Showing papers by "Pamela C. Ronald published in 2004"

Overexpression of (At)NPR1 in Rice Leads to a BTH- and Environment-Induced Lesion-Mimic/Cell Death Phenotype

Abstract: Systemic acquired resistance (SAR) is an inducible defense response that protects plants against a broad spectrum of pathogens. A central regulator of SAR in Arabidopsis is NPR1 (nonexpresser of pathogenesis-related genes). In rice, overexpression of Arabidopsis NPR1 enhances plant resistance to the bacterial pathogen Xanthomonas oryzae pv. oryzae. This report demonstrates that overexpression of (At)NPR1 in rice also triggers a lesion-mimic/cell death (LMD) phenotype. The LMD phenotype is environmentally regulated and heritable. In addition, the development of lesions and death correlates with the expression of rice defense genes and the accumulation of hydrogen peroxide. Application of the salicylic acid (SA) analog, benzo(1,2,3) thiadiazole-7-carbothioc acid S-methyl ester (BTH), potentiates this phenotype Endogenous SA levels are reduced in rice overexpressing (At)NPR1 when compared with wildtype plants, supporting the idea that (At)NPR1 may perceive and modulate the accumulation of SA. The association of (At)NPR1 expression in rice with the development of an LMD phenotype suggests that (At)NPR1 has multiple roles in plant stress responses that may affect its efficacy as a transgenic tool for engineering broad-spectrum resistance.

Bacterial genes involved in type I secretion and sulfation are required to elicit the rice Xa21-mediated innate immune response.

Abstract: Innate immunity to microorganisms relies on the specific sensing of pathogen-associated molecules by host recognition receptors. Whereas studies in animals have largely focused on the recognition of extracellular pathogen-associated molecules by the TLR (toll-like receptor) superfamily, few studies have been carried out in plants, and it is not understood how these molecules are secreted or modified. The rice Xa21 gene encodes a receptor-like kinase that provides immunity against strains of the bacterial pathogen Xanthomonas oryzae pv. oryzae carrying AvrXa21 activity. We identified four X. oryzae pv. oryzae genes that are required for AvrXa21 activity. raxA, raxB, and raxC encode proteins with similarity to a membrane fusion protein, an ATP-binding cassette transporter, and an outer membrane protein, respectively, of bacterial type I secretion systems. The fourth gene, raxST, encodes a sulfotransferase-like protein. Sequence analysis of three naturally occurring X. oryzae pv. oryzae strains no longer recognized by Xa21 revealed alterations in the raxST and raxA genes. The raxC gene complemented an Escherichia coli tolC mutant for secretion of a double glycine-leader peptide confirming the function of raxC in type I secretion. These results indicate that bacterial type I secretion is necessary for Xa21-mediated recognition and immunity and further suggest that type I secretion and modification of pathogen-associated molecules play an important role in triggering the innate immune response in rice.

RaxH/RaxR: a two-component regulatory system in Xanthomonas oryzae pv. oryzae required for AvrXa21 activity.

Abstract: Xanthomonas oryzae pv. oryzae is the causal agent of bacterial leaf blight, one of the most serious diseases in rice. X. oryzae pv. oryzae Philippine race 6 (PR6) strains are unable to establish infection in rice lines expressing the resistance gene Xa21. Although the pathogen-associated molecule that triggers the Xa21-mediated defense response (AvrXa21) is unknown, six rax (required for AvrXa21 activity) genes encoding proteins involved in sulfur metabolism and Type I secretion were recently identified. Here, we report on the identification of two additional rax genes, raxR and raxH, which encode a response regulator and a histidine protein kinase of two-component regulatory systems, respectively. Null mutants of PR6 strain PXO99 that are impaired in either raxR, raxH, or both cause lesions significantly longer and grow to significantly higher levels than does the wild-type strain in Xa21-rice leaves. Both raxR and raxH mutants are complemented to wild-type levels of AvrXa21 activity by introduction of expression vectors carrying raxR and raxH, respectively. These null mutants do not affect AvrXa7 and AvrXa10 activities, as observed in inoculation experiments with Xa7- and Xa10-rice lines. Western blot and raxR/gfp promoter-reporter analyses confirmed RaxR expression in X. oryzae pv. oryzae. The results of promoter-reporter studies also suggest that the previously identified raxSTAB operon is a target for RaxH/RaxR regulation. Characterization of the RaxH/RaxR system provides new opportunities for understanding the specificity of the X. oryzae pv. oryzae-Xa21 interaction and may contribute to the identification of AvrXa21.

Use of Pi5(t) markers in marker-assisted selection to screen for cultivars with resistance to Magnaporthe grisea.

Abstract: Identification of the PCR markers tightly linked to genes that encode important agronomic traits is useful for marker-assisted selection (MAS). The rice Pi5(t) locus confers broad-spectrum resistance to Magnaporthe grisea, the causal agent of rice blast disease. It has been hypothesized that the Pi5(t) locus carries the same gene as that encoded by the Pi3(t) and Pii(t) loci. We developed three PCR-based dominant markers (JJ80-T3, JJ81-T3, and JJ113-T3) from three previously identified BIBAC clones—JJ80, JJ81, and JJ113—that are linked to the Pi5(t) locus. PCR analysis of 24 monogenic lines revealed that these markers are present only in lines that carry Pi5(t), Pi3(t), and Pii(t). PCR and DNA gel-blot analysis of candidate resistance lines using JJ80-T3, JJ81-T3, and JJ113-T3 indicated that Tetep is the likely donor of Pi5(t). Of the 184 rice varieties tested, 34 carried the JJ80-T3-, JJ81-T3-, and JJ113-T3-specific bands. Disease evaluation of those 34 varieties revealed that all conferred resistance to PO6-6. The genomic structure of three of these resistant varieties (i.e., IR72, Taebaeg, Jahyangdo) is most similar to that of Pi5(t). Our results demonstrate the usefulness of the JJ80-T3, JJ81-T3, and JJ113-T3 markers for MAS for M. grisea resistance.

Development of Co-Dominant Amplified Polymorphic Sequence Markers in Rice that Flank the Magnaporthe grisea Resistance Gene Pi7(t) in Recombinant Inbred Line 29

Abstract: Pi7(t), a dominant blast resistance gene derived from the rice cultivar Moroberekan, confers complete resistance against the fungal pathogen Magnaporthe grisea. Pi7(t) previously was positioned on chromosome 11 by restriction fragment length polymorphism (RFLP) mapping of a recombinant inbred line population. One derivative of this population, recombinant inbred line (RIL)29, was designated as the representative line for Pi7(t). A segregating F2 population was created from RIL29 in order to determine the location of Pi7(t). The new mapping data indicate a position for Pi7(t) 30 centimorgans distal to the original location. Pi7(t) shares a common position with the previously mapped Pi1 M. grisea resistance gene. RIL29 carries DNA not derived from either parent used to create the RIL population at the newly assigned Pi7(t) locus. RFLP analysis has identified a possible donor source.

Chapter 21 Engineering Pathogen Resistance in Crop Plants

Abstract: As the world population continues to increase, food supplies must also grow to meet nutritional requirements. One means of ensuring the stability and plentitude of the food supply is to mitigate crop loss caused by plant pathogens. Strategies for combating disease include traditional technologies such as plant breeding and chemical applications; current technologies such as generating transgenic plants that express components of known defence signalling pathways and the adaptation of newer technologies such as RNA silencing of pathogen and plant transcripts. Breeding has been used to pyramid resistance (R) genes into many different plants including rice. Chemical strategies include application of salicylic acid (SA) analogues to stimulate systemic acquired resistance (SAR) responses. Genetic screens in Arabidopsis have identified genes controlling SAR and these genes have been manipulated and used to engineer crop plants. The diseases caused by plant viruses are being thwarted through the initiation of endogenous RNA silencing mechanisms. Many of these strategies show great promise, some limitations and exciting opportunities to develop many new tools for combating plant pests. Keywords: resistance; gene-pyramiding; RNAhomology-dependent; silencing; systemic acquired resistance (SAR); salicylic acid (SA)

Isolation and characterization of rice mutants compromised in Xa21 -mediated resistance to X. oryzae pv. oryzae

Abstract: The rice gene, Xa21, confers resistance to diverse races of Xanthomonas oryzae pv. oryzae (Xoo) and encodes a receptor-like kinase with leucine-rich repeats in the extra-cellular domain. To identify genes essential for the function of the Xa21 gene, 4,500 IRBB21 (Xa21 isogenic line in IR24 background) mutants, induced by diepoxybutane and fast neutrons, were screened against Philippine race six (PR6) Xoo for a change from resistance to susceptibility. From two greenhouse screens, 23 mutants were identified that had changed from resistant to fully (6) or partially (17) susceptible to PR6. All fully susceptible mutants carried rearrangements at the Xa21 locus as detected by PCR and Southern hybridization. For the partially susceptible mutants, no changes were detected at the Xa21 locus based on Southern and PCR analyses. However, two of these mutants were confirmed via genetic analysis to have mutations at the Xa21 locus. Partially susceptible mutants exhibited variation in level of susceptibility to different Xoo strains, suggesting that they may carry different mutations required for the Xa21-mediated resistance. The mutants identified in this study provide useful materials for dissecting the Xa21-mediated resistance pathway in rice.

Food for the Gods

Abstract: The Art of Rice Spirit and Sustenance in Asia by Roy W Hamilton [with 26 Contributors] UCLA Fowler Museum of Cultural History, Los Angeles, 2004 562 pp Paper, $60 ISBN 0-930741-98-6 The contributors to this profusely illustrated catalog explore beliefs and practices relating to rice as they are represented in the arts and cultures of south, southeast, and east Asia