A Novel QTL for Root-Knot Nematode Resistance is Identified from a South African Sweet Sorghum Line

Southern root-knot nematodes, Meloidogyne incognita, feed on the underground portions of hundreds of plant species and affect nutrient partitioning and water uptake of the host plants. Sorghum (Sorghum bicolor) is often not significantly damaged by southern root-knot nematodes (RKN) but some sorghum genotypes support greater population densities of RKN than other genotypes. These higher nematode populations increase the risk of damage to subsequently planted susceptible crops. A previous study identified a major quantitative trait locus (QTL) for RKN resistance on sorghum chromosome (chr.) 3. To maintain long-term resistance, multiple resistance genes should be pyramided in a cultivar. In this study, we identified a new source of RKN resistance, created a mapping population, and identified single-nucleotide polymorphism markers using genotyping-by-sequencing of the segregating population. Use of single-marker analysis and composite interval mapping identified a single QTL on chr. 5 that was associated with egg number and egg number per gram of root from the resistant sweet sorghum line PI 144134. This region on chr. 5 and the prior QTL on chr. 3 can be potentially moved from PI 144134 and Honey Drip, respectively, into elite sorghum germplasm via marker-assisted selection for more durable resistance.

Calidad fisiológica de la semilla de sorgo sometido a defoliación y déficit hídrico.

The effects of defoliation were studied in sorghum plants subjected to drought stress in pre and post-blooming stages. The quality was appraised on the seed obtained from a sorghum line evaluated under greenhouse conditions in 1991, at the Colegio de Postgraduados, Montecillo, Mexico. The pre-blooming drought stress consisted of withholding the water to the plants at their primary branch differentiation stage, whereas during the post-blooming drought stress, the water was withheld at theearly blooming stage. The defoliation levels (0, 40 and 80% of the total leaf area) were conducted during each drought stress treatment. In general, the germination was reduced as the seed size decreased. The germination percentage was above 80% for the 3 and 3.5 mm seed sizes; while for the 2.5 mm seed size, the germination decreased rapidly in plants that were not defoliated in both moisture conditions. This indicates that adverse factors such as drought stress and defoliation can reduce seed size, but it the same time it can also stimulate the seed germination.

DNA markers for downy mildew resistance genes in sorghum

The random amplified polymorphic DNA technique was used to find markers for a downy mildew resistance gene in sorghum. Of the 674 random primers screened for polymorphism, 2 amplified fragments were linked to a downy mildew resistance gene in sorghum line SC414. Utilization of an existing restriction fragment length polymorphism mapping population (IS3620C × BTx623) also revealed two markers that are linked to a different resistance gene in another sorghum line, BTx623.Key words: sorghum, downy mildew, RFLP, RAPD.