SR-059

Cheddar cheese made from milk containing Sr⁸⁹ and Ca⁴⁵ had a slightly greater Sr: Ca ratio in the curd than in the whey.This led to a question of the relation between the uptake of Sr vs. Ca in the growth of S. lactis.Nine trials were conducted; each trial consisted of an oxalate precipitate prepared from 2 samples of medium containing radioactivity but no inoculum, 4 samples of bacterial cells which had been collected, and 4 samples of supernatant.The bacterial cells gathered had a slightly greater ratio of Sr⁵⁹: Ca⁴⁵ than did the supernatants from the same samples.The average ratio in the collected cell fractions divided by that present in the medium was very close to that previously found in Cheddar cheese.This suggests that the preferential uptake of Sr over Ca in Cheddar cheese curd was augmented by the greater uptake of Sr by S. lactis.Removing the bacteria from the broth by filtration showed a greater Sr⁸⁹: Ca⁴⁵ ratio in the cell fraction than in the broth.

Control of stem rust, caused by Puccinia graminis f.sp. tritici, a highly destructive fungal disease of wheat, faces continuous challenges from emergence of new virulent races across wheat-growing continents. Using combinations of broad-spectrum resistance genes could impart durable stem rust resistance. This study attempted transfer of Sr59 resistance gene from line TA5094 (developed through CSph1bM-induced T2DS·2RL Robertsonian translocation conferring broad-spectrum resistance). Poor agronomic performance of line TA5094 necessitates Sr59 transfer to adapted genetic backgrounds and utility evaluations for wheat improvement. Based on combined stem rust seedling and molecular analyses, 2070 BC1F1 and 1230 BC2F1 plants were derived from backcrossing BAJ#1, KACHU#1, and REEDLING#1 with TA5094. Genotyping-by-sequencing (GBS) results revealed the physical positions of 15,116 SNPs on chromosome 2R. The adapted genotypes used for backcrossing were found not to possess broad-spectrum resistance to selected stem rust races, whereas Sr59-containing line TA5094 showed resistance to all races tested. Stem rust seedling assays combined with kompetitive allele-specific PCR (KASP) marker analysis successfully selected and generated the BC2F2 population, which contained the Sr59 gene, as confirmed by GBS. Early-generation data from backcrossing suggested deviations from the 3:1 segregation, suggesting that multiple genes may contribute to Sr59 resistance reactions. Using GBS marker data (40,584 SNPs in wheat chromosomes) to transfer the recurrent parent background to later-generation populations resulted in average genome recovery of 71.2% in BAJ#1*2/TA5094, 69.8% in KACHU#1*2/TA5094, and 70.5% in REEDLING#1*2/TA5094 populations. GBS data verified stable Sr59 introgression in BC2F2 populations, as evidenced by presence of the Ph1 locus and absence of the 50,936,209 bp deletion in CSph1bM. Combining phenotypic selections, stem rust seedling assays, KASP markers, and GBS data substantially accelerated transfer of broad-spectrum resistance into adapted genotypes. Thus, this study demonstrated that the Sr59 resistance gene can be introduced into elite genetic backgrounds to mitigate stem rust-related yield losses.