Pros and cons of utilizing major, race-specific   resistance genes versus partial resistance in          breeding rust res...
Resistance in wheat to rust pathogens                             Seedling reactions- stem rust Highly variable: immunity...
Pro and cons debate Not new and debated for almost 50 years since the publication  of famous book of van der Plank in 196...
Breeding PrioritiesCore traits (ideally should be      Additional traits for specific mega-present in all CIMMYT wheats)  ...
Utilization of R-genes in breeding: Pros Simpler to utilize:    High frequency of resistant plants with appropriate agro...
“Boom-and-Bust”: Race-Specific Genes for leaf rust           resistance in Northwestern Mexico                            ...
Utilization of R-genes in breeding: Cons “Boom-and-Bust” phenomenon    Evolution and selection for virulence is often ra...
Utilization of R-genes in breeding: cons Coping with “boom-and-bust” by breeding programs   Breeding materials must cont...
Deployment of R-genes: Cons Avoiding losses in farmers’ fields    Continuous releases, seed multiplication and distribut...
Better strategy: utilize effective R-genes in  combinations to enhance resistance longevity All public and private breedi...
Breeding for high levels of durable resistance to rust diseases % Rust 100                                                ...
Utilization of PR genes in breeding: challenges Small to intermediate effects of individual genes Dispersed presence of ...
Ug99 stem rust resistance in 464 wheat lines derived from crosses made in 2006 and distributed worldwide through various C...
Adult plant resistance (APR): PR- and R-genesNot all APR is based on PR-genesRace-specific APR genes with major, interme...
Interaction of moderately effective R-gene Lr42 and two PR-      genes in enhancing the resistance of Avocet x Quaiu3 RILs...
Deployment of PR-genes in breeding: ConsRelease of varieties with moderate but adequate resistance is challenging in some...
Deployment of PR-genes in breeding: Pros Breeding program    Breeding for yield and other important traits becomes easie...
Conclusions and future outlook● Both R- and PR-genes offer opportunities to achieve durable control of  wheat rusts if uti...
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Pros and cons of utilizing major, race-specific resistance genes versus partial resistance in breeding rust resistant wheat

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Ravi Singh, CIMMYT

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Pros and cons of utilizing major, race-specific resistance genes versus partial resistance in breeding rust resistant wheat

  1. 1. Pros and cons of utilizing major, race-specific resistance genes versus partial resistance in breeding rust resistant wheatR. P. Singh2012 BGRI Technical WorkshopBeijing, China
  2. 2. Resistance in wheat to rust pathogens Seedling reactions- stem rust Highly variable: immunity to small reduction in host reaction or severity Race-specific genes (R-genes)  large to small effect  Seeding and adult plant effective  Diverse hypersensitive host reactions Seedling reactions- yellow rust  Various genes; those with large and intermediate effects characterized Partial or slow rusting genes (PR-genes)  Intermediate to small effects Seedling reactions- leaf rust  More effective in post seedling growth stages  Compatible host reactions for LR & SR (chlorosis & necrosis in stripes for YR)  Associated with slower disease progress due to longer latent period, lower infection frequency, pustule size and spore production  Only a few genes characterized
  3. 3. Pro and cons debate Not new and debated for almost 50 years since the publication of famous book of van der Plank in 1963 Both types of resistance have played important role commercially Enhancing the resistance longevity or durability- main concern Significantly enhanced knowledge of the genetic basis should help in implementing better breeding and deployment strategies A successful variety must possesses various traits- rust resistance is just one of them
  4. 4. Breeding PrioritiesCore traits (ideally should be Additional traits for specific mega-present in all CIMMYT wheats) environments High and stable yield potential  Durable resistance to diseases and Durable resistance to Rusts- pests Stem (Ug99), Stripe and Leaf Septoria leaf blight (ME2) Water use efficiency/Drought Spot Blotch (ME5) tolerance Tan Spot (ME4) Heat tolerance Fusarium – head scab and myco- Appropriate end-use quality toxins (ME2/4/5) Karnal bunt (ME1) Root rots and nematodes (ME2) Enhanced Zn and Fe concentration (ME1/5) Most traits have complex inheritance
  5. 5. Utilization of R-genes in breeding: Pros Simpler to utilize:  High frequency of resistant plants with appropriate agronomic characteristics in segregating populations  Large effects and low host reactions eases selection under poor epidemics in field  Selection of seedlings in greenhouse possible- advantage for SSD method  Preferred targets for MAS, especially when located on alien chromosome segments  Large number of advanced lines likely possess them even if no selection carried out in segregating generations
  6. 6. “Boom-and-Bust”: Race-Specific Genes for leaf rust resistance in Northwestern Mexico YearVariety Resistance genes Released Breakdown RaceBread Wheat:Yecora 70 Lr1, 13 1970 1973 ?Tanori 71 Lr13, 17 1971 1975 ?Jupateco 73 Lr17, 27+31 1973 1977 TBD/TMGenaro 81 Lr13, 26 1981 1984 TCB/TBSeri 82 Lr23, 26 1982 1985 TCB/TDBaviacora 92 Lr14b, 27+31 1992 1994 MCJ/SPDurum Wheat:Altar 84 LrAlt 1984 2001 BBG/BNJupare 2001 LrAlt, 27+31 2001 2007 BBG/BP
  7. 7. Utilization of R-genes in breeding: Cons “Boom-and-Bust” phenomenon  Evolution and selection for virulence is often rapid: 3-5 years or less in many areas  Longer resistance remains effective greater the effect of “bust” in farmers’ fields (large area or several varieties) and on breeding programs (large proportion of breeding materials)  Continuous and relevant monitoring of avirulence/virulence to R- genes necessary in country where deployed and in the region (often worldwide)  Long distance, unpredicted migration of new races more common- globalization effect  Good communication amongst scientists of different disciplines necessary
  8. 8. Utilization of R-genes in breeding: cons Coping with “boom-and-bust” by breeding programs  Breeding materials must contain sufficient diversity for R-genes  Availability of only a few R-genes at a given time for utilization  Continuous search for new sources of resistance in wheat and related species and genera  Alien R-genes usually associated with undesirable traits due to linkage drag & available in poorly adapted backgrounds  Introduction in adapted backgrounds through backcrossing is necessary to promote utilization of new genes
  9. 9. Deployment of R-genes: Cons Avoiding losses in farmers’ fields  Continuous releases, seed multiplication and distribution of new varieties with diverse R-genes  Retrieving older varieties, when a new virulent race detected in significant frequency, not possible due to large seed volumes and farmers’ saved seed  Slow adoption rates of new resistant varieties in most countries  Post-epidemic demands for seed of resistant varieties is higher and this unexpected demand is hardly met
  10. 10. Better strategy: utilize effective R-genes in combinations to enhance resistance longevity All public and private breeding programs in a region must follow it if same R-genes are being utilized No legislation to stop releases of varieties with single R-gene Molecular markers necessary for pyramiding multiple R-genes Markers based selection could lead to a reduction in latent genetic diversity (less selection in field and increased reliance on genes with markers) Real life- rarely practiced & less likely to be practiced as wheat breeding programs worldwide are managed with limited resources
  11. 11. Breeding for high levels of durable resistance to rust diseases % Rust 100 Susceptible 80 1 to 2 minor genes 60 40 2 to 3 minor genes 20 4 to 5 minor genes 0 e.g. Kingbird 0 10 20 30 40 50 Disease progress over time (days) Relatively few additive genes, each having small to intermediate effects, required for satisfactory disease control Near-immunity (trace to 5% severity) can be achieved even under high disease pressure by combining 4-5 additive genes
  12. 12. Utilization of PR genes in breeding: challenges Small to intermediate effects of individual genes Dispersed presence of genes in different varieties and germplasm Field selection environment lacking uniform and high disease pressure Need for growing larger population sizes for selection Necessity of pyramiding 3-5 genes to achieve adequate to high resistance levels Presence of race-specific genes in parents used in crossing programs Difficulty in distinguishing small effect race-specific genes from slow rusting genes (especially for resistance to yellow rust) Higher G x E interaction on the expression and effectiveness of slow rusting genes Slow progress in identifying linked molecular markers if MAS to be used Despite numerous challenges, significant progress was made at CIMMYT for resistance to all three rusts
  13. 13. Ug99 stem rust resistance in 464 wheat lines derived from crosses made in 2006 and distributed worldwide through various CIMMYT international trials and nurseries in 2011/2012 Adult plant resistance Stem rust Entries Race-specific Entries Category severity (%) No. % genes No. % Near-Immune Resistant 1 85 18 Sr25 10 2 Resistant 1-10 116 25 Sr26 7 2 Resistant- Mod. Res. 15-20 121 26 SrHuw234 1 0 Moderately Resistant 30 35 8 SrSha7 14 3 SrTmp 35 8 Mod. Res.- Mod. Sus. 40 22 5 Sr? 3 1 Mod. Sus.- Susceptible 50-100 (dead plants) 14 376% entries distributed internationally possess high toadequate adult-plant resistance & another 16% carrydiverse race-specific resistance genes
  14. 14. Adult plant resistance (APR): PR- and R-genesNot all APR is based on PR-genesRace-specific APR genes with major, intermediate and small effects are commonTrue PR-genes likely associated with multi-pathogen resistanceDelineating PR-genes from small to moderate effect APR R-genes difficult for yellow rust due to similar host reactionsCombinations of PR-genes and small/int. effect R-genes can lead to high levels of longer lasting resistance
  15. 15. Interaction of moderately effective R-gene Lr42 and two PR- genes in enhancing the resistance of Avocet x Quaiu3 RILs Lr/Yr PR QTL PR gene R-gene Lr46/Yr29 Lr42 Lr42+ Lr46/Yr29+ Lr/Yr QTL on 3DSource: Bhoja Basnet (PhD thesis)
  16. 16. Deployment of PR-genes in breeding: ConsRelease of varieties with moderate but adequate resistance is challenging in some countries where regulations require releases of clean or highly resistant varieties onlySmall disease in field can be alarming and will require educating extension agencies and farmersMore PR-genes will need to be bred together to achieve near-immune levels of resistance
  17. 17. Deployment of PR-genes in breeding: Pros Breeding program  Breeding for yield and other important traits becomes easier once key parental materials enriched with multiple PR-genes Avoiding losses in farmers’ fields  Resistance is expected to be durable or long lasting- change of varieties necessary only when a better variety available  Farmers can continue using their saved seed  Emergency chemical control strategies not necessary  Planned multiplication, distribution and promotion of new varieties
  18. 18. Conclusions and future outlook● Both R- and PR-genes offer opportunities to achieve durable control of wheat rusts if utilized properly● “Boom-and-bust” experiences are unlikely to change in the near-future● Use of complex PR-gene resistance, or combinations of small effect R- and PR- genes, should be a more attractive strategy● High yielding wheat germplasm with complex APR to three rusts being distributed by CIMMYT should enhance the release and deployment of varieties with durable resistance● New PR-genes are expected to be characterized due to an increased research focus in recent years● New selection strategies, e.g. genome-wide selection, could be beneficial in pyramiding multiple R- and PR-genes● Development of multiple R- and PR-genes cassettes and acceptance of GM wheat can simplify breeding for durable resistance
  19. 19. Thank you

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