Four QTL in Rice Associated with Broad Spectrum Resistance to Blast Isolates from Rice and Barley

Pyricularia grisea is the most destructive and cosmopolitan fungal pathogen of rice and it can also cause disease on other agriculturally important cereals. We determined the number, location and interaction of quantitative trait loci (QTL) associated with resistance to P. grisea isolates obtained from rice (THL142 and THL222) and barley (TH16 and THL80) grown in Thailand. The isolates showed a spectrum of virulence when used to inoculate a series of differentials. We used a reference blast resistance mapping population of rice (IR64 × Azucena). IR64 was highly resistant, and Azucena was highly susceptible, to all four isolates. The numbers of resistant vs. susceptible progeny suggest that the resistance of IR64 is determined by two or three genes with additive effects. The correlation coefficients for all pairwise comparisons of disease severity were high and highest between barley isolates and between rice isolates. Four QTL were detected, one on each of the following chromosomes 2, 8, 9 and 10. IR64 contributed resistance alleles at three of the QTL (chromosomes 2, 8 and 9). Azucena contributed the resistance allele at the QTL on chromosome 10 in response to inoculation with isolate THL142. The results of the QTL analysis support interpretation of the phenotypic frequency distributions regarding the number of genes determining resistance to the four isolates in this population. Our results are novel in adding blast isolates from barley to the catalogue of pathogen specificities to which a gene, or genes, from IR64 confer resistance.     

Genetic Mapping of Magnaporthe grisea Avirulence Gene Corresponding to Leaf and Panicle Blast Resistant QTLs in Jao Hom Nin Rice Cultivar

The avirulence characteristic of Magnaporthe grisea isolate TH16 corresponding to Jao Hom Nin (JHN) rice cultivar was studied by mapping population of 140 random ascospore progenies derived from the cross between B1-2 and TH16 isolates. Segregation analyses of the avirulence characteristic performing on JHN rice at the seedling and flowering stages were performed in this mapping population. We used the reference map of Guy11/2539 to choose microsatellite DNA markers for mapping the avirulence gene. The genetic map of this population was constructed from 39-microsatellite markers. The genetic map was spanned by covering seven chromosomes with an average distance of 11.9 cM per marker. In mapping population the distribution of pathogenic and non-pathogenic progenies on JHN rice were found to be fitted to 1 : 1 ratio for two of the rice stages, seedling and flowering stages. The Quantitative Trait Loci (QTL) analysis for avirulence genes corresponding to two rice stages were located at the same region on chromosome 2 between markers Pyms305 and Pyms435. The LOD score and percentage of phenotypic variance explained (PVE) on two rice stages were 5.01/16.69 and 6.73/20.26, respectively. These loci were designated as Avr-JHN(lb) and Avr-JHN(pb) corresponding to leaf and panicle blast characteristics. The findings of this study can be the initial step for positional cloning and identifying any function of avirulence genes corresponding to leaf and panicle blast characteristics.     

Assessment of Genetic Diversity in Thai Isolates of Pyricularia grisea by Random Amplification of Polymorphic DNA

One hundred and seventy‐four isolates of Pyricularia grisea were collected from various hosts such as barley, rice, weed and wild rice in Thailand. Seven arbitrary decamer primers from the set of University of British Columbia were employed and nine lineages were classified. Lineages B, C and H were predominant, contributing up to 70% of total pathogens in this study. Analysis showed that the distribution of each lineage differs from the predominant lineages across Thailand in such that other lineages were restricted in particular area. For instance, lineage A was limited only in southern Thailand, whereas wide distribution of lineages B and C reflected an influence of both biological and physical effects on pathogen variation. Principal component analysis resulted in a total of four groups of blast pathogen with small distinctions between barley‐, rice‐, weed‐ and wild rice‐infected blast. Bridging relationships occurred among border isolates of weed and rice blast suggesting a chance of migrations between hosts. Higher diversity was observed in northern, north‐eastern and central Thailand while eastern and southern parts were rather low. Genetic diversity indices elucidated an abundance of pathogen lineages existing in northern Thailand suggesting that it should be the centre of diversity.     

Amplified Fragment Length Polymorphism Fingerprinting Differentiates Genetic Diversity of Xanthomonas oryzae pv. oryzae from Northern Thailand

Thirty isolates of Xanthomonas oryzae pv. oryzae were collected from different rice‐producing area in northern Thailand. For the assessment of genetic variation of bacterial blight pathogen, 19 primer combinations of amplified fragment length polymorphism primer system were screened to evaluate the genetic diversity and five combinations were selected according to their producibility, number of scorable bands and differences detected among representative isolates. Six lineages of X. oryzae pv. oryzae were identified in northern Thailand base on location. Lineage A composed of members from two provinces, Phitsanulok and Chainat. Lineage B was from various provinces as Sukhothai, Phetchaboon, Phicit, Phayao and Phrae. Lineage C was from Phitsanulok and Phrae. Lineage D comprised of members from Phrae, Chiangmai and Chiangrai while the lineage E composed of isolates from Sukhothai and Phitsanulok. The final lineage, lineage F, was from Lampang. Lineages B and D were the most widely distributed while lineage E seemed to be restricted to specific planting area. Wide distribution of the pathogen might be due to seed allocation and germplasm exchanged. Analysis showed that diversity of pathogen is due to single field and cultivars‐specific effects. The results of this study will facilitate the use of effective bacterial blight resistance gene in northern Thailand.     

Quantitative trait loci associated with leaf and neck blast resistance in recombinant inbred line population of rice (Oryza sativa).

Blast is an economically important disease of rice. To map genes controlling blast resistance, recombinant inbred lines (RIL) were developed from Khao Dawk Mali 105, an aromatic, blast-susceptible cultivar and the blast resistance donor, CT 9993-5-10-M (CT). A linkage map encompassing 2112 cM was constructed from 141 RILs using 90 restriction fragment length polymorphisms (RFLPs) and 31 simple sequence repeats (SSR). Virulent isolates of blast fungus were identified by screening differential host sets against 87 single-spore isolates collected from the north and northeast of Thailand. Fifteen virulent blast isolates were selected for leaf blast screening. Neck blast was evaluated both under natural conditions and controlled inoculations. Quantitative trait loci (QTLs) for broad resistance spectrum (BRS) to leaf blast were located on chromosomes 7 and 9. In particular, the QTL(ch9) was mapped near the Pi5(t) locus. The QTL(ch7) was located close to a previously mapped partial resistance QTL. Both loci showed significant allelic interaction. Genotypes having CT alleles at both QTL(ch7) and QTL(ch9) were the most resistant. Two neck-blast QTLs were mapped on chromosomes 5 and 6. The inconsistent map locations between the leaf and neck blast QTLs indicate the complexity of fixing both leaf and neck blast resistance. The coincidence of BRS and field resistance QTLs on chromosome 7 supports the idea that BRS may reflect the broad resistance spectrum to leaf blast in rice. These findings laid the foundation for the development of a marker-assisted scheme for improving Khoa Dawk Mali 105 and the majority of aromatic Thai rice varieties that are susceptible to blast.     

Ascherxanthone B from Aschersonia luteola, a new antifungal compound active against rice blast pathogen Magnaporthe grisea

Aims:  Fungicide resistance now exists in the rice blast fungus, Magnaporthe grisea , necessitating the need for new active agents. Fungi isolated from habitats in Thailand were screened with reference to this problem.
Methods and Results:  A new, reliable in vitro screening system based on a microdilution plate format was set up using a virulent strain of M. grisea THL 16. Culture broth extracts from approximately 800 fungal strains were investigated, one of these, Aschersonia luteola BCC 8774, was found to produce an active fungicidal compound, ascherxanthone B, with an IC₉₀ value of 0·58 μg ml⁻¹ (0·95 μmol l⁻¹). An in vivo study of anti-blast efficacy of ascherxanthone B showed a positive effect in disease reduction.
Conclusions:  Previous report has shown that a species of Aschersonia produces ascherxanthone A. Research on the species, A. luteola BCC 8774, led to the discovery of related novel metabolite, ascherxanthone B with fungicidal properties.
Significance and Impact of the Study:  Current methods of rice blast control seem to fail leading to increase in crop losses. Our discovery of the anti-blast activity shown by ascherxanthone B is the first step in the development of a potentially novel fungicide.