RECOMBINANT AND MUTANT GENES FROM THE Rp1 RUST RESISTANCE COMPLEX OF MAIZE

¹ Dept.of Plant Pathology, Kansas State University, Manhattan, KS66506-5502

² CSIRO Plant Industry, PO BOX 1600, Canberra ACT 2601

Rp1 is a complex rust resistance locus of maize comprising of a number of tandemly arranged homologous genes. The rphd haplotype is composed of Rp1-D and eight homologs. Members of the rphd haplotype and several mutant and recombinant alleles were sequenced. Seven of the rphd genes coded for predicted NBS-LRR proteins similar to the Rp1-D gene (DNA identities 91% to 97%), but with significant polymorphism, especially in the C-terminal LRR domain. One haplotype member was truncated and had no LRR domain. Seven of the nine rphd members were transcribed, including the truncated gene. Sequence comparisons between the genes provided evidence for intragenic recombination and diversifying selection. Isolation of genes from variants that completely or partially lost resistance indicated they were usually associated with intragenic crossing over. Recombination breakpoints occurred throughout the genes. Crossovers that occurred while the genes were mispaired during meiosis generated novel haplotypes. Many of the recombination events occurred between the Rp1-D gene and an untranscribed homolog. One recombinant gene with a complete LRR from Rp1-D, but the amino terminal half from another homolog, conferred the Rp1-D specificity but with a reduced level of resistance. This is consistent with the idea that the LRR domain confers race-specificity in this class of genes. One mutant allele that was not associated with crossing over was found to be identical to Rp1-D except for an in-frame deletion corresponding to 11 aa in the 10th LRR, indicating this LRR is essential for Rp1-D gene function.