Expression of defense genes in sorghum grain mold and tagging and mapping a sorghum anthracnose resistance gene

Sorghum grain mold and anthracnose are two major diseases of sorghum (Sorghum bicolor) that constrain sorghum production worldwide. Grain mold is caused by several species of fungi, but the two most common are Curvularia lunata and Fusarium thapsinum. Isolates of these two species were used to inoculate panicles of selected sorghum cultivars in green house and field experimentations. Panicles were sprayed at the time of anthesis with conidial suspensions of the two fungal species individually or in a mixture and with water to serve as a control. Samples were collected 48 hours after inoculation for RNA extraction. In greenhouse studies, four cultivars (Tx2911, Sureno, SC170 and RTx430) were used while thirteen cultivars were grown in the field experiments. Gene expression was measured for the following genes using real time polymerase chain reactions (rt-PCR): PR10, ?-glucanase, chitinase, thaumatin, sormatin, phenyalanine ammonia lyase (PAL), obtusifoliol 14?-demethylase (Obtus), antifungal protein (AFP), apoptosis related protein (Apop) and leucine rich repeat (LRR). Seed germination tests in field grown cultivars indicated that germination rates for SC279-14E, SC660 and Sureno were not greatly influenced by grain mold. Covering the panicles with bags served to protect them against grain mold pathogens. The seed mycoflora test showed that Fusarium thapsinum was the most frequently recovered species and there were more species present in non-covered panicles. The response of sorghum cultivars to grain mold infection involves multiple defense genes. Real time PCR used to study the expression of sorghum defense in greenhouse grown plants showed that mRNA encoding PR-10, a small 10 kDa protein, was highly expressed in the glumes and spikelets of resistant cultivars Tx2911 and Sureno and constitutively in leaves. The expression of some other defense genes like beta-glucanase, chitinase and AFP was variable. Sormatin was not expressed. Expression of ?-glucanase, chitinase, and PR10 was higher in field than in greenhouse experiments. A second area of research involved tagging of a resistance gene for sorghum anthracnose. Three AFLP markers (Xtxa607, Xtxa3181 and Xtxa4327) and three SSRs (Xtxp3, Xtxp55 and Xtxp72) were identified. These markers were loosely linked to the resistance genes. The markers are located on linkage group B. The results suggest that markers located 20-30 cM on one side or the other of those tested should provide useful tags for the resistance gene.