CHARACTERIZATION OF AN NBS-LRR GENE FAMILY AT THE RP3 LOCUS IN MAIZE

¹ Department of Plant Pathology Kansas State University Manhattan, KS 66056

² Division of Plant Industry C.S.I.R.O Canberra ACT 2601 Australia

The rp3 locus maps near the centromere on the short arm of maize chromosome three. Rp3 confers resistance to common rust caused by the fungal pathogen Puccinia sorghi. Genetic experiments indicate that rp3 is a complex locus like rp1. The PIC13 probe identifies an nucleotide binding site-leucine rich repeat (NBS-LRR) gene family that maps to the rp3 locus (Collins et.al., MPMI Vol.11, No.10,1998, pp.968-978). Two fine-structure mapping populations totaling 2,623 individuals were constructed for RFLP analyses with PIC13. Sixteen recombinants were identified between rp3 and either the gl6 or ys3 morphological markers. No recombination was detected between the PIC13 hybridizing restriction fragments and rp3. Susceptible variants from Rp3 homozygotes usually showed non-parental combinations of flanking RFLP markers, indicating they arise by unequal crossing-over. When these variants were analyzed using PIC13, they show losses of PIC13 homologous DNA fragments when compared to the resistant parents. 3' RACE experiments indicate that there are at least five transcribed family members in Rp3 lines. Transgenic maize plants carrying an antisense construct are being generated and will be tested for suppression of Rp3 mediated resistance to verify that we have cloned the correct gene family. Strong similarities to known resistance gene sequences, mapping data indicating tight/coincidental linkage to the rp3 locus, and the non-parental banding patterns of susceptible rp3 variants collectively suggest that PIC13 identifies the rp3 gene family.