Marker-assisted pyramiding of four QTL/genes for Asian rust (Phakopsora pachyrhizi) resistance in soybean

Abstract

ABSTRACTGene pyramiding (assembling multiple desirable genes into a single genotype) via conventional methods to combat Asian rust resistance in soybean (Glycine max (L.) Merrill) and developing soybean varieties with durable resistance to this disease continues to be a challenge. Therefore, our objectives were to pyramid four Asian rust (caused by Phakopsora pachyrhizi) resistance (Rpp) genes, viz., Rpp1, Rpp2, Rpp3, and Rpp4 from their respective donors, PI200492 (Komata), PI230971, PI462312 (Ankur), and PI459025 (Bing Nan). Two single crosses (PI200492 × PI230971; PI462312 × PI459025) and a double-cross [(PI200492 × PI230971) × (PI462312 × PI459025)] were made for present study. The scoring of parents and crosses was done according to a 0–9 grading scale, where 0 = 0% disease intensity (absolutely resistant), 1 = 1% disease intensity (highly resistant), 3 = 1.1–10% disease intensity (moderately resistant), 5 = 10.1–25% disease intensity (moderately susceptible), 7 = 25.1–50% disease intensity (susceptible), and 9 = more than 50% disease intensity (highly susceptible). Out of four parents used, Rpp1 gene-donor plant introduction (PI)200492 (Komata) was immune to rust, with a disease grade of 0.92. Rpp2 and Rpp4 gene donors, PI230971 and [PI459025 (Bing Nan)], had reddish-brown (RB) lesions. Rpp3 gene-donor PI462312 (Ankur) showed a few, highly localized patches of tan lesions; however, it was not as susceptible as the susceptible check JS335. In cross-A (PI200492 × PI230971), all the F1s were rust resistant, whereas in the F2, 93.9% (845 plants) were either immune (11.0%, 99 plants) showing no infection or had RB lesions (82.9%, 746 plants) with resistant reaction and the remainder 6.1% (55 plants) had susceptible reaction. In cross-B (PI462312 × PI459025), 80.8% (63 plants) of the F1 had RB lesions and resistant reaction, whereas 19.2% (15 plants) had susceptible reaction. In its F2 population, 24.2% (217 plants) were susceptible. In cross-C [(PI200492 × PI230971) × (PI462312 × PI459025)], i.e., double-cross hybrid (DCH), 85.5% (77 plants) were either immune or had RB lesions, whereas the rest of the plants 14.5% (13 plants) showed susceptible reaction. Based on parental survey, four simple sequence repeat primers were short-listed for amplification of individual rust-resistance gene-specific markers in ten plants of each cross, viz., Satt191-210bp (Rpp1 gene), Sat_361-245bp (Rpp2 gene), Satt263-195bp (Rpp3 gene), and Rpp4TM-128bp (Rpp4 gene). In the single-cross hybrid (SCH)-A (PI200492 × PI230971), six plants had both Rpp1 and Rpp2 genes, whereas in another SCH-B (PI462312 × PI459025), seven plants had both Rpp3 and Rpp4 genes. In DCH [(PI200492 × PI230971) × (PI462312 × PI459025)], a single plant had all four Rpp genes, five plants had two Rpp genes, whereas a single susceptible plant had only Rpp3 gene. The homozygous rust-resistant segregants from these crosses can be used for improving rust resistance of otherwise adapted, high-yielding soybean genotypes.