Rice Bacterial Blight Resistance Gene Research Articles

Efficient protein tagging and cis-regulatory element engineering via precise and directional oligonucleotide-based targeted insertion in plants.

Efficient and precise targeted insertion holds great promise but remains challenging in plant genome editing. An efficient non-homologous end-joining-mediated targeted insertion method was recently developed by combining CRISPR/SpCas9 gene editing with phosphorothioate modified double-stranded oligodeoxynucleotides (dsODNs). Yet this approach often leads to imprecise insertions with no control over the insertion direction. Here, we compared the influence of chemical protection of dsODNs on efficiency of targeted insertion. We observed that CRISPR/SpCas9 frequently induced staggered cleavages with 1-nucleotide 5´overhangs; we also evaluated the effect of donor end structures on the direction and precision of targeted insertions. We demonstrate that chemically protected dsODNs with 1-nucleotide 5´ overhangs significantly improved the precision and direction control of target insertions in all tested CRISPR targeted sites. We applied this method to endogenous gene tagging in green foxtail (Setaria viridis), and engineering of cis-regulatory elements for disease resistance in rice (Oryza sativa). We directionally inserted two distinct transcription activator-like effector binding elements into the promoter region of a recessive rice bacterial blight resistance gene with up to 24.4% efficiency. The resulting rice lines harboring heritable insertions exhibited strong resistance to infection by the pathogen Xanthomonas oryzae pv. oryzae in an inducible and strain-specific manner.

Molecular Marker Assisted Breeding of a New <i>Japonica</i> Hybrid Rice

The development of Japonica  hybrid rice is of great significance to ensure supply of good quality Japonica  rice and food security in China. Aiming at the problems of late growth period, unstable yield, and the need to improve rice quality and seed production yield of Japonica  hybrid rice, a new Japonica  hybrid rice combination 'Shenyou28' with early maturity, good quality, blast resistance, stable and high yield and easy seed production was developed by using molecular breeding techniques including high-density rice gene chip. 'Shenyou28', bred by Shanghai Academy of Agricultural Sciences, is a three-line Japonica  hybrid rice with 'Shen21A' as sterile line and 'Shenhui26-28' as restorer line. For two-year regional trials from 2018 to 2019, the average output were 10 588.5 kg/hm 2 , which was 11.6% higher than the control variety 'Huayou14'. For the one-year production trial, the output reached 10 473.0 kg/hm 2 , which was 4.8% higher than the control variety 'Huayou14'. 'Shenyou28' has good quality, reaching the second grade of national standard, and has fragrance because it carries fragrance gene badh2-p-5 ’ UTR . It also has good resistance to rice blast and bacterial blight because it aggregates the rice blast resistance genes Pi2 , Pita , Pib , Pi9 , Pi54 , Pikm , Pit , and bacterial blight resistance gene Xa21 . It was approved by the Crop Variety Certification Committee of Shanghai in 2020. The breeding process, variety characteristics and key points of high yield mechanized seed production technology of 'Shenyou28' are introduced in this study.

Variation and Functional Markers of Xa23 Promoter of Rice Bacterial Blight Resistance Gene

Xa23  is a dominant gene with broad spectrum and strong resistance to bacterial blight of rice. The functional difference between  Xa23  and the allele susceptible gene  xa23  lies in the 28 bp core sequence on the promoter that is recognized and activated by the avirulent effector avrXa23 of  Xanthomonas oryzae pv. Oryzae  (EBEavrXa23) and development of  Xa23  functional markers can speed up the process of rice breeding. In this study, the  Xa23  promoters of 43 wild rice and 7 representative conventional rice were tested, and the 200 bp sequence upstream of the promoter was analyzed using Vector NTI .  The results showed that only 9 common wild rice and 2 conventional rice could amplify about 200 bp sequence upstream of promoter, and the detection rate was only 22.0%, indicating that  Xa23  was not widespread in rice. Further sequence analysis showed that there were abundant variations in the sequence of EBE avrXa23 , with at least seven haplotypes in its allele sequence, but EBE avrXa23  only exists in rice CBB23. Finally, the  Xa23  dominant functional marker was developed based on the sequence of EBE avrXa23 . It was verified that the marker could clearly distinguish whether there was  Xa23  gene in rice. This study will be helpful to further study the evolution mechanism of  Xa23  and molecular mark assisted breeding.&nbsp

Linkage analysis of rice bacterial blight resistance gene xa20 in XM6, a mutant line from IR24.

Bacterial blight caused by Xanthomonas oryzae pv. oryzae (Xoo) is an important disease constraining rice (Oryza sativa L.) production worldwide. The XM6 line was induced by N-methyl-N-nitrosourea from IR24, an Indica cultivar that is susceptible to Philippine and Japanese Xoo races. XM6 was confirmed to carry a recessive gene named xa20, resistant to six Philippine and five Japanese Xoo races. The chromosomal gene location was found using 10 plants with the shortest lesion length in an F2 population consisting of 298 plants from a susceptible Japonica variety Koshihikari × XM6. Analysis using PCR-based DNA markers covering the whole rice genome indicated the gene as located on the distal region of the long arm of chromosome 3. The IKC3 line carries IR24 genetic background with Koshihikari fragment on chromosome 3 where a resistance gene was thought to be located. The F2 population from IKC3 × XM6 clearly showed a bimodal distribution separating resistant and susceptible plants. Further linkage analysis conducted using this F2 population revealed that xa20 is located within the 0.8 cM region flanked by DNA markers KIC3-33.88 (33.0 Mb) and KIC3-34.06 (33.2 Mb). This study yields important findings for resistance breeding and for the genetic mechanism of Xoo resistance.

Introgression of rice blast and bacterial leaf blight resistance genes into HPR2143 rice variety through marker assisted selection and Anther culture

The use of resistant rice cultivars is the most proficient way to overcome rice blast and bacterial blight diseases in rice. Cultivars, however, experience a rapid breakdown in their resistance, predominantly due to the advent of new pathotypes. Therefore, for durable resistance, it is important to combine multiple genes that impart resistance to more than one pathotypes into one genetic background. Foreground selection of BC2F2 plants of two crosses (HPR2143/DHMAS164 and HPR2143/PB1) was done to detect the presence of blast resistance genes i.e. Pita and Pi9 in these plants. Plants homozygous for gene Pita were screened by a pair of gene specific markers YL87/155 (coupling dominant) and YL87/183 (repulsion dominant). Likewise, homozygous positive plants for gene Pi9 were identified using the co-dominant marker Pi9STS-1. For bacterial blight (BLB) resistance genes, BC2F2 plants of two crosses (HPR2143/IRBB54 and HPR2143/PR114) were screened for genes Xa21 and Xa38 using gene-derived markers BLB248 and Oso4g53050-1, respectively. Anther culture of developed F1s was attempted for fixation of genes.

TaXa21, a Leucine-Rich Repeat Receptor-Like Kinase Gene Associated with TaWRKY76 and TaWRKY62, Plays Positive Roles in Wheat High-Temperature Seedling Plant Resistance to Puccinia striiformis f. sp. tritici.

Puccinia striiformis f. sp. tritici causes wheat stripe rust, one of most important diseases of wheat worldwide. High-temperature seedling plant (HTSP) resistance of wheat to P. striiformis f. sp. tritici is one specific type of host resistance, induced by high temperature (HT). Receptor-like kinases (RLKs) play key roles in regulating plant development and signaling networks, but there have been no reports on possible roles played by RLKs in wheat HTSP to P. striiformis f. sp. tritici. In the present study, a leucine rich repeat (LRR)-RLK gene, TaXa21, with a high homology with rice bacterial blight resistance gene Xa21, was cloned from wheat cultivar Xiaoyan 6 (XY 6). TaXa21 expression was up-regulated by the exposure to HT (20°C) for 24 h at 8 days postinoculation with P. striiformis f. sp. tritici and was induced by ethylene (ET) and hydrogen peroxide (H2O2). Knocking down TaXa21 using virus-induced gene silencing reduced HTSP resistance to P. striiformis f. sp. tritici compared with the control plants. In addition, the expression level of TaCAT in the H2O2 pathway was induced and TaACO in the ET signal pathway was reduced in the HT-treated TaXa21-silenced plants. Transient expression of TaXa21 in tobacco leaves confirmed its subcellular localization in plasma membrane, consistent with the prediction from bioinformatics analysis. The transmembrane and kinase domain of TaXa21 can interact with TaWRKY76 in the nucleus and cell membrane, which is different from the localization of Xa21 in rice. The interaction between TaWRKY76 and TaWRKY62 (positively involved in the HTSP resistance of XY 6) were observed. Together, these results indicated that TaXa21 is a RLK associated with TaWRKY76 and TaWRKY62 and functions as a positive regulator of wheat HTSP resistance to P. striiformis f. sp. tritici. Furthermore, the host defense is mediated by the H2O2 and ET signal pathways.

Breeding Hybrid Rice with Genes Resistant to Diseases and Insects Using Marker-Assisted Selection and Evaluation of Biological Assay

Developing elite hybrid rice varieties is one important objective of rice breeding programs. Several genes related to male sterilities, restores, and pollinators have been identified through map-based gene cloning within natural variations of rice. These identified genes are good targets for introducing genetic traits in molecular breeding. This study was conducted to breed elite hybrid lines with major genes related to hybrid traits and disease/insect resistance in 240 genetic resources and F1 hybrid combinations of rice. Molecular markers were reset for three major hybrid genes (S5, Rf3, Rf4) and thirteen disease/insect resistant genes (rice bacterial blight resistance genes Xa3, Xa4, xa5, Xa7, xa13, Xa21; blast resistance genes Pita, Pib, Pi5, Pii; brown planthopper resistant genes Bph18(t) and tungro virus resistance gene tsv1). Genotypes were then analyzed using molecular marker-assisted selection (MAS). Biological assay was then performed at the Red River Delta region in Vietnam using eleven F1 hybrid combinations and two control vatieties. Results showed that nine F1 hybrid combinations were highly resistant to rice bacterial blight and blast. Finally, eight F1 hybrid rice varieties with resistance to disease/insect were selected from eleven F1 hybrid combinations. Their characteristics such as agricultural traits and yields were then investigated. These F1 hybrid rice varieties developed with major genes related to hybrid traits and disease/insect resistant genes could be useful for hybrid breeding programs to achieve high yield with biotic and abiotic resistance.

A Strain of an Emerging Indian Xanthomonas oryzae pv. oryzae Pathotype Defeats the Rice Bacterial Blight Resistance Gene xa13 Without Inducing a Clade III SWEET Gene and Is Nearly Identical to a Recent Thai Isolate.

The rice bacterial blight pathogen Xanthomonas oryzae pv. oryzae (Xoo) injects transcription activator-like effectors (TALEs) that bind and activate host “susceptibility” (S) genes important for disease. Clade III SWEET genes are major S genes for bacterial blight. The resistance genes xa5, which reduces TALE activity generally, and xa13, a SWEET11 allele not recognized by the cognate TALE, have been effectively deployed. However, strains that defeat both resistance genes individually were recently reported in India and Thailand. To gain insight into the mechanism(s), we completely sequenced the genome of one such strain from each country and examined the encoded TALEs. Strikingly, the two strains are clones, sharing nearly identical TALE repertoires, including a TALE known to activate SWEET11 strongly enough to be effective even when diminished by xa5. We next investigated SWEET gene induction by the Indian strain. The Indian strain induced no clade III SWEET in plants harboring xa13, indicating a pathogen adaptation that relieves dependence on these genes for susceptibility. The findings open a door to mechanistic understanding of the role SWEET genes play in susceptibility and illustrate the importance of complete genome sequence-based monitoring of Xoo populations in developing varieties with effective disease resistance.

High-resolution mapping and characterization of xa42, a resistance gene against multiple Xanthomonas oryzae pv. oryzae races in rice (Oryza sativa L.).

Improvement of resistance against rice bacterial blight (BB) disease is an important breeding strategy in breeding programs across the world, especially in Africa and southern Asia where BB is more prevalent. This report describes a high-resolution map and characterization of xa42 at XA42 locus, a rice BB resistance gene in XM14, a mutant line originating from IR24. The candidate gene region was narrowed down from 582 kb, which had been obtained in our previous study, to 57 kb. XM14 shows brown spots in its leaves like lesion mimic mutants. This line also shows a shorter stature than the original cultivar IR24. In XA42 gene segregating populations, homozygotes of xa42 allele were consistently resistant to the six Japanese Xanthomonas oryzae pv. oryzae races used for this study. They also showed brown spots and markedly short stature compared with the other genotypes, suggesting that xa42 gene exhibits pleiotropic effects.

Marker-assisted breeding of the rice restorer line Wanhui 6725 for disease resistance, submergence tolerance and aromatic fragrance.

BackgroundRice is a staple food crop in the world. With the increase in world population and economic development, farmers need to produce more rice in limited field. However, the rice production is frequently affected by biotic and abiotic stresses. The use of natural disease resistance and stress tolerance through genetic breeding is the most efficient and economical way to combat or acclimate to these stresses. In addition, rice with aromatic fragrance can significantly increase market value for its good grain quality. Mianhui 725 (MH725) is an elite restorer line that has been widely used to produce three-line hybrid rice in China. We previously introduced rice bacterial blight resistance genes Xa4 and Xa21 into MH725 and obtained an introgression rice line Wanhui 421 (WH421), which theoretically possesses 96.9% genetic background of MH725.ResultsHere we report the introduction and pyramiding of disease resistance genes Xa27 and Pi9, submergence tolerance gene Sub1A and aromatic fragrance gene badh2.1 in WH421 through backcrossing and marker-assisted selection. The newly developed introgression rice line was designated as Wanhui 6725 (WH6725), which theoretically possesses 95.0% genetic background of MH725. WH6725 and its hybrid rice conferred disease resistance to both blast and bacterial blight diseases and showed tolerance to submergence for over 14 days without significant loss of viability. Compared with non-aromatic rice MH725, WH6725 has strong aromatic fragrance. The major important agronomic traits and grain quality of WH6725 and its hybrid rice obtained in field trials were similar to those of MH725 and the control hybrid rice, indicating that WH6725 is as good as MH725 when it is used as a restorer line for three-line hybrid rice production.ConclusionWe have successfully developed a new restorer line WH6725 with disease resistance to rice blast and bacterial blight, tolerance to submergence and aromatic fragrance, which can be used to replace MH725 for hybrid rice production.

Pyramiding of two rice bacterial blight resistance genes, Xa3 and Xa4, and a closely linked cold-tolerance QTL on chromosome 11.

We fine mapped the Xa4 locus and developed a pyramided rice line containing Xa3 and Xa4 R - alleles and a cold-tolerance QTL. This line will be valuable in rice breeding. Bacterial blight (BB) caused by Xanthomonas oryzae pv. oryzae (Xoo) is a destructive disease of cultivated rice. Pyramiding BB resistance genes is an essential approach for increasing the resistance level of rice varieties. We selected an advanced backcross recombinant inbred line 132 (ABL132) from the BC3F7 population derived from a cross between cultivars Junam and IR72 by K3a inoculation and constructed the mapping population (BC4F6) to locate the Xa4 locus. The Xa4 locus was found to be delimited within a 60-kb interval between InDel markers InDel1 and InDel2 and tightly linked with the Xa3 gene on chromosome 11. After cold (4°C) treatment, ABL132 with introgressions of IR72 in chromosome 11 showed lower survival rate, chlorophyll content, and relative water content compared to Junam. Genetic analysis showed that the cold stress-related quantitative trait locus (QTL) qCT11 was located in a 1.3-Mb interval close to the Xa4 locus. One line, ABL132-36, containing the Xa3 resistance allele from Junam, the Xa4 resistance allele from IR72, and the cold-tolerance QTL from Junam (qCT11), was developed from a BC4F6 population of 250 plants. This is the first report on the pyramiding of Xa3 and Xa4 genes with a cold-tolerance QTL. This region could provide a potential tool for improving resistance against BB and low-temperature stress in rice-breeding programs.

Identification and linkage analysis of a new rice bacterial blight resistance gene from XM14, a mutant line from IR24.

Bacterial blight caused by Xanthomonas oryzae pv. oryzae (Xoo) is a chief factor limiting rice productivity worldwide. XM14, a rice mutant line resistant to Xoo, has been obtained by treating IR24, which is susceptible to six Philippine Xoo races and six Japanese Xoo races, with N-methyl-N-nitrosourea. XM14 showed resistance to six Japanese Xoo races. The F2 population from XM14 × IR24 clearly showed 1 resistant : 3 susceptible segregation, suggesting control of resistance by a recessive gene. The approximate chromosomal location of the resistance gene was determined using 10 plants with shortest lesion length in the F2 population from XM14 × Koshihikari, which is susceptible to Japanese Xoo races. DNA marker-assisted analysis revealed that the gene was located on chromosome 3. IAS16 line carries IR24 genetic background with a Japonica cultivar Asominori segment of chromosome 3, on which the resistance gene locus was thought to be located. The F2 population from IAS16 × XM14 showed a discrete distribution. Linkage analysis indicated that the gene is located around the centromeric region. The resistance gene in XM14 was a new gene, named XA42. This gene is expected to be useful for resistance breeding programs and for genetic analysis of Xoo resistance.

Nucleotide diversity analysis of three major bacterial blight resistance genes in rice.

Nucleotide sequence polymorphisms among R gene alleles influence the process of co-evolutionary interaction between host and pathogen by shaping the response of host plants towards invading pathogens. Here, we present the DNA sequence polymorphisms and diversities present among natural alleles of three rice bacterial blight resistance genes, Xa21, Xa26 and xa5. The diversity was examined across different wild relatives and cultivars of Oryza species. Functional significance of selected alleles was evaluated through semi-quantitative reverse transcription polymerase chain reaction and real time PCR. The greatest nucleotide diversity and singleton variable sites (SVS) were present in Xa26 (π = 0.01958; SVS = 182) followed by xa5 and Xa21 alleles. The highest frequency of single nucleotide polymorphisms were observed in Xa21 alleles and least in xa5. Transition bias was observed in all the genes and ‘G’ to ‘A’ transitions were more favored than other form of transitions. Neutrality tests failed to show the presence of selection at these loci, though negative Tajima’s D values indicate the presence of a rare form of polymorphisms. At the interspecies level, O. nivara exhibited more diversity than O. sativa. We have also identified two nearly identical resistant alleles of xa5 and two sequentially identical alleles of Xa21. The alleles of xa5 showed basal levels of expression while Xa21 alleles were functionally not expressed.

Identification of molecular markers linked to rice bacterial blight resistance genes from Oryza meyeriana

Y73 is a progeny of asymmetric somatic hybridization between Oryza sativa cv. Dalixiang and the wild rice species Oryza meyeriana. Inoculation with a range of strains of Xanthomonas oryzae pv. oryzae showed that Y73 had inherited a high level of resistance to rice bacterial blight (BB) from its wild parent. An F2 population of 7125 individuals was constructed from the cross between Y73 and a BB-susceptible cultivar IR24. After testing 615 SSR and STS markers covering the 12 rice chromosomes, 186 markers were selected that showed polymorphism between Y73 and IR24. Molecular markers linked to the BB resistance genes in Y73 were scanned using the F2 population and the polymorphic markers. The SSR marker RM128 on chromosome 1, the STS marker R03D159 on chromosome 3 and the STS marker R05D104 on chromosome 5 were found to be linked to the rice BB resistance genes in Y73.

Genetic diversity analysis in a set of Caricaceae accessions using resistance gene analogues.

BackgroundIn order to assess genetic diversity of a set of 41 Caricaceae accessions, this study used 34 primer pairs designed from the conserved domains of bacterial leaf blight resistance genes from rice, in a PCR based approach, to identify and analyse resistance gene analogues from various accessions of Carica papaya, Vasconcellea goudotiana, V. microcarpa, V. parviflora, V. pubescens, V. stipulata and, V. quercifolia and Jacaratia spinosa.ResultsOf the 34 primer pairs fourteen gave amplification products. A total of 115 alleles were identified from 41 accesions along with 12 rare and 11 null alleles. The number of alleles per primer pair ranged from 4 to 10 with an average of 8.21 alleles/ primer pair. The average polymorphism information content value was 0.75/primer. The primers for the gene Xa1 did not give any amplification product. As a group, the Indian Carica papaya accessions produced a total of 102 alleles from 27 accessions. The similarity among the 41 accessions ranged from 1% to 53%. The dendrogram made from Jaccard’s genetic similarity coefficient generated two major clusters showing that the alleles of Jacaratia spinosa and Vasconcellea accessions were distinctly different from those of Carica papaya accessions. All the alleles were sequenced and eleven of them were allotted accession numbers by NCBI. Homology searches identified similarity to rice BLB resistance genes and pseudogenes. Conserved domain searches identified gamma subunit of transcription initiation factor IIA (TFIIA), cytochrome P450, signaling domain of methyl-accepting chemotaxis protein (MCP), Nickel hydrogenase and leucine rich repeats (LRR) within the sequenced RGAs.ConclusionsThe RGA profiles produced by the 14 primer pairs generated high genetic diversity. The RGA profiles identified each of the 41 accessions clearly unequivocally. Most of the DNA sequences of the amplified RGAs from this set of 41 accessions showed significant homology to the conserved regions of rice bacterial leaf blight resistance genes. These information can be used in future for large scale investigation of tentative disease resistance genes of Carica papaya and other Caricaceae genus specially Vasconcellea. Inoculation studies will be necessary to link the identified sequences to disease resistance or susceptibility.Electronic supplementary materialThe online version of this article (doi:10.1186/s12863-014-0137-0) contains supplementary material, which is available to authorized users.

Update on Bacterial Blight of Rice: Fourth International Conference on Bacterial Blight

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High-resolution genetic mapping of rice bacterial blight resistance gene Xa23

Bacterial blight (BB) caused by Xanthomonas oryzae pv. oryzae (Xoo) is the most devastating bacterial disease of rice (Oryza sativa L.), a staple food crop that feeds half of the world's population. In management of this disease, the most economical and effective approach is cultivating resistant varieties. Due to rapid change of pathogenicity in the pathogen, it is necessary to identify and characterize more host resistance genes for breeding new resistant varieties. We have previously identified the BB resistance (R) gene Xa23 that confers the broadest resistance to Xoo strains isolated from different rice-growing regions and preliminarily mapped the gene within a 1.7cm region on the long arm of rice chromosome 11. Here, we report fine genetic mapping and in silico analysis of putative candidate genes of Xa23. Based on F2 mapping populations derived from crosses between Xa23-containing rice line CBB23 and susceptible varieties JG30 or IR24, six new STS markers Lj36, Lj46, Lj138, Lj74, A83B4, and Lj13 were developed. Linkage analysis revealed that the new markers were co-segregated with or closely linked to the Xa23 locus. Consequently, the Xa23 gene was mapped within a 0.4cm region between markers Lj138 and A83B4, in which the co-segregating marker Lj74 was identified. The corresponding physical distance between Lj138 and A83B4 on Nipponbare genome is 49.8kb. Six Xa23 candidate genes have been annotated, including four candidate genes encoding hypothetical proteins and the other two encoding a putative ADP-ribosylation factor protein and a putative PPR protein. These results will facilitate marker-assisted selection of Xa23 in rice breeding and molecular cloning of this valuable R gene.

In vitro Screening and Molecular Characterization of a Bacterial BlightResistance Gene in Rice

Bacterial blight (BB) is one of the most serious bacterial diseases of rice in the world. Since 1986, our laboratory has conducted in vitro screening and application of BB resistance somaclonal mutants for rice breeding. In this paper, we review our nearly 30-year research on BB resistance somaclonal variation, including the development of an in vitro selection system, discovery of BB resistance mutants, evaluation of BB resistance in a somaclonal mutant named HX-3, genetic analysis and molecular mapping of the new BB resistance gene Xa-25. In addition, we also used the new resistance gene to develop BB resistance cultivar and hybrid rice. Our long-term research contributed a novel resistance gene in rice, it also provided an example for new gene creation and discovery using somaclonal variation.

Addition of transcription activator‐like effector binding sites to a pathogen strain‐specific rice bacterial blight resistance gene makes it effective against additional strains and against bacterial leaf streak

Xanthomonas transcription activator-like (TAL) effectors promote disease in plants by binding to and activating host susceptibility genes. Plants counter with TAL effector-activated executor resistance genes, which cause host cell death and block disease progression. We asked whether the functional specificity of an executor gene could be broadened by adding different TAL effector binding elements (EBEs) to it. We added six EBEs to the rice Xa27 gene, which confers resistance to strains of the bacterial blight pathogen Xanthomonas oryzae pv. oryzae (Xoo) that deliver the TAL effector AvrXa27. The EBEs correspond to three other effectors from Xoo strain PXO99(A) and three from strain BLS256 of the bacterial leaf streak pathogen Xanthomonas oryzae pv. oryzicola (Xoc). Stable integration into rice produced healthy lines exhibiting gene activation by each TAL effector, and resistance to PXO99(A) , a PXO99(A) derivative lacking AvrXa27, and BLS256, as well as two other Xoo and 10 Xoc strains virulent toward wildtype Xa27 plants. Transcripts initiated primarily at a common site. Sequences in the EBEs were found to occur nonrandomly in rice promoters, suggesting an overlap with endogenous regulatory sequences. Thus, executor gene specificity can be broadened by adding EBEs, but caution is warranted because of the possible coincident introduction of endogenous regulatory elements.

Gene silencing using the recessive rice bacterial blight resistance gene xa13 as a new paradigm in plant breeding

Resistant germplasm resources are valuable for developing resistant varieties in agricultural production. However, recessive resistance genes are usually overlooked in hybrid breeding. Compared with dominant traits, however, they may confer resistance to different pathogenic races or pest biotypes with different mechanisms of action. The recessive rice bacterial blight resistance gene xa13, also involved in pollen development, has been cloned and its resistance mechanism has been recently characterized. This report describes the conversion of bacterial blight resistance mediated by the recessive xa13 gene into a dominant trait to facilitate its use in a breeding program. This was achieved by knockdown of the corresponding dominant allele Xa13 in transgenic rice using recently developed artificial microRNA technology. Tissue-specific promoters were used to exclude most of the expression of artificial microRNA in the anther to ensure that Xa13 functioned normally during pollen development. A battery of highly bacterial blight resistant transgenic plants with normal seed setting rates were acquired, indicating that highly specific gene silencing had been achieved. Our success with xa13 provides a paradigm that can be adapted to other recessive resistance genes.