Ground Cover Supplement : GC Supplement - Wheat breeding
5 WHEAT BREEDING GROUND COVER GENE DISCOVERY PRE-BREEDERS HAVE LONG known that the wilder the gene pool, the more genetic diversity from which to select valuable traits. So for decades, techniques were developed to transfer DNA from wild relatives into cultivated crops to spice up their performance. That process has just become more sophisticated with a CSIRO Plant Industry innovation that recently transferred three disease-resistance genes from two different wild species into bread wheat. The bonus is that the traits, once introduced and combined into the wheat genome, are thereafter inherited together as a single package. GM technology can then be used to add additional genes and breeders need only one marker to select for the entire package. Led by Dr Phil Larkin and Dr Ligia Ayala- Navarrete, the CSIRO team targeted the Lr19, Sr25 and Bdv2 genes that are effective against leaf rust, stripe rust and barley yellow dwarf virus. In tests against all races of the new stem rust pathotype Ug99, the Sr25 gene has provided total resistance while Bdv2 is the first gene to provide true resistance to barley yellow dwarf virus, the most widespread and economically significant cereal virus in Australia. The technique is based on manipulating chromosome 'translocations' -- situations where DNA from the chromosome of a wild grass replaces a corresponding chunk of a bread wheat chromosome. The ability to induce these translocations has been around for years, but was of limited use to breeders if deleterious genes (that erode yield or quality traits) came through with beneficial genes. Until recently, there were few options to refine which wild genes are retained or discarded ... until Dr Larkin found a way to manipulate the content of the translocated DNA. The CSIRO strategy involves two wild relatives of bread wheat: Thinopyrum ponticum (the donor of Lr19 and Sr25) and Th. intermedium (the donor of Bdv2). For each wild species, the team selected translocations that moved a chunk of wild chromosome (containing the disease-resistance genes) to wheat chromosome 7D. These translocated chromosomes were then crossed so that during sexual reproduction -- when chromosomes pair up to exchange DNA -- the Th. intermedium and Th. ponticum fragments were also brought together. Normally Th. intermedium and Th. ponticum do not exchange DNA because bread wheat contains a gene (called Ph1) that prevents DNA from different Wild disease-resistance genes stir the wheat gene pool Three disease-resistance genes have made their way into bread wheat from wild grasses using a new pre-breeding technique that allows the genes to be inherited together as a single package By Gio Braidotti A METHOD HAS BEEN FOUND TO MANIPULATE THE CONTENT OF THE TRANSLOCATED DNA. sources from pairing and exchanging DNA. "The important point for our project is that since the early 1960s, a mutation in Ph1 has been available that relaxes the control of chromosome pairing," Dr Larkin says. "In the presence of that mutation, chromosomes that normally would not pair are allowed to and they can then recombine their DNA, even across wild and domesticated genomes." The scientists allowed Th. intermedium and Th. ponticum DNA to recombine and then screened for new arrangements that bring together the desirable resistance genes while losing a deleterious gene that causes an extreme yellow colour in flour. "The upshot now is that we have beautiful new combinations of these valuable disease-resistance genes," Dr Larkin says. Once the translocation is crossed back to a normal Ph1 gene, the new gene combination is locked in and thereafter all the resistance genes are inherited together as a single package. That means just one marker is needed by breeders to select for all the resistance genes. The first batch of CSIRO germplasm -- called 'Pontin lines' (Figure 1) -- is now available to wheat breeders and more lines are on the way as the team continues to refine this new pre-breeding technology and apply it to other resistance genes. This includes ongoing work targeting barley yellow dwarf virus and a recent arrival in Australia that could pose a major threat to wheat productivity -- wheat streak mosaic virus. □ GRDC Research Code CSP00090 More information: Dr Philip Larkin, CSIRO Plant Industry, 02 6246 5060, firstname.lastname@example.org This wild relative of wheat might not look very impressive but its gene pool is providing diversity for improved rust resistance. Researchers have developed novel techniques to transplant the wild genes into productive bread wheat lines. FIGURE 1 A diagrammatic representation of the movement of beneficial chunks of chromosomes from wild grasses to wheat germplasm Pontins PARENTS RECOMBINANT LINES 1234567 3P3/3P4 BF145935 gwm37 STS Lr19130 Leaf rust BYDV PSY-E Stem rust Ug99 BYAgi T4 (Lr19) T. aestivum Th. ponticum Th. intermedium TC5 (Bdv2) wheat intermedium ponticum Lr19 Bdv2 Lr19 Bdv2 Sr25 ? ? ?
GC Supplement - Pulse breeding
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