Functional Characterization Of TaAbc1and Sr33-related Genes In The Interaction Between Wheat And Rust

Plant hypersensitive response (HR), visualized as rapid death of host plant cells at theinfection sites, is a means of plant defense response to microbial pathogens. In plants, HR istriggered by incompatible interactions of host plants and corresponding pathogens. Stripe rust(Puccinia striiformis f. sp. tritici，Pst) can cause wheat stripe rust disease, and is aworldwide problem threatening wheat production. Therefore, searching for genes involved inwheat (Triticum aestivum L.) defense response to the stripe rust pathogen Pst is of greatlyimportance to elucidate the molecular mechanisms of HR and the signal pathway in theinteractions between the wheat and stripe rust.To search for genes involved in wheat (Triticum aestivum L.) defense response to thestripe rust pathogen Pst, we identified and cloned a new wheat gene similar to the Abc1-likefamily. We present the results on molecular characteristics of this newly identified wheatAbc1-like gene, its expression profiles in response to Pst infection and abiotic elicitors and itsinvolvement in wheat defense response to a group of rust fungal pathogens. To test if TaAbc1has any functional role in wheat defense response to stripe rust, we knocked down theendogenous TaAbc1gene in Suwon11using the barley stripe mosaic virus induced genesilencing (BSMV-VIGS) assay.1. From a cDNA library constructed from the wheat cultivar Suwon11infected with avirulent stripe rust pathotype CYR31，we selected an EST clone for further characterization inthis study due to its strong up-regulated expression in the incompatible interaction ofSuwon11/CYR23. The gene was successfully amplified from Suwon11after in silico. Thenew gene, designated as TaAbc1, encodes a717-amino acid,80.35kD protein. The TaAbc1protein contains two conserved domains shared by Abc1-like proteins, two trans-membranedomains at the C-terminal and a36-amino acid chloroplast targeting presequence at theN-terminal. Sequence analysis showed that TaAbc1were highly homologous to proteins inOryza sativa and Arabidopsis etc, and these proteins formed a subgroup.2. Characterization of TaAbc1expression revealed that gene expression was tissue-specific. The highest TaAbc1transcript abundances were detected in stems. In contrast,roots had the lowest--only about11%of the level in leaves. To study the expression profilesof TaAbc1during the course of rust infections in both incompatible and compatibleinteractions, we challenged Suwon11with two Pst pathotypes, CYR23and CYR31. Inaddition, we included in the study two additional incompatible interactions with a USA springwheat genotype Scholar with a leaf rust resistance gene Lr47, inoculated with pathotype PBJLof the leaf rust pathogen (P. triticina) and pathotype QFCSC of the stem rust pathogen (P.graminis f. sp. tritici). TaAbc1was up-regulated and peaked at24hpi with rust challenge inboth incompatible and compatible interactions. High-fold induction was associated with thehypersensitive response (HR) triggered only by avirulent stripe rust pathotypes, suggestingTaAbc1is a rust-pathotype specific HR-mediator. An exogenous treatment of JA/ETup-regulated TaAbc1transcription but the increase was not as much as that detected in thedirectly wounded plants. The expression of TaAbc1was not affected by SA treatment,suggesting that TaAbc1was positioned either up-stream of SA or in an SA-independentmanner.3. A significant increase in TaAbc1transcript level in the H2O2treated plant as early as at2hpt. Down-regulating TaAbc1expression by virus-induced gene silencing reduced necroticarea at infection sites but not the overall resistance level, suggesting TaAbc1was involved inHR against stripe rust, but overall host resistance is not HR-dependent. Our study suggestedthat signaling pathways to HR and other defense responses induced by CYR23in wheat arebranched. These observations invited the hypothesis that TaAbc1could be a commonly sharedcell-death signal used in wheat defense response to rust fungal pathogens. Our results tend tosuggest that TaAbc1-mediated HR was rust species and pathotype specific only in theincompatible interactions triggered by stripe rust.Stem rust (Puccinia graminis f. sp. tritici) of wheat is a major threat to global foodsecurity and necessitates the continued development of new stem rust resistant varieties. Stemrust resistance gene Sr33derived from Aegilops tauschii and transferred into bread wheatconfers resistance against all stem rust pathogen derivatives of Ug99and diverse global wheatstem rust races. They identified the member Sr33responsible for stem rust resistance functionfrom the RGA Gene Cluster（Lagudah unpublished）.Characterization of Sr33-mediated stemrust resistance indicated that Sr33protein function is independent of RAR1, SGT1and HSP90chaperones by silencing genes encoding these chaperones in the hexaploid wheat line CSexpressing Sr33. Moreover, as the BSMV:Sr33treated plants displayed an increased susceptibility to stem rust infection, these data further validate the notion of this gene asproviding wheat with Sr33-dependent stem rust resistance. We also showed that silencing ofthe adjacent member carrying the exocyst70subunit did not compromise resistanceindicating the gene is not required for Sr33mediated resistance.