Genetic mapping of the Pc gene in sorghum

The Pc gene in sorghum (Sorghum bicolor (L.) Moench) controls susceptibility to the root rot fungus Periconia circinata (Mangin) Sacc. and response to peritoxin, a host-selective toxin produced only by pathogenic strains of the fungus. Susceptibility to the fungus is strictly correlated with sensitivity to peritoxin, which, in the absence of the fungus, reproduces the identical symptoms of milo disease. Thus, the specificity of peritoxin and its causal role in milo disease may simplify the studies of complex host-pathogen interactions which can be reduced to the interaction between a single metabolite and a single genetic locus or gene product. To better understand the mechanisms of the sorghum-P. circinata interactions, the ultimate goal is to clone and characterize the Pc gene. Towards map-based cloning of the Pc gene, DNA markers tightly linked to Pc are a prerequisite. To uncover these markers, bulked DNA pools from the susceptible (PcPc; toxin-sensitive) or resistant (pcpc; toxin-insensitive) F$\sb2$s of the cross (S)-Colby (PcPc) x Shanqui Red (pcpc) were used. Five random amplified polymorphic DNA (RAPD) markers were identified in a screen of 1,600 arbitrary primers (10-mers). Of these markers, two cosegregated with the susceptible phenotype (in coupling) and three with the resistant phenotype (in repulsion). To reliably locate the Pc gene, sequence-tagged-site (STS) primers converted from these RAPD markers were designed and used to screen the same F$\sb2$ population and the recombinant inbred (RI) lines derived from the cross of SRN39 x Shanqui Red. The STSB12 and STSO10 markers were closely linked to Pc, 7.9 cM and 5.9 cM, respectively. For unknown reason, STSB12 and STSO10 were mapped to two different sorghum linkage groups in these RI lines. Four maize clones flanking STSO10 on sorghum linkage group I were chosen for RFLP analysis. UIU101 was found tightly linked (0.98 cM) to Pc when 51 homozygous susceptible F$\sb3$ families of S-Colby x Shanqui Red were examined; the other three clones were not polymorphic in the RI lines. For high-resolution mapping of the Pc locus, UIU101 was mapped to chromosome 6 in maize, which contains markers colinear with sorghum linkage group I