in the South African cultivar Kariega. In 1998 a second South African pathotype was identified, 6E222. Fortunately the APR in Kariega remained effective against this new pathotype. It was decided to analyse the genetics of the APR within Kariega.

Although single genes controlling yellow rust APR have been identified3, many sources of APR are controlled by more than one gene4. Such sources of multigenic or quantitative resistance (quantitative trait loci QTLs) are difficult to transfer to new cultivars in their entirety. To ensure that all the genes contributing to the APR are transferred to a new cultivar, molecular markers, closely linked to each gene, are used to screen for the gene's presence. A DNA mapping program was initiated to find molecular markers linked to the genes contributing to the yellow rust APR in cultivar Kariega.

Dr Prins and Prof. Pretorius have made a Doubled Haploid (DH) population from a cross between Kariega and the yellow rust susceptible cultivar Avocet S. This population consists of more than 200 individual DH lines. In June 2000, 150 of the DH lines were sown in field trials at Pannar, Greytown, South Africa. These field trials were inoculated with the current South African P.s. f.sp. tritici pathotypes.

 A DNA marker map of this population is being developed, using microsatellites to provide chromosome anchors, and Amplified Fragment Length Polymorphisms (AFLPs) to fine map. The disease phenotype of each DH line, obtained from the field trial, will be superimposed on the DNA molecular map using suitable statistical software for QTL analysis. This will identify DNA markers linked to the genes/QTLs contributing to the yellow rust APR seen in Kariega and locate their chromosomal position. These DNA markers provide the raw materials from which simple marker selection systems to identify the resistance genes during a wheat breeding program can be developed. These tools will allow us to transfer the complete APR found in Kariega to new wheat cultivars.

Dr Prins has already mapped a number of microsatellite markers in the Kariega x Avocet S DH population, some of which have been supplied by The Comparative Genetics Unit at JIC. Dr Prins' M.Sc. student, Mr Viresh Ramburan, is currently screening for, and mapping polymorphic AFLP in this DH population. As part of this UK/South African collaboration funds are currently being sought for Mr Viresh Ramburan to work at JIC for 3 months to complete the AFLP fine mapping and use the facilities and expertise in QTL mapping available at the JIC.
    In addition to the Kariega APR, the yellow rust APRs of Yr16 and Yr18 are also being assessed under South African conditions. Cappelle Desprez, as a source of Yr16, has been crossed to the susceptible South African cultivar Palmiet. F3 families from this cross were also planted at the Pannar site in June, 2000.

 A series of F4 Recombinant Inbred Lines (RILs), developed by me in the UK and carrying the gene Yr18, have been assessed in field tests in the UK for yellow rust APR and have proven very effective against UK pathotypes. A number of these UK tested RILs were included in the South African trial so that the performance of Yr18 against South African pathotypes and under South African environmental conditions could be directly compared to its performance under UK field conditions.

Work Undertaken:

Kariega x Avocet S Doubled Haploid Population
Four reps of 150 DH lines, made from the F1 of the cross Kariega x Avocet S were planted in June, 2000. The parents, Kariega and Avocet S were included at regular intervals as controls. The field trial was sown at Pannar, Greytown, 70 km N/W of Durban, South Africa. Spreaders were planted in each replicate, infected with current South African P.s. f.sp. tritici pathotypes. By September 2000 a good yellow rust infection was observed on the plants.

To perform a QTL analysis it is essential that a careful and detailed infection score is made for the population. The 150 DH lines were scored on two occasions during the disease cycle. For comparison of  disease assessment the trial was scored independently by me. Prof. Pretorius and Willem Boshoff.

The phenotypic disease score data has yet to be analysed statistically, and future QTL analysis awaits the construction of a DNA, molecular map for the cross Kariega x Avocet S. However, phenotypes ranging from a resistant fleck to intermediate levels of sporulation and fully susceptible (resembling the Avocet S parent) were observed. DH lines, slightly less susceptible than Avocet S were also found. The yellow rust APR in this cross would therefore appear to involve both genes having a large effect on resistance and possibly some of minor effect, making the population suitable for QTL analysis. In addition to yellow rust infection the population was also scored for Leaf Tip Necrosis (LTN), a character associated with Yr18 resistance.

A number of microsatellites have been mapped in the DH population, but more are needed to provide chromosome anchors for the AFLP markers. Dr. R. Prins's M.Sc. student is making some progress with the AFLP mapping, but many more AFLP markers are required to give a good coverage of a genome the size of hexaploid wheat.

Yr16 and Yr18 Adult Plant Resistance
Yr16, derived from Cappelle Desprez, was assessed in F3 families from a cross with the South African cultivar Palmiet. Variation in APR was seen between the F3 families and these will now be tested with microsatellites linked to Yr16 to determine the presence of this gene in each F3 family.

The Yr18 APR is not complete, but appeared to perform equally as well under South African conditions as it did in the UK field trials.

Future Work and Benefits:

This collaboration has enabled us to assess diverse wheat germplams for potentially useful sources of yellow rust resistance effective against P.s. f.sp. tritici populations in the UK and South Africa. This work therefore has potential value for both the UK and South African plant breeding industries. The program of research has concentrated on examining APR, with the potential this may hold for durability. Because of the difficulties encountered with breeding for APR i.e. (1) late developmental stage of expression, (2) potential multigenic control and (3) high environmental variation, the program seeks to use molecular markers to tag the genes responsible for yellow rust APR, and develop these as simple molecular tools for breeders to use in the introduction of this APR into new wheat cultivars.

Dr. Lesley A. Boyd
Cereal Rust Pathologist, John Innes Centre, Norwich, NR4 7UH

P.S. A very pleasant bonus to my visit to South Africa was the opportunity to meet Dr. Colin R. Wellings, Rust Pathology, Plant Breeding Institute, University of Sydney, Cobbitty, Sydney, Australia. Colin was at the time visiting research groups in Africa working on cereal rusts are a problem. While knowing of Colin and his excellent work, we had not previously met. Consequently we took the opportunity to conduct our own mini, international cereal rust meeting. Therefore, I must doubly thank BSPP for providing the funding to allow me to make this trip.

South African Collaborators:
(1) Dr. Renee Prins, Dept. of Genetics, J.C. Smuts Building,  Uni. of Stellenbosch, Stellenbosch, South Africa
(2) Mr Willem Boshoff and Mr Danie Van Niekerk, Small Grains Institute, Private Bag X29, Bethlehem, 9700, South Africa.
(3) Prof. Sakkie Pretorius, Dept. of Pathology,  Uni. of the Free State,  Bloemfontein, 9300, South Africa.
(4) Dr. Rikus Kloppers, Pannar Ltd, PO Box 19, Greytown, 3250, South Africa

References:
1) Z.A. Pretorius (1997) First report of Puccinia striiformis f.sp. tritici on wheat in South Africa. Plant Disease 81:424
2) W.H.P. Boshoff (1999) A new pathotype of Puccinia striiformis f.sp. tritici on wheat in South Africa. Plant Disease 83:591
3) Wheat rusts: an atlas of resistance genes. (1995) R.A. McIntosh, C.R. Wellings and R.F. Park (Eds). Pub: CSIRO, Sydney, Australia.
4) H.S. Bariana and R.A. McIntosh (1995) Genetics of adult plant stripe rust resistance in four Australian wheats and the French cultivars 'Hybride-de-Bersee'. Plant Breeding 114:485-491.