Combining bacterial blight resistance and Basmati quality characteristics by phenotypic and molecular marker-assisted selection in rice

Bacterial Blight (BB) caused by Xanthomonas oryzae pv. oryzae is a major disease of rice in tropical Asia. Since all the Basmati varieties are highly susceptible and the disease is prevalent in the entire Basmati growing region of India, BB is a severe constraint in Basmati rice production. The present study was undertaken with the objective of combining the important Basmati quality traits with resistance to BB by a combination of phenotypic and molecular marker-assisted selection (MAS). Screening of 13 near-isogenic lines of rice against four isolates of the pathogen from Basmati growing regions identified the Xa4, xa8, xa13 and Xa21 effective against all the isolates tested. Two or more of these genes in combination imparted enhanced resistance as expressed by reduced average lesion length in comparison to individual genes. The two-gene pyramid line IRBB55 carrying xa13 and Xa21 was found equally effective as three/four gene pyramid lines. The two BB resistance genes present in IRBB55 were combined with the Basmati quality traits of Pusa Basmati-1 (PB-1), the most popular high yielding Basmati rice variety used as recurrent parent. Phenotypic selection for disease resistance, agronomic and Basmati quality characteristics and marker-assisted selection for the two resistance genes were carried out in BC₁F₁, BC₁F₂ and BC₁F₃ generations. Background analysis using 252 polymorphic amplified fragment length polymorphism (AFLP) markers detected 80.4 to 86.7% recurrent parent alleles in BC₁F₃ selections. Recombinants having enhanced resistance to BB, Basmati quality and desirable agronomic traits were identified, which can either be directly developed into commercial varieties or used as immediate donors of BB resistance in Basmati breeding programs.     

Proteomic analysis of bacterial-blight defense-responsive proteins in rice leaf blades

Plants exhibit resistance against incompatible pathogens, via localized and systemic responses as part of an integrated defense mechanism. To study the compatible and incompatible interactions between rice and bacteria, a proteomic approach was applied. Rice cv. Java 14 seedlings were inoculated with compatible (Xo7435) and incompatible (T7174) races of Xanthomonas oryzae pv. oryzae (Xoo). Cytosolic and membrane proteins were fractionated from the leaf blades and separated by 2-D PAGE. From 366 proteins analyzed, 20 were differentially expressed in response to bacterial inoculation. These proteins were categorized into classes related to energy (30%), metabolism (20%), and defense (20%). Among the 20 proteins, ribulose-1,5-bisphosphate carboxylase/oxygenase large subunit (RuBisCO LSU) was fragmented into two smaller proteins by T7174 and Xo7435 inoculation. Treatment with jasmonic acid (JA), a signaling molecule in plant defense responses, changed the level of protein accumulation for 5 of the 20 proteins. Thaumatin-like protein and probenazole-inducible protein (PBZ) were commonly up-regulated by T7174 and Xo7435 inoculation and JA treatment. These results suggest that synthesis of the defense-related thaumatin-like protein and PBZ are stimulated by JA in the defense response pathway of rice against bacterial blight.     

Pyramiding Xa23 and Rxo1 for resistance to two bacterial diseases into an elite indica rice variety using molecular approaches

Rice bacterial leaf blight (BB) caused by Xanthomonas oryzae pv. oryzae and bacterial leaf streak (BLS) caused by X. oryzae pv. oryzicola (Xoc) are two important diseases of rice that often outbreak simultaneously and constrain rice production in much of Asia and parts of Africa. Developing resistant cultivars has been the most effective approach to control BB, however, most single resistance genes have limited value in breeding programs because of their narrow-spectrum of resistance to the races of the pathogen. By contrast, there is little progress in breeding varieties resistant to Xoc since BLS resistance in rice was a quantitative trait and so far only a few quantitative resistance loci have been identified. We reported here the development of a high yield elite line, Lu-You-Zhan highly resistant to both BB and BLS by pyramiding Xa23 with a wide-spectrum resistance to BB derived from wild rice and a non-host maize resistance gene, Rxo1, using both marker assisted selection (MAS) and genetic engineering. Our study has provided strong evidence that non-host R genes could be a valuable source of resistance in combating those plant diseases where no single R gene controlling high level of resistance exists and demonstrated that MAS combined with transgenic technologies are an effective strategy to achieve high level of resistance against multiple plant diseases. Y-L Zhou and J-L Xu contributed equally to this work.     

Pyramiding three bacterial blight resistance genes (xa5, xa13 and Xa21) using marker-assisted selection into indica rice cultivar PR106

Bacterial blight (BB) of rice caused by Xanthomonas oryzae pv. oryzae (Xoo) is a major disease of rice in several countries. Three BB resistance genes, xa5, xa13 and Xa21, were pyramided into cv. PR106, which is widely grown in Punjab, India, using marker-assisted selection. Lines of PR106 with pyramided genes were evaluated after inoculation with 17 isolates of the pathogen from the Punjab and six races of Xoo from the Philippines. Genes in combinations were found to provide high levels of resistance to the predominant Xoo isolates from the Punjab and six races from the Philippines. Lines of PR106 with two and three BB resistance genes were also evaluated under natural conditions at 31 sites in commercial fields. The combination of genes provided a wider spectrum of resistance to the pathogen population prevalent in the region; Xa21 was the most effective, followed by xa5. Resistance gene xa13 was the least effective against Xoo. Only 1 of the BB isolates, PX04, was virulent on the line carrying Xa21 but avirulent on the lines having xa5 and xa13 genes in combination with Xa21. Received: 26 May 2000 / Accepted: 16 August 2000     

Suppression of bacterial infection in rice by treatment with a sulfated peptide

The rice XA21 receptor kinase confers robust resistance to bacterial blight disease caused by Xanthomonas oryzae pv. oryzae (Xoo). A tyrosine‐sulfated peptide from Xoo, called RaxX, triggers XA21‐mediated immune responses, including the production of ethylene and reactive oxygen species and the induction of defence gene expression. It has not been tested previously whether these responses confer effective resistance to Xoo. Here, we describe a newly established post‐inoculation treatment assay that facilitates investigations into the effect of the sulfated RaxX peptide in planta. In this assay, rice plants were inoculated with a virulent strain of Xoo and then treated with the RaxX peptide 2 days after inoculation. We found that post‐inoculation treatment of XA21 plants with the sulfated RaxX peptide suppresses the development of Xoo infection in XA21 rice plants. The treated plants display restricted lesion development and reduced bacterial growth. Our findings demonstrate that exogenous application of sulfated RaxX activates XA21‐mediated immunity in planta, and provides a potential strategy for the control of bacterial disease in the field.     

QTL analysis of field resistance to Xanthomonas axonopodis pv. manihotis in cassava

We evaluated cassava bacterial blight (CBB) infection in an pair-cross population of 150 individuals derived from an intra-specific cross between two non-inbred cassava (Manihot esculenta Crantz) lines. The replicated trials were carried out in the field under high disease pressure over two consecutive crop cycles. Evaluations were conducted at 4 and 7 months after planting for the two cycles. Simple regression analysis and the nonparametric Kruskal-Wallis rank-sum test revealed that eight quantitative trait loci (QTLs) were involved in resistance. We detected changes in QTLs from crop cycle to crop cycle. The pathogen population (Xanthomonas axonopodis pv. manihotis) was also monitored over the period, using a restriction fragment length polymorphism probe and pathogenic tests. Changes in QTL detection over the 2 years could be correlated with changes in pathogen population structure. One QTL, located in linkage group D, was conserved over the two crop cycles, and in field to greenhouse evaluations. This study thus identified molecular markers useful for marker assisted-selection, a technique that can accelerate the long, multiple-season process of breeding for CBB resistance. Received: 1 January 2000 / Accepted: 25 June 2000     

A mutation in an exoglucanase of Xanthomonas oryzae pv. oryzae,which confers an endo mode of activity,affects bacterial virulence,but not the induction of immune responses,in rice

Xanthomonas oryzae pv. oryzae (Xoo) causes bacterial blight, a serious disease of rice. Xoo secretes a repertoire of cell wall‐degrading enzymes, including cellulases, xylanases and pectinases, to degrade various polysaccharide components of the rice cell wall. A secreted Xoo cellulase, CbsA, is not only a key virulence factor of Xoo, but is also a potent inducer of innate immune responses of rice. In this study, we solved the crystal structure of the catalytic domain of the CbsA protein to a resolution of 1.86 Å. The core structure of CbsA shows a central distorted TIM barrel made up of eight β strands with N‐ and C‐terminal loops enclosing the active site, which is a characteristic structural feature of an exoglucanase. The aspartic acid at the 131st position of CbsA was predicted to be important for catalysis and was therefore mutated to alanine to study its role in the catalysis and biological functions of CbsA. Intriguingly, the D131A CbsA mutant protein displayed the enzymatic activity of a typical endoglucanase. D131A CbsA was as proficient as wild‐type (Wt) CbsA in inducing rice immune responses, but was deficient in virulence‐promoting activity. This indicates that the specific exoglucanase activity of the Wt CbsA protein is required for this protein to promote the growth of Xoo in rice.     

农杆菌介导将白叶枯病抗性基因Xa21导入水稻获得转基因植株

利用农杆菌介导的高效遗传转化系统,将白叶枯病抗性基因Xa21转入黄淮稻区主栽品种豫粳6号的胚性愈伤组织,获得转基因植株,GUS染色和PCR分析证明Xa21基因已整合到水稻基因组中,其自交T1代植株经GUS染色和白叶枯病接种鉴定呈现3：1分离,研究为培育抗白叶枯病水稻品种奠定了基础。     

小麦杂种世代生育期与产量的遗传关系

用二个杂交组合的F2-F4代材料, 研究了小麦生育期性状的遗传变异以及与产量的关系。各生育期的变异如下:开花-成熟期>拔节-抽穗期>播种-拔节期>抽穗-开花期。各生育期性状在杂种早期世代(F2、F3)具较高遗传力。单株产量与开花-成熟期正相关,与拔节-开花期负相关,与全生育期相关不显著。对开花-成熟期正向选择,拔节-开花期负向选择,产量可获较大遗传改进。