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Materials and methods

Triticum aestivum cv. WL 711 (a widely adapted and agronomically superior Indian spring wheat cultivar) was crossed as female with a leaf rust, Karnal bunt, powdery mildew and cereal cyst nematode resistant accession (Table 1) of Ae. triuncialis (Acc.3549). WL 711 is susceptible to these diseases/pests. The sterile F₁ was backcrossed to T aestivum cv. WL 711. The desirable progenies were selfed after two to three backcrosses. Data on chromosome number and meiotic chromosome pairing were recorded in F₁ and the subsequent backcross/selfed progenies. The observations on field reaction to leaf rust we re recorded at adult plant stage in each generation by using modified Cobb's scale (Paterson et al. 1949). However, seedling response to an individual pathotype of leaf rust was recorded in early backcross generations (up to BC₂) and a part of advanced progenies. The standard procedure for inoculation of seedlings (Nagarajan et al. 1986) was followed and seedling response to rust was recorded on 0-4 scale (Knott 1989). The addition/substitution/translocation of the alien chromosome(s) or chromosome part (s) were investigated through Giemsa C-banding (Friebe et al. 1992) and/or genomic in situ hybridization (Mukai and Gill 1991). Advanced progenies were scored for reaction to leaf rust, Karnal bunt, powdery mildew (one Indian isolate from Keylong and a Japanese isolate) and cereal cyst nematode. To record reaction to Karnal bunt, 10-15 plants from each progeny were inoculated at boot stage (3-5 tillers per plant) using artificial inoculation method of Aujla et al. (1982) and percent incidence of disease was recorded on tiller basis at maturity. Reaction to powdery mildew was recorded on first leaf of 7- 10 days old seedlings. Seedlings were inoculated by dusting conidia and inoculated seedlings were incubated in a growth chamber at 20 plus or minus 2 ^OC, 70-80% relative humidity and 14 hours day light. The infection types were recorded on 0-4 scale (Smith and Blair 1950) after 8-14 days of inoculation. Reaction to cereal cyst nematode, Heterodera avenae, was recorded under artificial inoculation conditions following the method used by Singh et al. (1991).

Results and discussion

A set of derivatives from the cross T. aestivum cv. WL 711 x Ae. triuncialis Acc.3549 had spelta type head, a characteristic associated with monosomy for 5A. C-banding of mitotic metaphase chromosomes showed that in these derivatives chromosome 5U was substituted for 5A (Fig. 1). Genomic in situ hybridization further confirmed the presence of this satellited chromosome (Fig. 2a). These derivatives with 42 chromosomes had 0 to 4 univalents and occasionally a quadrivalent. The occurrence of a quadrivalent suggested the homoeology of 5U with one of the chromosomes of group 5 of wheat (5B or 5D) The progenies gave a high reduction in cyst population of Heterodera avenae as compared to the initial cyst population under artificial screening (Table 2). The significant reduction in the cyst population in Ae. triuncialis and increase in the susceptible recurrent parent (WL 711) suggested that the nematode resistance of these derivatives has been derived from chromosome 5U of Ae. triuncialis. These derivatives also exhibited moderate resistance to leaf rust under field conditions as compared to highly susceptible recurrent parent (90S to 100S). One of the progenies (L98.99-1180) that segregated for leaf rust resistance also segregated for spelta/normal head. The plants with spelta head possessed moderate resistance (20X) and those with normal head were susceptible (80S). This suggested that the 5U chromosome of Ae. triuncialis in these derivatives is also carrying the gene(s) for moderate leaf rust resistance. However, this leaf rust resistance may be of adult plant type resistance as the BC₂ plant from which this set of progenies were derived (BC₂ plant No.20), exhibited 3+ seedling reaction to pathotype 77A- 1 (Avirulence/virulence formula: Lr 9, Lr20, Lr23, Lr26/Lr1, W, Lr10, Lr13, Lr15). Also, two progenies, L98.99-1183 and L98.99-1184 gave 1+ to 3 and 2 to X reactions, respectively, to pathotype 77-2 (Avirulence/ virulence formula: Lr9, Lr26/Lr1, Lr3, Lr10, Lr13, Lr15, Lr20, Lr23) whereas T. aestivum cv. WL 711 exhibited 3+ to 4 (susceptible) reaction and Ae. triuncialis was resistant (reaction=O) in this test. These two progenies exhibited 3+ to 4 and 3 to 3+ reactions, respectively, to pathotype 77-5 (Avirulence/ virulence formula Lr9/Lr1, Lr3, Lr10, Lr13, Lr15, Lr20, Lr23, Lr26). T. aestivum cv. WL 711 was fully susceptible (reaction = 4) and Ae. triuncialis was resistant (reaction = 0 to 0;) to pathotype 77-5. The intermediate seedling responses of these progenies to pathotypes 77-2 and 77-5 further supported the presence of low adult plant resistance which is presumably due to the rust resistance gene(s) on 5U. These derivatives also carried moderate to high seedling resistance to an Indian isolate (Keylong isolate) of powdery mildew. One of the progenies that segregated for resistance to powdery mildew (L98.99 1184) also segregated for resistance to leaf rust.

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