On the Structure of a Gene for Disease Resistance in Maize

Abstract

Disease resistance is an extremely valuable control measure of rusts in cereal crops and is based, almost entirely, on simple procedures of identification and incorporation of major genes for resistance into economic varieties. Usually, several loci that carry these genes are known in each plant species. Frequently, several alleles at a locus can be distinguished on the basis of their reaction to various races of the pathogen. In spite of the vast array of resistance genes now known, their use has been sharply limited by the restricted number of loci in each species. The result has been a never-ending cycle of (1) releasing a variety resistant to prevalent races, (2) finding it susceptible to new races of the pathogen appearing a few years later, (3) breeding resistance to the new races, and (4) releasing the new variety only to find it susceptible, a few years later, to yet new races of the pathogen. Thus, breeding for rust resistance in cereals has been so repetitious that the value and use in disease control of genes that condition resistance to only a portion of the possible rust races is now widely questioned.' Although major genes for rust resistance have been used extensively, little is known about their fine structure. This is probably due to the fact that in crops, where resistance genes have been of maximum importance, genetic analyses requiring large test-cross progenies are not feasible. Economic and biological considerations, however, provide compelling reasons why studies on the structure of genes for rust resistance should be made. The system of genetic resistance to rust, Puccinia sorghi Schw., in maize, Zea mays L., is apparently parallel to that found in other cereal crops. In maize, there are at least five loci (Rpl, Rp3, Rp4, Rp5, and Rp6) at which dominant genes for rust resistance occur.2-6 Rp, is located in the short arm of chromosome 10 and was considered to occupy the terminal position on the genetic map.3'7 Recently, Rpi has been shown to be located between Rp5 and Rpe which are 1.1 and 2.1 map units, respectively, from it.' Orientation of these genes with respect to the centromere is not known. Rp3 and Rp4 are independent of Rpi. On the basis of genetic studies and disease reaction, 14 alleles, Rpia, Rp b... to

Comparative Sequence Analysis of the Sorghum RphRegion and the Maize Rp1 Resistance Gene Complex

“…The Rp1 region in maize has been placed near the telomere of the short arm of chromosome 10 by both cytogenetic and recombinational mapping (Rhoades, 1935;Saxena and Hooker, 1968;Hulbert and Bennetzen, 1991). Although recombination rates in the region can vary a great deal in different haplotype combinations, the complex Rp1 locus usually is found to cover about 0.4 centiMorgans.…”

Molecular Characterization of the Maize Rp1-D Rust Resistance Haplotype and Its Mutants

“…The rp1 genes described by Saxena and Hooker (1968) are available in a series of near-isogenic lines in the R168 back- Maize Rp1-D Rust Resistance 1371 ground. DNA gel blot analysis with Rp1-D probes was used to examine DNAs of 13 of these R168 lines following digestion with a range of restriction enzymes (e.g., Figure 6).…”

“…Different rp1 rust resistance specificities can be genetically recombined, and chromosomes recombinant for two different rp1 specificities (either double resistant or double susceptible) have been selected from testcross progeny by using appropriately chosen rust biotypes (Saxena and Hooker, 1968;Hulbert et al, 1993). Rp1-D/Rp1-J recombinants subjected to DNA gel blot analysis with an Rp1-D probe for the NBS region display combinations of fragments from both parents, indicating that the recombination events had occurred within the cluster of Rp1-D homologous genes from each parent (Figure 7).…”

“…Fourteen different resistances have been given the rp1 designation on the basis of map position (Saxena and Hooker, 1968), and a number of these have been genetically recombined, suggesting that they are encoded by members of a gene cluster (Saxena and Hooker, 1968;. Different rp1 genes spontaneously mutate to susceptibility at frequencies between 0.016 and 0.5% (Pryor, 1987;Bennetzen et al, 1988).…”

Resistance gene analogs are conserved and clustered in soybean

“…Genetic linkage of resistance genes has been reported in maize for the Rpl cluster (14)(15)(16), in barley for the Mla cluster (17)(18)(19)(20), in lettuce for a wide range of Dm loci (21,22), in oat for the Pc cluster (23), and in flax for the L gene cluster (24)(25)(26). Clustering of resistance genes suggests that a common genetic mechanism has been involved in their evolution (16).…”