Genetic and ontogenic analysis of anthracnose resistance in Sorghum bicolor (L.) Moench

Abstract

Experiments were designed to understand the genetic and ontogenic characteristics of resistance to Colletotrichum graminicola (Ces.) Wils. in Sorghum bicolor (L.) Moench. Resistant lines SC326-6 and SC599-11E were crossed to susceptible lines BTx623 and B35-6 to derive F1, F2, and F3 progenies, and the disease response of these materials was tested against one avirulent and two virulent isolates of the pathogen. Resistant and susceptible lines did not differ in their ability to respond to infection. However, symptom development varied with plant genotype and age at inoculation. The response of SC326-6 was typically hypersensitive whereas the reaction of SC599-11E was of a slow symptom development type. Contrasting with these resistant responses was the behavior of the susceptible lines, which lost the capacity to restrict lesion growth in older plants. Three 3-deoxyanthocyanidin phytoalexins were shown to accumulate in resistant lines and their Fi progenies to significantly greater concentrations than in the susceptible lines in response to attempted infection by the pathogen, regardless of the developmental stage of plants. This response was due to earlier onset and faster rate of synthesis of the phytoalexins in resistant lines compared to susceptible lines. Phytoalexin concentration was considerably lower in plants that had grown beyond the juvenile stage. Of particular interest was the antimicrobial activity of these phytoalexins since they inhibited the formation of appressoria by fungal propagules in vitro. Earlier availability of these toxic compounds in resistant lines may explain their capacity to restrict infection compared to susceptible lines. Resistance in SC326-6 was not the same as in SC599-11E, the latter line displaying reversal of its phenotype from resistant to susceptible as plants aged. Reversal of resistance also occurred in SC599-11E-derived progenies. In contrast, the phenotype of SC326-6 and its progenies was stable throughout all growth stages. This and the bimodal distribution for anthracnose severity index frequencies suggested that resistance was controlled by dominance at a single multiallelic locus. Restriction analysis of DNA from SC326-6, BTx623, and 60 F2's produced a syntenic group containing the anthracnose resistance locus, which was 23.8 cM and 25.9 cM away from flanking RFLP loci Xtam309 and Xtam1090, respectively.