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2010 expression of a truncated form of yeast ribosomal protein l3
 

2010 expression of a truncated form of yeast ribosomal protein l3

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    2010 expression of a truncated form of yeast ribosomal protein l3 2010 expression of a truncated form of yeast ribosomal protein l3 Document Transcript

    • Plant Science 178 (2010) 374–380 Contents lists available at ScienceDirect Plant Science journal homepage: www.elsevier.com/locate/plantsciExpression of a truncated form of yeast ribosomal protein L3 in transgenicwheat improves resistance to Fusarium head blightRong Di a,*, Ann Blechl b, Ruth Dill-Macky c, Andrew Tortora d, Nilgun E. Tumer aa Biotechnology Center for Agriculture and the Environment, The School of Environmental and Biological Sciences, Rutgers University, New Brunswick, NJ 08901-8520, USAb Western Regional Research Center, United States Department of Agriculture, Agricultural Research Service, Albany, CA 94710-1105, USAc Department of Plant Pathology, University of Minnesota, St. Paul, MN 55108-6030, USAd Undergraduate Biology Program, Rutgers University, New Brunswick, NJ 08901-8520, USAA R T I C L E I N F O A B S T R A C TArticle history: Fusarium head blight (FHB) is a disease that causes major economic losses in wheat and barleyReceived 7 December 2009 production worldwide. Contamination of food with the trichothecene mycotoxin deoxynivalenol (DON)Received in revised form 2 February 2010 produced by Fusarium is a major health concern for humans and animals because trichothecenes areAccepted 3 February 2010 potent cytotoxins of eukaryotic cells. Trichothecene mycotoxins inhibit translation by targetingAvailable online 11 February 2010 ribosomal protein L3 at the peptidyltransferase center. We previously showed that expression of an N- terminal fragment of yeast L3 (L3D) in transgenic tobacco plants reduced the toxicity of DON. Here, weKeywords: produced transgenic wheat plants that express the same yeast L3 (L3D) fragment and evaluated theirFusarium head blightWheat and barley scab susceptibility to Fusarium graminearum infection and their ability to accumulate DON. Following F.Trichothecene mycotoxin graminearum infection in greenhouse tests, two transgenic wheat lines expressing the highest levels ofDeoxynivalenol L3D showed reductions in disease severity and kernel DON levels, compared to non-transformed plants.Transgenic wheat plants In a field test, a transgenic wheat line with the highest L3D expression controlled by the maize Ubi1Ribosomal protein L3 promoter had significant reductions in visually scabby kernels and kernel DON levels. These results demonstrate that expression of a modified form of the ribosomal protein that is the target of DON can improve FHB resistance in wheat. ß 2010 Elsevier Ireland Ltd. All rights reserved.1. Introduction Trichothecene mycotoxins interact with the peptidyltransfer- ase center of eukaryotic ribosomes and inhibit protein synthesis Fusarium head blight (FHB) caused by several Fusarium species, [7]. Trichothecenes have been reported to have diverse roles in theprimarily Fusarium graminearum Schwabe in the U.S., is an cell that are not limited to the inhibition of protein synthesis. Aeconomically important disease of wheat and barley worldwide recent genome-wide screen in Saccharomyces cerevisiae revealed a[1]. It was estimated that several severe FHB outbreaks in U.S. critical role for the mitochondria in the toxicity of a trichothecenewheat and barley from 1991 to 1997 resulted in $4.8 billion of total mycotoxin [8]. The tcm1 mutation that conferred resistance to thedirect losses [2]. The contamination of wheat, barley and maize trichothecene, trichodermin was identified in the yeast RPL3 gene,related products with the trichothecene mycotoxin deoxynivale- which encodes the ribosomal protein L3 [7,9,10]. L3 participates innol (DON) due to the infection with F. graminearum poses a health the formation of the peptidyltransferase center [11,12]. There havethreat to humans and animals [3,4]. Disruption of the F. been attempts to over-express the RPL3 gene carrying the tcm1graminearum gene Tri5, which encodes the trichodiene synthase mutation (W255C) in transgenic plants to achieve resistance tothat catalyzes the first step in the DON biosynthetic pathway, DON. A modified rice RPL3 cDNA containing the W255C mutationreduces FHB severity on greenhouse- and field-grown wheat [5,6]. was transformed into tobacco [13]. The transgenic tobacco calliThe Tri5 revertant strains of F. graminearum regain their aggres- and protoplasts displayed greater regeneration efficiency andsiveness on field-grown wheat [6]. These results have provided viability in the presence of DON. The expression of a tomato RPL3evidence that DON acts as a virulence factor in FHB. cDNA with the tcm1 mutation in transgenic tobacco plants improved the ability of these plants to adapt to DON, but did not result in constitutive resistance, possibly because the mutant protein did not accumulate in the transgenic plants [14]. * Corresponding author at: Biotechnology Center for Agriculture and the We cloned the two different L3 genes (RPL3A and RPL3B) fromEnvironment, The School of Environmental and Biological Sciences, RutgersUniversity, 59 Dudley Rd., Rm 127 Foran Hall, New Brunswick, NJ 08901-8520, tobacco and showed that their expression is coordinately regulatedUSA. Tel.: +1 732 932 8165x183; fax: +1 732 932 6535. [15]. Increasing L3 levels in transgenic tobacco resulted in leaf E-mail address: di@aesop.rutgers.edu (R. Di). overgrowth and mottling, and led to an increase in cell number and0168-9452/$ – see front matter ß 2010 Elsevier Ireland Ltd. All rights reserved.doi:10.1016/j.plantsci.2010.02.003
    • R. Di et al. / Plant Science 178 (2010) 374–380 375a decrease in cell size. The rRNA precursor and the mature rRNAs previously [19], with forward primer UbiA2 [19] or Lem752 [50 -accumulated in these plants, suggesting that ribosome biogenesis GACAGTGGGAGTGGGGTTTG-30 ] in combination with reversewas up-regulated. In contrast, L3 deficiency led to a reduction in primer L3-L3d [50 -CGACGTAACCGACAACACC-30 ] and an annealingcell number and an increase in cell size [15]. Since altering temperature of 62 8C.endogenous L3 expression led to an abnormal phenotype intobacco, we expressed the full length and a truncated form of the 2.2. Real-time PCR analysis to determine the gene expression levels inyeast L3 gene corresponding to the first 99 amino acids of L3 in transgenic wheat plantstobacco to determine if expression of the yeast L3 gene confersresistance to DON. Transgenic tobacco plants expressing yeast L3D Total RNA was isolated from the florets and immature seeds orwere phenotypically normal and, in a germination test, showed leaves of transgenic wheat plants using TRIzol1 Reagent (Invitro-resistance to DON and to pokeweed antiviral protein (PAP), which gen) according to the manufacturer’s protocol. The RNA samplesalso targets L3 [16]. PAP is a 29-kDa ribosome inactivating protein were treated with RQ1 RNase-Free DNase (Promega) following the(RIP) isolated from pokeweed plants that inhibits translation by manufacturer’s instructions and further purified with phenol/binding to L3 and catalytically removing a specific purine residue chloroform extraction and ethanol precipitation. SuperScript1from the highly conserved a-sarcin/ricin loop (SRL) of the large reverse transcriptase (Invitrogen) and oligo dT were used to producerRNA [17]. Ribosomes from tobacco plants expressing PAP and the first strand cDNA from 5 mg of total RNA. ABI PRISM 7000yeast L3D were not depurinated, even though PAP was associated Sequence Detection System (Applied Biosystems) was used towith ribosomes. These results demonstrated that yeast L3D perform real-time PCR analysis using gene-specific primersconferred resistance possibly by protecting ribosomes from the designed by ABI Primer Express software, following the manufac-translation inhibitory effects of DON and PAP [16]. turer’s protocols. The primers for yeast L3D were L3d67F, 50 - In this study, we aimed to test whether expression of the yeast GCCTCCATCAGAGCTAGAGTTAAGG-30 and L3d145R, 50 -AACCCAA-L3D gene could protect wheat ribosomes from DON and thus GAAGGAAGTTAGAGCAA-30 . The primers used to measure the levelsreduce FHB severity in wheat plants carrying L3D transgenes. of wheat TaRPL3A were WL3A829F, 50 -CACCGCACAGAGATGAA-Bread wheat (Triticum aestivum = Ta) is hexaploid and thus CAAA-30 and WL3A900R, 50 -AGTGCAGGCCTCGTGAGACTC-30 . Thecontains six different RPL3 genes: TaRPL3A1, TaRPL3A2, TaRPL3A3, primers for the wheat TaRPL3B were WL3B829F, 50 -CACCGAACT-TaRPL3B1, TaRPL3B2 and TaRPL3B3 [18]. We transformed hexaploid GAGATGAACAAG-30 and WL3B900R, 50 -GGTAGAGGCATCAT-wheat cultivar Bobwhite with yeast L3D under the control of two GAGTTTC-30 . The relative gene expression levels of TaRPL3 weredifferent promoters and assayed the resultant transformants for measured using wheat a-tubulin (GenBank Access #U76558) as anRPL3 and L3D expression levels and for FHB resistance and DON internal control with primers WTub942F, 50 -CAGCTGAGAAGGCT-accumulation in the greenhouse and field. TACCATGA-30 and WTub1000R, 50 -AAAGGCGCTGTTGGTGATCT-30 . The 2ÀDDC T method [23] was used to determine the relative gene2. Materials and methods expression levels.2.1. Production of transgenic wheat plants 2.3. Greenhouse screening of transgenic wheat lines for resistance to F. graminearum The coding sequence of the N-terminal 99 amino acids of yeastribosomal protein L3 was cloned in place of the bar coding region in Plants were grown in pots (15 cm  15 cm  16.5 cm; Beldenthe wheat expression vector pUBK [19]. The resulting construct, Plastics, Roseville, MN) containing commercial potting mediumcalled pLem-L3d, contained the L3D gene between the barley (Metromix 200, Scotts Co.) in a greenhouse maintained at 20 Æ 2 8Ctissue-specific Lem1 promoter [20] and the nopaline synthase with a 16-h light and 8-h dark cycle. Six to seven seeds were planted(NOS) 30 transcription terminator (Fig. 1). The L3D gene was also in each pot. Five pots per line were thinned to six plants at Zadokscloned into a similar wheat expression vector containing the maize growth stage (GS) 13–14. In addition to the transgenic lines, twoUbi1 promoter/first intron [21], resulting in construct pUbi-L3d check lines were included in the greenhouse screening. The FHB(Fig. 1). checks used were Wheaton, a hard red spring wheat, released Embryogenic calli of a hexaploid FHB-susceptible spring wheat cooperatively by the Minnesota Agricultural Experiment Station and(T. aestivum L. em. Thell. cv Bobwhite) were co-bombarded with a USDA-ARS in 1984, which is highly susceptible to FHB, and Alsen, a2:1 (pLem-L3d) or 4:1 (pUbi-L3d) molar ratio of the L3D constructs hard red spring wheat released from North Dakota State Universityand pUBK containing the bar selectable marker gene under the and moderately resistant to FHB. The FHB resistance in Alsen iscontrol of Ubi1. Regenerants were selected with bialaphos as derived from the Chinese wheat Sumai 3. Bobwhite was included asdescribed previously [19,22], except that the callus recovery media an untransformed control. The planting was replicated. Each pot wasafter bombardment contained 5 mM CuSO4 and the regeneration fertilized with 5.8 g of slow release 14–14–14 (N–P–K) fertilizermedia contained 5 mM CuSO4, 0.1 mg lÀ1 6-benzylamino purine, (Osmocote Classic, Scotts Co.) at GS 13–14. Insect pests wereand 3 mg lÀ1 bialaphos. T0 plants containing pLem-L3d or pUbi-L3d controlled with an application of imidacloprid (Marathon 60WP,were identified using PCR of wheat genomic DNA, as described Olympic Hort. Products). Powdery mildew was controlled with a single application of tridimefon (Bayleton 50% DF, Bayer CropScience). Plants were inoculated when the main spike from each plant reached anthesis (GS 60–65). A single spikelet at the central node of the main spike of each plant was inoculated with 10 ml of a macroconidial spore suspension (100,000 conidia mlÀ1) of F. graminearum using a repeating 500-ml Hamilton 700 syringeFig. 1. Constructs used to generate the transgenic wheat plants. A gene fragment (model 80830), fitted with a PB600 dispenser (model 83700)from yeast that encodes the N-terminal 99 amino acids (L3D) of the ribosomal (Hamilton Co.). Twelve isolates of F. graminearum collected fromprotein L3 was cloned into wheat expression vectors under the control of either thebarley Lem1 (pLem-L3d) or the maize Ubi1 promoter/first intron (pUbi-L3d). The commercial fields of wheat and barley in 2002 and 2003, whichnopaline synthase (NOS) 30 transcription termination sequence was used as the were naturally infected with F. graminearum, were used to producetranscription terminator in both constructs. the inoculum according to the previously described procedure
    • 376 R. Di et al. / Plant Science 178 (2010) 374–380[24]. Immediately after inoculation, the plants were placed in a a selection marker to identify the stably transformed T0 plants.dew chamber for 72 h. Following the dew period, the plants were Several bialaphos resistant T0 wheat plants were regenerated fromreturned to the greenhouse bench. Disease development was one bombardment experiment with each expression vector. Someassessed visually 21 days after the inoculation. FHB incidence is of these T0 plants also contained Lem1::L3D or Ubi1::L3D, asdefined as the percentage of spikes with visually symptomatic determined by PCR with oligonucleotide primers specific for thespikelets and FHB severity as the percentage of symptomatic promoters and L3D (data not shown). The integration of the L3Dspikelets from the total spikelets observed. The DON content of transgenes was confirmed by PCR analyses of genomic DNA fromgrain was determined on the kernels harvested from the four T1 plants, demonstrating that the transgene locus was inheritedspikelets adjacent to the inoculated spikelet of inoculated heads. (data not shown). All (12/12) the progeny of one T0 plantHeads were harvested at maturity and threshed by hand. containing Lem1::L3D inherited a Lem1::L3D transgene, suggest- ing that the original transformant contained at least two2.4. Field testing of transgenic wheat lines for resistance to F. independent transgenic loci. Three homozygous T2 sublinesgraminearum derived from this event were identified by PCR (data not shown) and named 771, 772 and 774. The progeny of two independent T0 The 2008 field screening nursery was located at UMore Park, plants containing Ubi1::L3D segregated 3:1 (8/10 and 11/15) forRosemount, MN. In addition to the transgenic wheat lines, the transgene, suggesting they each contained a single transgeneuntransformed Bobwhite, the moderately resistant wheat Alsen locus. Homozygous T2 plants from these two transformants wereand the FHB-susceptible wheat Wheaton were included as checks. identified by PCR and were named 8133 and 8153. Eight to tenThe experimental design was a randomized block with four homozygous T3 plants from each of the five lines were grown in thereplicates. Plots were 2.4 m long single rows. All plots, except a greenhouse for expression and resistance analyses. No develop-non-inoculated Wheaton check, were inoculated twice. The first mental or seed set differences were observed between theinoculation was applied at anthesis and the second 3 days after transgenic plants and their parent.anthesis. Inoculum was a composite of 41 F. graminearum isolatesat a concentration of 100,000 macroconidia mlÀ1. Polysorbate 20 3.2. Expression of yeast L3D in transgenic wheat plants(Tween 20) was added at 2.5 ml lÀ1 to the inoculum as a wettingagent. The inoculum was applied using a CO2-powered backpack For each of the five homozygous transgenic wheat lines,sprayer fitted with a SS8003 TeeJet spray nozzle with an output of expression of the L3D transgene was measured at anthesis, the10 ml sÀ1 at a working pressure of 275 kPa. FHB incidence and stage at which they would be most susceptible to Fusariumseverity were assessed visually 20–25 days after inoculation on 20 infection. The florets and immature seeds were sampled since mostarbitrarily selected spikes per plot. FHB incidence and severity of the natural infection occurs in these organs. Total RNA waswere defined as stated above. The harvested seeds from each plot extracted from these samples and used in real-time polymerasewere split using a Borrner divider (Seedburo) to obtain a 50 g sub- chain reaction (PCR) assays to measure the expression level of thesample, which was then cleaned by hand. These samples were used yeast L3D gene, using a set of primers specifically designed for theto estimate the percentage of visually scabby kernels (VSK%), 50 end of the yeast RPL3 gene, corresponding to the N-terminalaccording to method of Jones and Mirocha [25], and then analyzed amino acids +22 to +48 of L3. As shown in Fig. 2A, these amino acidsfor deoxynivalenol (DON). are significantly divergent among the yeast and TaRPL3 proteins. Experimental results showed that these primers did not hybridize2.5. Determination of DON level in kernels of the inoculated wheat to the endogenous wheat TaRPL3 genes (data not shown). Theplants relative L3D expression levels were determined using the wheat a- tubulin mRNA as an internal control. Real-time PCR analysis was Grain samples were analyzed for DON according to the performed on RNA from four to five different plants of eachpublished protocol [26]. Briefly, the samples were ground for homozygous line with two replicates per plant and results were2 min with a Stein Laboratories Mill (model M-2, Stein Laboratories averaged. As shown in Fig. 3A, the L3D expression was detected atInc.). DON was extracted from 4 g of the ground sample in 16 ml of high levels in the florets/immature seeds of transgenic wheatacetonitrile–water (84:16, vol/vol) placed on an Eberbach recipro- plants. Transgenic lines 772 and 8153 contained the highest levelscal shaker (model 6010, Eberbach) for 1 h. A 1-ml sample eluted of the yeast L3D transcripts.through a specially prepared cleanup column [27] was evaporatedto dryness with nitrogen. DON was derivatized and analyzed using 3.3. Relative steady state levels of the endogenous wheat TaRPL3gas chromatography–mass spectrometry (GC/MS) (Model QP- transcripts in transgenic wheat plants2010, Shimadzu Ltd.) in selected ion monitoring (CSIM) mode. DONconcentration was determined based on retention times and peak We have shown previously that the endogenous TaRPL3 genesareas by comparing to standards. were up-regulated in the transgenic tobacco plants expressing the yeast L3D [16]. In order to determine if expression of the yeast L3D3. Results affected expression of the endogenous TaRPL3 genes in wheat, we measured the TaRPL3 expression levels relative to those of a-3.1. Generation of transgenic wheat plants expressing yeast ribosomal tubulin in the homozygous lines of transgenic wheat plants. Theprotein L3D TaRPL3 gene family consists of three alleles of both TaRPL3A and TaRPL3B [18]. The following six different wheat TaRPL3 gene To determine if expression of L3D in transgenic wheat would sequences from the GenBank were used in this study: TaRPL3A1confer resistance to DON, embryogenic calli of susceptible spring (accession #AY343327), TaRPL3A2 (accession #AY343328), TaR-wheat (T. aestivum L. em. Thell. cv Bobwhite) were transformed via PL3A3 (accession #BK001237), TaRPL3B1 (accession #BK001235),particle gun bombardment with genes encoding L3D under the TaRPL3B2 (accession #BK001234) and TaRPL3B3 (accessioncontrol of either the barley Lem1 [20] (pLem-L3d) or the maize Ubi1 #AY347532). Comparison of the amino acid sequences among[21] promoter (pUbi-L3d) (Fig. 1). The bar gene, conferring the three wheat TaRPL3A genes and three wheat TaRPL3B genesresistance to the herbicides bialaphos and BASTA, under control showed that TaRPL3A and TaRPL3B protein sequences are veryof Ubi1, was co-bombarded with the L3D constructs into the calli as similar within the A and B groups. However, the sequences of the
    • R. Di et al. / Plant Science 178 (2010) 374–380 377Fig. 2. Alignment of the amino acids of yeast L3D with the wheat (Triticum aestivum) L3 proteins. Amino acids that differ among the aligned sequences are boxed. (A)Comparison of the N-terminal 99 amino acids of the yeast with the wheat L3 encoded by TaRPL3A1, TaRPL3A2, TaRPL3A3, TaRPL3B1, TaRPL3B2 and TaRPL3B3. The region used todesign primers specific for the yeast L3D transgene transcripts is over-lined. (B) Diversity of amino acids 277–300 among the wheat L3 proteins.L3 proteins encoded by the TaRPL3B1/B2/B3 genes are approxi- T3 generation homozygous lines and non-transformed Bobwhitemately 5% divergent from the sequences of the proteins encoded by plants were evaluated for resistance in a greenhouse. Wheaton,the TaRPL3A1/A2/A3 genes. Most of the divergence occurs between a hard red spring wheat, was used as the susceptible check andamino acids 277 and 300 as shown in Fig. 2B. Therefore, two sets of Alsen, another hard red spring wheat, was used as the resistantprimers corresponding to this most divergent region were check. Twenty-one days after the single spikelet at the centraldesigned to distinguish between the expression levels of TaR- node of the main spike was inoculated with a macroconidialPL3A1/A2/A3 and TaRPL3B1/B2/B3 by real-time PCR analysis. As spore suspension, whole spikes of wt plants had turned brownshown in Fig. 3B, the relative expression levels of TaRPL3A1/A2/A3 (Fig. 4A). In the transgenic plants, except in line 774, browningwere higher in 772 and 8153 transgenic wheat lines. Fig. 3C also was mainly confined to the inoculated spikelets (Fig. 4A). Table 1shows that the relative TaRPL3B1/B2/B3 expression levels were lists the averages and ranges of incidence of F. graminearumhigher in 772, 8133 and 8153 transgenic wheat lines. In line 772 infection in all the wheat lines including Alsen and Wheatonthe TaRPL3A1/A2/A3 expression levels were about 2.5-fold and the from two separate experiments. All transgenic wheat linesTaRPL3B1/B2/B3 expression levels were 3.5-fold higher than the except 774 exhibited a significantly lower incidence of infectionwild type (wt) wheat plants, while there was an approximately 4- spread than the untransformed wt Bobwhite as assessed by thefold increase in the TaRPL3A1/A2/A3 expression levels and 2.5-fold Student t-test (excluding Alsen and Wheaton). The average FHBincrease in the TaRPL3B1/B2/B3 expression levels in 8153 incidences for the five transgenic lines in these greenhouse testscompared to the wt plants. The increases observed in the were inversely correlated with their levels of the L3D mRNAendogenous TaRPL3 expression level in the transgenic wheat (Fig. 3A). The FHB severity parameter measures the percentageplants were of similar magnitude to those previously documented of spikelets with symptoms. Fig. 4B indicates that on average ain transgenic tobacco plants [16]. 50% reduction in disease severity was observed in line 772, a 49% reduction in line 8153, and a 48% reduction in line 8133,3.4. Greenhouse test of transgenic wheat plants for FHB resistance as compared to the wt Bobwhite. These results demonstrated that transgenic wheat plants expressing the yeast L3D To determine if the transgenic wheat plants expressing showed improved resistance to FHB over the untransformedthe yeast L3D were resistant to infection by F. graminearum, Bobwhite plants.Table 1Averages and ranges of incidence of Fusarium graminearium infection from two separate experiments. Line Number of plants tested Percentage of infected spikesa Range of percentage b 771 24 22.3 Æ 18.3 11.7–54 772 25 18.7 Æ 5.9b 10.1–25.9 774 24 43.3 Æ 19.8 20.3–60.5 8133 24 31.4 Æ 16.9b 17.4–51.5 8153 25 20.5 Æ 16.9b 10.8–35.1 wt Bobwhite 23 42.0 Æ 17.2 22.2–55 Alsen (resistant check) 25 10.8 Æ 5.7b 6.5–20.8 Wheaton (susceptible check) 16 65.7 Æ 29.4 27.7–95.7 a Average Æ standard deviation. b The percentage of infected spikelets in these transgenic plants was significantly different from the wt Bobwhite as assessed by the Student t-test (p < 0.05).
    • 378 R. Di et al. / Plant Science 178 (2010) 374–380Fig. 3. Real-time PCR analysis of the L3D and TaRPL3 mRNA levels in the transgenicwheat lines. Yeast L3D mRNA (A), endogenous TaRPL3A (B) and TaRPL3B (C) mRNAlevels were quantified by real-time PCR analysis using gene-specific primers. Total Fig. 4. Susceptibility of transgenic wheat plants to FHB in the greenhouse andRNA was isolated from the florets and immature seeds of the wheat plants at the accumulation of DON in the kernels. (A) The central spikelets of the main spike ofstage just prior to when they would be inoculated with Fusarium graminearum for transgenic and wt wheat plants, 20–25 plants per line, were inoculated with 10 mlthe greenhouse resistance tests. The relative gene expression levels were of a macroconidial spore suspension (100,000 conidia mlÀ1) of F. graminearumdetermined using the 2ÀDDC T method and wheat a-tubulin mRNA as an internal where indicated by the arrows. Disease spread was recorded 21 days after thecontrol. The relative gene expression levels were measured from four to five plants inoculation. Representative plants are shown. (B) FHB severity was assessed byper line, averaged and presented as bar graphs with standard errors. counting the number of symptomatic spikelets for each spike and expressing the infection level as the percentage of the total number of spikelets for each spike. FHB severity was averaged from two separate experiments for each line and presented with standard errors. (C) The mycotoxin DON was extracted from kernels harvested3.5. Reduction of DON toxin levels in some transgenic wheat plants in from the four spikelets adjacent to the inoculated spikelet of inoculated heads. DONgreenhouse test was analyzed using GC/MS. DON levels from each line were averaged from two separate experiments and shown in the bar graph with standard errors. To determine if the FHB resistance of L3D transgenic wheatplants would result in reduction of the fungal toxin DON, themature kernels above and below 20–25 inoculated spikelets were with untransformed Bobwhite and resistant and susceptible checkextracted, and their DON levels were measured by GC/MS. Since cultivars (Section 2). The disease pressure in this trial was verythe susceptible Wheaton was heavily infested (Table 1 and Fig. 4B), high, as indicated by FHB incidence of 91% for moderately resistantwe were not able to harvest sufficient seeds to be able to determine check Alsen. The FHB incidence was 100% for all the transgenicthe DON levels in this line. However, compared to the untrans- wheat lines, the non-transformed wt Bobwhite plants, and theformed wt Bobwhite, there was a 58% reduction in DON levels in susceptible check Wheaton. However, plants from line 8153 had athe transgenic line 772, a 46% reduction in DON levels in line 8153, significantly lower percentage of visually scabby kernels (VSK%)and a 36% reduction in line 771 (Fig. 4C). These were also the lines and DON levels, compared to the untransformed wt Bobwhitewith the highest L3D mRNA levels. Surprisingly, lines 774 and plants (Table 2).8133 had higher DON levels than their non-transformed parent inthe greenhouse tests. 4. Discussion3.6. Field testing of transgenic wheat plants for FHB resistance Outbreaks of FHB over the last 15 years have caused significant yield losses in wheat and barley in the United States [2] and the In order to assess resistance to FHB in the field, the transgenic resultant DON contamination in small grain crops is a major healthwheat lines were planted in the field during spring of 2008 along concern for human food and animal feed. Control of FHB disease is
    • R. Di et al. / Plant Science 178 (2010) 374–380 379Table 2Field testing of transgenic wheat lines in 2008. The mean percentage of visually scabby kernels (VSK%) was assessed with 4 replicates after the seeds were harvested. The DONlevels in the kernels were analyzed as described in Section 2. Line VSK% Range of VSK% DONa (ppm) Range of DON level 771 26.3 Æ 7.8 17.5–35.0 39.2 Æ 8.5 29.3–47.5 772 24.4 Æ 5.5 17.5–30.0 36.5 Æ 2.5 33.9–39.2 774 25.6 Æ 9.4 17.5–35.0 35.8 Æ 9.8 28.1–49.2 8133 26.3 Æ 13.6 12.5–45.0 34.5 Æ 5.9 29.7–42.8 8153 16.9 Æ 3.2b 12.5–20.0 28.0 Æ 4.9b 23.5–34.9 wt Bobwhite 28.8 Æ 1.4 27.5–30.0 35.3 Æ 8.2 27.4–46.6 Alsen (resistant check) 5.8 Æ 2.4b 4.0–9.0 13.0 Æ 6.4b 5.7–21.1 Wheaton (susceptible check) 58.1 Æ 19.5 40.0–75.0 53.3 Æ 14.5 42.6–74.7 a Average Æ standard deviation. b The values were significantly different from those of wt Bobwhite plants as assessed by the Student t-tests (p < 0.05).a proactive way to prevent DON from getting into the food supply. transgene showed reduction of multiple disease indices in the field,Although wheat cultivars with partial resistance are available, including DON accumulation, percentage visually scabby kernels,development of completely resistant cultivars by conventional and disease severity, compared with the non-transformed parentbreeding methods has proven to be difficult. The fungicides that [31]. Improved Type II resistance in greenhouse tests was alsohave been approved to control FHB are also only partially effective. reported by Makandar et al. [32], who expressed the ArabidopsisThus, additional sources of host plant resistance, from breeding or thaliana NPR1 (AtNPR1) gene in Bobwhite wheat. Since AtNPR1genetic engineering, are needed to protect small grains from FHB in regulates the activation of systemic acquired resistance, thethe field. heightened resistance in their wheat plants was attributed to In the present study, we report the production of transgenic the transgenic plants being more responsive to an endogenousBobwhite wheat plants expressing the N-terminal 99 amino acids activator of plant defense. Recently, an antibody fusion proteinof the yeast L3 protein under the control of either the barley Lem1 consisting of a Fusarium specific recombinant antibody derivedor the maize Ubi1 promoter. Endogenous wheat TaRPL3 genes were from chicken and an antifungal peptide from Aspergillus giganteusup-regulated in most of the transgenic wheat lines. Most conferred Type I and Type II resistance to FHB [33].noticeably, 772 expressing pLem-L3d and 8153 expressing pUbi- Here we use a novel approach for FHB resistance by expressing aL3d had higher gene expression levels of L3D compared to the modified form of the trichothecene target in transgenic plants. Ourother transgenic wheat lines. These two lines demonstrated the results demonstrate that expression of yeast L3D in transgenichighest resistance to the spread of FHB (Type II resistance) and the wheat plants can result in improved FHB resistance and alowest DON levels when adult plants were challenged with F. reduction in DON accumulation in wheat kernels. We postulategraminearum in greenhouse tests. These transgenic lines also that introduction of a modified form of L3 into transgenic plantsshowed higher levels of accumulation of the endogenous TaRPL3As. may prevent DON from targeting ribosomal protein L3 at theEven though the FHB incidence in the field test reported here was peptidyltransferase center. Consequently, ribosome-bound L3 mayvery high, plants from line 8153 demonstrated the lowest VSK be protected from the inhibitory effects of DON. The tryptophanpercentage and lowest level of DON, indicating that high levels of residue at position 255 in the yeast L3 that is mutated in tcm1L3D and/or endogenous RPL3 can protect plants from FHB and makes the closest approach of any amino acid to the peptidyl-reduce DON accumulation in wheat seeds. transferase center of the large subunit [34]. Previous studies The difference in the behavior of 8153 and 772 in the field may be showed that substitution of W255 with cysteine (W255C)due to the different activities of their promoters. The Ubi1 promoter conferred resistance to trichodermin, anisomycin, and affectedis expressed throughout the plant, so any protective protein encoded translational fidelity by decreasing the peptidyltransferase activityby Ubi1::L3D would be present during all phases of plant [34]. Since expression of L3D in yeast has similar effects ondevelopment [28]. Thus 8153 could be protected both from the resistance to anisomycin and on translational fidelity as the tcm1spread of FHB (Type II resistance), as measured by the greenhouse mutation [35], an alternative model is that L3D may be making thetests, and the recurring infection by FHB that occurs in the field (Type ribosomes less sensitive to DON by decreasing their peptidyl-I resistance). The Lem1 promoter has much more restricted transferase activity. Further studies are needed to investigate theexpression [20] and it may be that the L3D does not accumulate observed resistance.in 772 in all the tissues in which it is needed for Type I protection. Lucyshyn et al. reported that the amino acids encoded by the Our resistance test results are comparable to those reported by TaRPL3 genes were the same in wheat cultivars differing inothers using the transgenic approach to engineer FHB resistance by Fusarium resistance [18], ruling out qualitative mutations inexpressing genes with different modes of action. The Fusarium TaRPL3 alleles that could account for resistance in varietiesTRI101 gene, which encodes a trichothecene 3-O-acetyltransferase, Frontana and CM-82036. In the latter cultivar, the locus forwas expressed in transgenic wheat plants, where it conferred TaRPL3A3 was mapped in a low recombination region that includedpartial protection against the spread of F. graminearum in an FHB resistance QTL [27]. In light of our observations that TaRPL3greenhouse tests [19]. Several groups have expressed individual levels were increased in some of our L3D transgenic plants, itplant defense genes under control of constitutive promoters in would be interesting to measure the levels of TaRPL3A3 transcriptstransgenic wheat [29–31]. Greenhouse tests of plants carrying in CM-82036 to determine whether quantitative differences insuch constructs showed that Type II resistance was improved 20– TaRPL3 levels could account for this variety’s resistance.50% compared with their non-transformed parents. However, Regardless of the mechanism, protection of wheat ribosomeswhen several of these transgenic plants were challenged with from the inhibitory effects of DON would rob Fusarium fungi of oneFusarium infection in field experiments [19,29,31], only those over- of their most potent virulence factors, thus decreasing the severityexpressing a wheat alpha-thionin, a barley beta-1,3-glucanase, or a of FHB. Further field testing is needed to confirm the potential ofbarley thaumatin-like-protein showed any improvement in this strategy for increasing both Type I and Type II resistance andresistance [31]. Among these, only a line carrying the glucanase decreasing mycotoxin accumulation in cereals.
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