No abstract
Recessively inherited gene Sr2 has provided the basis of durable resistance to stem rust (caused by Puccinia graminis tritici) in wheat (Triticum aestivum L.) worldwide. The associated earhead and stem melanism or Ôpseudo-black chaffÕ is generally used as a marker for this gene. Sr2 has been postulated in many wheat cultivars of India including ÔLok 1Õ, based on associated pseudo-black chaff in adult plants, and leaf chlorosis in seedlings. However, dominant inheritance of the resistance factor operating in ÔLok 1Õ, and a 13 : 3 (resistant : susceptible) F 2 segregation in the ÔSr2-lineÕ (ÔChinese SpringÕ 6 · ÔHopeÕ 3B) · ÔLok 1Õ cross confirmed that Sr2 was absent in ÔLok 1Õ. Susceptible plants with a pseudo-black chaff phenotype were observed in F 2 populations of ÔAgra LocalÕ (susceptible) · ÔLok 1Õ, and the ÔSr2-lineÕ · ÔLok 1Õ crosses. Most of the F 3 families derived from the susceptible F 2 segregants with pseudo-black chaff phenotypes were true breeding for the expression of pseudo-black chaff with susceptibility to stem rust. Thus, linkage of pseudo-black chaff with Sr2 in wheat can be broken, and hence, caution may be exercised in using pseudo-black chaff as a marker for selecting Sr2 in breeding programmes.
The bread wheat cultivar ÔThatcherÕ is documented to carry the gene Lr22b for adult-plant resistance to leaf rust. Seedling-resistance to leaf rust caused by Puccinia triticina in the bread wheat cultivar ÔThatcherÕ, the background parent of the near-isogenic lines for leaf rust resistance genes in wheat, is rare and no published information could be found on its genetic basis. The F 2 and F 3 analysis of the cross ÔAgra LocalÕ (susceptible) · ÔThatcherÕ showed that an apparently incompletely dominant gene conditioned seedling-resistance in ÔThatcherÕ to the three ÔThatcherÕ-avirulent Indian leaf rust pathotypes -0R8, 0R8-1 and 0R9. Test of allelism revealed that this gene (temporarily designated LrKr1) was derived from ÔKanredÕ, one of the parents of ÔThatcherÕ. Absence of any susceptible F 2 segregants in a ÔThat-cherÕ · ÔMarquisÕ cross confirmed that an additional gene (temporarily designated LrMq1) derived from ÔMarquisÕ, another parent of ÔThat-cherÕ, was effective against pathotype 0R9 alone. These two genes as well as a second gene in ÔKanredÕ (temporarily designated LrKr2), which was effective against all the three pathotypes, but has not been inherited by ÔThatcherÕ, seem to be novel, undocumented leaf rust resistance genes.
The gene Lr34 has contributed to durable resistance to leaf rust caused by Puccinia triticina in wheat worldwide. The closely associated leaf tip necrosis is generally used as the gene's marker. Lr34 has been postulated in many Indian bread wheat cultivars including ÔC 306Õ, based on the associated leaf tip necrosis and a few other field and glasshouse observations. The present study showed monogenic control of adult-plant resistance in ÔC 306Õ to leaf rust pathotype 77-5 (121R63-1). The F 2 segregation in the crosses between ÔC 306Õ and the two known carriers of Lr34, ÔLine 897Õ and ÔJupateco 73Õ ÔRÕ fitted a digenic ratio. The F 3 families derived from the susceptible F 2 segregants were true breeding for susceptibility, proving the absence of Lr34 in ÔC 306Õ. The cross between ÔLine 897Õ and ÔJupateco 73Õ ÔRÕ did not segregate for susceptibility. Resistance in the cross ÔAgra LocalÕ (susceptible) · ÔC 306Õ was associated with leaf tip necrosis, showing that the leaf rust resistance gene in ÔC 306Õ was associated with leaf tip necrosis, but was different from Lr34. This gene is being temporarily designated as LrÔC 306Õ. Hence, leaf tip necrosis cannot be considered as an exclusive marker for selecting Lr34 in wheat improvement.
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