JOHN INNES CENTRENorwich Research Park, Colney, Norwich NR4 7UH, United Kingdom.
Septoria tritici blotch is currently the major foliar disease of wheat in most of Europe, North Africa, South America, and several other parts of the world. Until recently, little was known about the genetics of resistance to this disease in comparison to, for example, the better-studied rust diseases and mildew. New sources of resistance and improved knowledge about the genetics of resistance would be of great value to breeders in improving resistance to Septoria tritici blotch.
Specific interactions between wheat cultivars and isolates of M. graminicola have been known since 1973, but their relevance to field conditions has been controversial. In a series of trials, we found strong, specific resistance of wheat genotypes to M. graminicola isolates at the adult plant stage. We, and our collaborators in The Netherlands and Switzerland (G. Kema and H-R. Forrer, respectively), tested 71 cultivars and breeding lines, mostly from Europe, to six isolates of M. graminicola. The isolate-by-variety interactions were stable over six field trials, which were grown in diverse conditions of climate and soil. This study confirms that isolate-specific resistance is stable over environments. We also identified several lines with good quantitative resistance of an apparently isolate-nonspecific nature from seven European countries, Brazil, and the U.S.
Screening for disease resistance in glasshouse experiments is often laborious and expensive, whereas field disease testing can only be done once a year. We therefore developed a method of testing resistance and detecting isolate-by-cultivar interactions in detached seedling leaves, a method that is widely used by mildew workers. Isolate-by-cultivar interactions were expressed in a consistent manner in different experimental conditions. There was a good correlation between the results of detached leaf tests and those of tests on whole-seedlings and of field trials.
The existence of interactions such as these suggested the existence of a gene-for-gene relationship between wheat and M. graminicola. To test this, we crossed Flame and Hereward, which are resistant to the isolate IPO323, with each other and with the susceptible cultivar Longbow. In tests of the F1, F2, and F3 generations, a single, semidominant resistance gene was identified as controlling resistance to IPO323 in Flame, whereas there was no recombination between genes for resistance to IPO323 in Flame and Hereward. The gene was mapped to the distal end of the short arm of chromosome 3A by linkage to microsatellite markers and is named Stb6. Tests on the pathogen have shown that a gene for avirulence in IPO323 is complementary to the resistance of both Flame and Hereward as well as several other wheat lines, representing the first demonstration of a gene-for-gene relationship in S. tritici blotch.
We have further shown that Dr. E. Sears' synthetic hexaploid wheat (T. dicoccoides/Ae. tauschii) is resistant to 12 of the 13 isolates of M. graminicola tested. We located this resistance to chromosome 7D of synthetic 6x in tests on intervarietal chromosome substitution lines of Synthetic 6x in Chinese Spring. A gene for resistance to isolate IPO94269 was mapped near the centromere of the short arm of chromosome 7D in a population of single homozygous chromosome recombinant lines and is named Stb5.
Lesley Boyd, Phil Smith, Peter Minchin, Tony Worland, Clare Ellenbrook, Jacqueline Garrood, and Robert Koebner.
Genetics of adult plant resistance to yellow rust in wheat. Adult plant resistance (APR) is being studied in wheat through an analysis of mutants showing enhanced field resistance to yellow rust. Mutants have been selected from mutant populations in the cultivars Hobbit 'sib' and Guardian. In the Hobbit 'sib' mutant I3-54, a single, dominant mutant locus segregates with enhanced yellow rust resistance, whereas in I3-48 at least two loci, one cosegregating with a deletion on chromosome 4DL, are associated with the altered yellow rust-resistant phenotype. Seven mutants have been selected from Guardian, four showing an increase in yellow rust resistance and three an increase in susceptibility. In mutant M66, a single, dominant locus confers the enhanced resistance. Mapping populations are being developed from these lines to find molecular markers for the mutant loci.
The genetics of yellow rust APR is also being examined in a number of wheat cultivars, many in collaboration with other research groups. Mapping populations of the Nickerson Seeds cultivars Claire and Buster are being made to map the QTLs controlling APR. A number of cultivars forming part of the Claire pedigree also are being studied. QTLs for yellow rust APR also are being identified in the South African cultivar Kariega (collaboration; Dr. RenJe Prins, Small Grains Institute, Bethlehem, and Prof. Sakkie Pretorius, University of the Free State, Bloemfontein, South Africa) and the Chinese cultivar Fan 6 (collaboration; Dr. Anmin Wan, Institute of Plant Protection, Beijing, PR China).
Development of markers for marker-assisted selection breeding. PCR-based molecular markers have been developed from cloned cosegregating AFLP bands for the wheat, yellow rust-resistance gene Yr10. Similar PCR-based markers also are being designed for Yr5. Little use has been made of either yellow rust-resistance gene in U.K.-wheat breeding and virulence to these resistance genes is unknown in the U.K. P. striiformis f. sp. tritici population. These markers have been developed so that Yr5 and Yr10 can be pyramided into new wheat cultivars.
Elizabeth Chandler, Richard Draeger, Nick Gosman, Martha Thomsett, Duncan Simpson, Andy Steed, Lorenzo Covarelli, and Paul Nicholson.
Fusarium research. The study of the genetic basis of FHB in wheat is continuing. Resistance of the cultivars Arina and RL4137 has been analyzed by spray inoculation of a DH population and RILs, respectively. Mapping and QTL analysis of resistance also is underway. The resistance of chromosome 4A of T. macha reported previously has been investigated further in single-chromosome recombinant DH lines. Resistance on this chromosome appears to be conferred by a single gene.
Molecular diagnostics. Molecular diagnostics (species-specific
competitive PCR) are being used to study interactions between
stem-base disease pathogens on cultivars differing in susceptibility
to these diseases. In addition, the efficacy of the eyespot resistance
genes Pch1 and Pch2 against stem-based diseases
other than eyespot also is under investigation. The molecular
diagnostics also are being used in studies to determine the effect
of fungicides upon species involved in FHB and the consequences
for accumulation of toxins in grain.
Simon Griffiths, Steve Reader, Tracie Foote, and Graham Moore.
The Ph1 locus, on wheat chromosome 5B, is defined by its ability to affect chromosome pairing as scored at metaphase I. Two deletions of this locus have long been known, one in hexaploid wheat (ph1b) and the other in tetraploid wheat (ph1c). The deleted segment of the chromosome 5B in the ph1b line is 70 Mb. Comparative analysis with the gene content of the equivalent region of the rice genome (chromosome 9) suggests that at least 200 genes have been deleted. Recently, we reported the isolation of a set of new fast-neutron irradiation-induced deletions of the region encompassing the Ph1 locus. Scoring the chromosome pairing of these lines at metaphase I and in wide hybrids with rye or Ae. variabilis has identified which have lost the Ph1 locus. By locating the breakpoints of these deletions with respect to the ne order in the equivalent region of rice, the Ph1 locus has now been delimited to a region defined by seven rice genes. Currently, a second round of screening for deletions will delimit the region further.
This work benefited from a collaboration with Scott Tingey
(Dupont, Wilmington DE, USA).
John Flintham, Manoel Bassoi, Rachel Adlam, and Mike Gale.
The map location for a novel major dormancy gene has been determined, close to the ancestral 4A/5A translocation point on the long arm of chromosome 4A. A fine-scale, genetic map and PCR markers for this locus are in development. In other work, QTL mapping has identified a number of dormancy genes that may be of use in combating preharvest sprouting in Brazilian germ plasm, in a Ph.D. program sponsored by EMBRAPA.