A number of genes that protect our wheats against rust are derived from grass relatives or from cereal rye. The work involved original interspecific crossings followed by years of conventional breeding to select out undesirable traits that came from the grass relative. One example is Lr24 (leaf rust resistance) and Sr24 (for stem rust resistance) in wheat varieties Sunco, Janz, Datatine, Krichauff and others.
The origin of these wheats started with a cross between a 70-chromosome Thinopyrum elongatum plant and wheat in the USA in the 1930s.
The 56-chromosome sterile hybrid was crossed and backcrossed to wheat (42 chromosomes), eventually giving a stable 56-chromosome line with all the wheat chromosomes and seven added pairs from the grass.
Further backcrosses produced a stable hybrid. However, all the resultant wheats with rust resistances were red seeded. A gene for red-seededness was introduced from the grass donor.
Meanwhile, the famous US wheat geneticist, E R Sears built on the knowledge that the absence of one particular wheat chromosome (5B) allowed related wheat chromosomes (wheat has three sets of related chromosomes) as well as identical chromosomes to recombine with each other. This induced chromosome pairing also extends to equivalent chromosomes from related species.
Among 20 resistant recombinants (all red-seeded) produced by Dr Sears, National Cereal Rust Control Program researchers found just two that gave some white-seeded progeny when backcrossed to Australian wheats.
These two lines are the progenitors of Australian varieties with Lr24 and Sr24 genes.
The genes continue to protect Australian wheats from leaf rust and stem rust, but Lr24 has lost effectiveness in several overseas countries and Sr24 has been overcome in India.
All the genetic materials were shared among researchers free of charge. So were similar materials that allowed Australian breeders to develop varieties with Sr26 stem rust resistance (Eagle, Kite, Currawong) also derived from Thinopyrum elongatum and originally developed in Canada, as well as varieties with stripe rust resistance genes now in Sunbri, Sunlin, Sunstate, Sunvale and Camm derived in France from another grass, Aegilops ventricosa.
The information for this article was provided by Professor Bob Mcintosh, Director of the National Cereal Rust Control Program, which is supported by growers and the Federal Government through the GRDC
Program 2.6.4 Contact: Professor Bob Mcintosh 02 9351 8802