Cases have been reported of PTGS spreading in the host (systemically acquired silencing) the mobile signal possibly being RNAs molecules. PTGS is responsible for: (i) a natural form of resistance known as ‘recovery‘; (ii) "cross protection"; (iii) "RNA-mediated resistance" in transgenic plants. Recovery occurs in plants infected by some nepoviruses and caulimoviruses. With cross protection, infection with a mild viral isolate prevents the secondary accumulation of isolates that are closely related to the primary infecting virus. In plants engineered with viral genes (parasite-derived resistance) RNA-directed RNA degradation is targeted not only to transgene transcripts, but also to any viral RNA possessing high sequence homology with it, hence inducing resistance. Many plant viruses counteract PTGS-driven host resistance by expressing proteins that suppress gene silencing through a number of different mechanisms which are beginning to be unravelled.
SESSION 3 - Development – controlling the pathogen
Pre-emptive breeding for rust resistance in cereals - an Australian perspective
Robert F. Park1, Harbans Bariana1, and Colin Wellings1,2
1The University of Sydney, Plant Breeding Institute, PMB 11, Camden. NSW. 2570. Australia, 2 on secondment from NSW Department of Primary Industries.
The Australian grains industry is worth about AUD$9 billion, and cereals are an important component of this industry. In 2002–03, wheat, barley and oats were sown on 11 million ha, 3.8 million ha and 908,000 ha, respectively, with an overall average yield of each crop of about 1 tonne per ha. Rust diseases have caused significant damage to Australian cereal crops since the first attempts by Europeans to grow them. Controlling cereal rust diseases in Australia by genetic resistance began in the early 1900s, and the first rust resistant wheat cultivar was released in 1938 (cv Eureka, Sr6). Since then, rust resistant cultivars of principally wheat but also barley, oats and triticale have been developed and deployed. Estimates of the value of wheat rust control in Australia, primarily through resistance breeding, have been placed at about $289 million. Breeding plants for disease resistance is essentially a pre-emptive process, with plant breeders striving to keep one step ahead of changing plant pathogen populations. The ability to anticipate changes in pathogen populations and to incorporate resistance accordingly, is however dictated by how well the interactions between a plant and a target pathogen are understood. Pre-emptive breeding has been used in cereal rust resistance breeding to incorporate resistance to rust diseases that are not present in Australia. It has also been used to breed for resistance to endemic diseases, by anticipating pathogenic changes that could threaten the resistance sources being used. Pre-emptive breeding against a disease that is not present in a given region can only be effective if it is ongoing, because it is not known when or indeed if the disease being targeted will be introduced. Difficulties can also arise because it is not known what genotype of a pathogen may be introduced. This is particularly important with cereal rust diseases, which are known to display significant variability in virulence for resistance genes in their respective hosts. To be successful, pre-emptive breeding to endemic pathogens requires a good understanding of variability in the target pathogen, a knowledge of the resistance genes present in deployed cultivars, an effective system for screening all breeding materials with pathotypes posing the greatest threat, and a program aimed at identifying new sources of resistance and incorporating them into germplasm adapted to local conditions. A key feature of pre-emptive breeding for resistance to endemic cereal rust pathogens in Australia has been long-term ongoing surveys of pathogenic variability. An overview will be provided of how this strategy has been used to incorporate resistance to rust diseases of wheat and barley in Australia. Three examples will be discussed: breeding barley for resistance to Puccinia striiformis f. sp. hordei, not present in Australia, and breeding barley for resistance to P. hordei and wheat for resistance to P. triticina, both endemic rust pathogens. A second exotic incursion of wheat stripe rust that was detected in Western Australia in 2002 will also be discussed to illustrate how pathogen genotype can dictate the effectiveness of pre-emptive breeding.
Comparative fitness of virulent and avirulent isolates of Leptosphaeria maculans (phoma stem canker) on oilseed rape
Yong-Ju Huang 1, Marie-Hélène Balesdent 2, Ziqin Li1, Neal Evans 1, Bruce Fitt 1
1 Rothamsted Research, Harpenden, Herts, AL5 2JQ, UK
2 INRA-PMDV unit, Route de St Cyr, F-78026 Versailles, Cedex, France
Phoma stem canker, caused by Leptosphaeria maculans, is a serious disease of oilseed rape word-wide. New sources of major gene resistance usually break down in a few seasons. Durability of resistance is affected by the ability of the pathogen to shift from avirulence to virulence. This shift may potentially cause loss in fitness which affects the ability of the pathogen to complete