The expression of leaf rust resistance in wheat lines containing Lr12 and Lr13

[摘要] Monogenic resistance in wheat (Triticum aestivum L.) to leaf rust, caused by Pucciniarecondita Rob. ex. Desm. f. sp. tritici, has generally not been durable. One strategy ofattaining durable resistance to rust diseases of wheat is the combination of severalresistance genes in a single genotype. Interactions among Lr genes have been definedas the combination of two or more genes resulting in higher resistance levels than thatconferred by the genes individually. It has been suggested that Lr12 and Lr13, bothadult-plant resistance genes, in combination with other genes, form the basis of durableresistance in several cultivars. In this study, the assumption that Lr12 and Lr13 mayinteract to condition improved resistance to leaf rust, was investigated.Four Thatcher (Tc) F3 lines (13/12-3, 13112-9, 13/12-19 and 13/12-40),homozygous for both Lr13 and Lr12, were selected and compared with the parents(CT263 [=TcLr13] and RL6011 [=TcLr12]), the single gene lines Tc/13-22 and Tc/12-16, and Thatcher. In addition to infection type studies in seedlings and adult plants,lines were compared according to several histological and macroscopic componentsof resistance, as well as disease ratings in the field.Flag leaf infection type studies showed that Lr12 is effective against mostpathotypes of P. recondita f. sp. tritici occurring in South Africa. Conversely, Lr13 isineffective against the dominant pathotypes, implying that the gene has no value as amonogenic source of resistance. Both Lr12 and Lr13 were inherited dominantly.Based on the fact that several pathotypes are avirulent to these genes, they should bemanipulated with relative ease in local breeding programmes directed at utilising thesesources in combination with other Lr genes. Considering the microscopic components, effects of Lr12 and/or Lr13 resistanceon the prepenetration stages were not determined. Results of aborted penetration,consisting of nonpenetrating appressoria and aborted substomatal vesicles, showedthat inhibition of fungal growth in wheat lines containing Lr12 and/or Lr13, wasactivated, to a certain degree, before haustoria were formed. Determination of colonysize 240 hours after inoculation indicated that fungal colonies in the combination lineswere generally smaller than in the parents, but not necessarily smaller than those in themonogenic line Tc/13-22. Host cell necrosis was more frequently associated withinfection sites, specifically of UVPrt2, in the combination lines than in the parents.Hypersensitivity index values (calculated by dividing the area of leaf necrosis with thearea of the respective colony), indicated that host cell necrosis was more severefollowing infection of the combination lines with UVPrt2. Quantification of cell wallappositions showed that fewer papillae occurred in Thatcher than in the other hostgenotypes. The number of haustoria observed per colony did not indicate any clear,repeatable differences between lines.The common occurrence of host cell necrosis observed during histologicalexaminations was also reflected in the macroscopic components. Infection types onthe flag leaves of lines carrying both Lr12 and Lr13 often displayed chlorosis andnecrosis. These ratings on primary and flag leaves, as well as the quantitativecomponents including latent period, uredium density and uredium size, did not indicateclear differences between the digenic lines and the most resistant parent. In theabsence of a pathotype virulent to both genes, the combination lines were resistant inthe field. Field tests with a pathotype virulent to both Lr12 and Lr13 would have beenmore valid in evaluating whether the genes interacted. Data obtained were not conclusive in suggesting pronounced resistanceenhancement due to combining Lr12 with Lr13. Therefore, the assumption of durability,resulting from a novel resistance mechanism conditioned by this combination, was notconfirmed.

[发布机构] University of the Free State