Important Rice Pathogens Research Articles

Infection of Rice Seed Grown in Arkansas by Pyricularia grisea and Transmission to Seedlings in the Field.

Rice blast, caused by Pyricularia grisea, is an important and serious disease of rice (Oryza sativa) in the southeastern United States. The disease sporadically reaches epidemic proportions on susceptible cultivars within fields and over large areas within Arkansas. The main overwintering sources of inoculum reportedly include infected rice stubble, related host species, and infected seed. The objectives of the research were to (i) determine whether rice seed grown in Arkansas were infected with P. grisea, (ii) investigate the relationship between seed infection and seedling disease, and (iii) determine if planting naturally infected seed could lead to the subsequent development of rice blast on seedlings in the field. The results of seed assays showed that P. grisea was detected in samples of foundation, certified, and production seed. Estimated levels of infection by P. grisea of rice seed from 66 samples of rice seed grown in Arkansas ranged from 0 to 10.5%. Planting infected seed in the greenhouse and the field resulted in seedling infection. Planting naturally infected seed may result in disease development (i) from seedlings grown from infected seed planted beneath the soil surface, (ii) from seedlings grown from germinating seed left on the soil surface, (iii) from seed coats, or (iv) from nongerminated seed left on the soil surface after planting. Additional research is necessary to establish the mechanisms of infection of seedlings and to establish disease thresholds for this important fungal pathogen of rice.

A Simple Method of Mass Inoculation of Rice Effective for Both Pathovars ofXanthomonas oryzae, and the Construction of Comparable Sets of Host cDNA Libraries Spanning Early Stages of Bacterial Leaf Blight and Bacterial Leaf Streak

AbstractXanthomonas oryzaepv.oryzae(Xoo) andX. oryzaepv.oryzicola(Xoc) are important bacterial pathogens of rice (Oryza sativa). Despite being very closely related, they infect in different ways and cause distinct diseases. Xoo invades the xylem to cause bacterial blight, whereas Xoc colonizes the leaf mesophyll to cause bacterial leaf streak. As the modes of infection of these two pathogens are different, traditional methods of inoculation effective for Xoo are not effective for Xoc and vice versa. We discovered that dipping plants in bacterial suspension is an effective inoculation method both for Xoo and Xoc and can be used to achieve uniform and synchronized infection of multiple plants. Using this method, comparable sets of rice Gateway®cDNA libraries spanning five time points in the early stages of bacterial blight and bacterial leaf streak were constructed. Prepared under identical conditions, the libraries are ideal for probing differential rice transcription during development of the diseases. Furthermore, by capturing transcripts expressed in response to these unique sources of biotic stress, the libraries are likely to contain cDNA clones not available elsewhere.

Origins of host-specific populations of the blast pathogen Magnaporthe oryzae in crop domestication with subsequent expansion of pandemic clones on rice and weeds of rice.

Rice, as a widely and intensively cultivated crop, should be a target for parasite host shifts and a source for shifts to co-occurring weeds. Magnaporthe oryzae, of the M. grisea species complex, is the most important fungal pathogen of rice, with a high degree of host specificity. On the basis of 10 loci from six of its seven linkage groups, 37 multilocus haplotypes among 497 isolates of M. oryzae from rice and other grasses were identified. Phylogenetic relationships among isolates from rice (Oryza sativa), millet (Setaria spp.), cutgrass (Leersia hexandra), and torpedo grass (Panicum repens) were predominantly tree like, consistent with a lack of recombination, but from other hosts were reticulate, consistent with recombination. The single origin of rice-infecting M. oryzae followed a host shift from a Setaria millet and was closely followed by additional shifts to weeds of rice, cutgrass, and torpedo grass. Two independent estimators of divergence time indicate that these host shifts predate the Green Revolution and could be associated with rice domestication. The rice-infecting lineage is characterized by high copy number of the transposable element MGR586 (Pot3) and, except in two haplotypes, by a loss of AVR-Co39. Both mating types have been retained in ancestral, well-distributed rice-infecting haplotypes 10 (mainly temperate) and 14 (mainly tropical), but only one mating type was recovered from several derived, geographically restricted haplotypes. There is evidence of a common origin of both ACE1 virulence genotypes in haplotype 14. Host-haplotype association is evidenced by low pathogenicity on hosts associated with other haplotypes.

PRODUCTION OF TRANSGENIC RICE WITH RICE RAGGED STUNT VIRUS SYNTHETIC RESISTANCE GENES

Rice ragged stunt virus (RRSV) is an important pathogen of rice affecting its cultivation in South and South East Asia. An approach based on pathogen derived resistance (PDR) was used to produce RRSV resistant rice cultivars. Sequences from the coding region of RRSV genome segments 7 and 10 (non-structural genes), and 5, 8 and 9 (structural genes) were placed in sense or antisense orientation behind the plant expression promoters CaMV35S, RolC, Ubil, Actl and RBTV. Rice cultivars Taipei 309 and Chinsurah Boro II were transformed by biolistic and/or Agrobacterium-mediated delivery of one or more of these PDR gene constructs. A large number of transgenic lines were produced from calli derived from mature or immature embryos, co-bombarded with the marker gene hph encoding hygromycin resistance and RRSV PDR genes or co-cultivated with strains having the binary vector containing these two genes. Both Mendelian and non-Mendelian segregations were observed in transgenic progeny, especially with transgenic lines produced by biolistics. Preliminary tests conducted in China on selected transgenic lines indicate that plants with RRSV segment 5 antisense PDR gene confer RRSV resistance.

Genetic variability in Gibberella fujikuroi and some related species of the genus Fusarium based on random amplification of polymorphic DNA (RAPD)

One of the most important rice pathogens is Fusarium moniliforme (perfect stage: Gibberella fujikuroi), the causal agent of the super-elongation ("bakanae") disease. Thirty-seven strains of this species from different geographical regions were analyzed for their ability to produce gibberellins (GA) and for genetic relatedness by random amplified polymorphic DNA (RAPD). All GA-producing isolates showed nearly identical RAPD patterns using 51 oligonucleotide nona- and deca-mers as arbitrary primers. On the other hand, large differences between GA-nonproducing isolates were obtained. Comparison of the RAPD patterns with those of the tester strains of the six known mating populations (A, B, C, D, E, F) of G. fujikuroi showed that all producer strains belong to mating population C and all nonproducer isolates to other mating populations. Evidence for the usefulness of the RAPD technique to distinguish between mating populations was provided by sexual crossings. Consensus phylogenetic trees based on RAPDs were constructed by the Phylogenetic Analysis Using Parsimony (PAUP) system. In combination with morphological analysis, RAPD can distinguish between different species of the genus Fusarium. These investigations may find an application in the diagnosis of unknown Fusarium spp. and in distinguishing isolates of G. fujikuroi within the section Liseola.

Identification of mycotoxin-producing fungal strains: A step in the isolation of compounds active against rice fungal diseases

The simultaneous occurrence of very toxic mycotoxins with other useful fungal metabolites is a major obstacle in assays for new antifungal molecules. Screening of 67 fungal extracts in ethyl acetate to identify those with high contents of a known mycotoxin, patulin, has been undertaken. The extracts were earlier found to inhibit the in vitro growth on solid malt extract medium of three important rice pathogens: Pyricularia oryzae, Drechslera oryzae, and Gerlachia oryzae. Eupenicillium javanicum, Penicillium cyaneum, Penicillium diversum, Penicillium' duclauxii, Penicillium echinulatum, Penicillium glabrum, Penicillium lignorum, Alternaria tenuissima, Chaetomium atrobrunneum, Scopulariopsis flava, and a Stemphylium sp. are reported for the first time as patulin producers. Pure patulin was observed to inhibit the growth of D. oryzae and G. oryzae at minimum concentrations of 0.50 and at 0.25 micrograms/ml, respectively, for P. oryzae. The diameter of the zones of inhibition increased with patulin concentration. There were low correlations between the quantity of patulin in the extracts and the level of in vitro inhibition of mycelial growth of each target phytopathogen.