Studies On Genetic Differentiation Of The Pathogens Causing Rice Sheath Blight In Guangxi

Rice sheath blight is a destructive disease of rice worldwide, and the disease hasbecome one of the major constraints for rice productivity and yield stability of rice inrecent years. It is very difficult to control the disease because of the diversity andvariability of the pathogen. In order to clarify the genetic mutations and diversity of thepathogens causing rice sheath blight in Guangxi, 160 pathogenic isolates derived fromtypical leaf lesions were analyzed for classification and genetic differentiation, and a PCRassay for detection of some of the strains has been established in the dissertation. To learnthe relationship between the directional selection of rice and the evolution of the pathogen,the genetic variations in the pathogen strains after host-pathogen interaction were studiedin the levels of vegetative compatibility, protein and DNA, which provided basic evidencesfor developing new strategies against this disease through reasonable cultivating measures.The main results were summarized as follows.1. Based on several classification and identification methods of Rhizoctonia, 160pathogenic isolates were divided into 5 groups, which were Rhizoctonia solani AG1-IA,with a percentage of 90.62%; Rhizoctonia oryzae-sativae, 2.5%; Rhizoctonia oryzae, 1.25%;Rhizoctonia sp AG-A, 1.25%; and Sclerotium hydrophilum, 4.38%, respectively. Among the5 groups, R. oryzae and S. hydrophilum were the first records in Guangxi. R. solani AG1-IAwas the preponderant pathogenic group and showed the strongest virulence, andRhizoctonia sp AG-A showed no virulence on rice, while others did infect rice but showedlower virulence. The results suggested that PCR-RFLP of rDNA was one of the mostpowerful tools to study the genetic variation of these Rhizoctonia fungi throughcomparison of 3 identification methods based on morphological features, hyphal fusions and sequencing analysis of rDNA, respectively.2. Genetic differences among the strains were studied comparatively with theirbiological characteristics and rep-PCR fingerprinting. The results indicated thatdifferences were obvious among the strains in their biological characteristics, such asoptimal initial pH, optimum temperature for hyphal growth, and so on. The rep-PCRfingerprinting technique utilized in this study was effective for distinguishing strains, andeach strains showed its own rep-PCR fingerprinting bands, which would be used as thespecific DNA fingerprints for distinguishing these strains.3. A UPGMA phylogenetic tree based on the rDNA ITS sequences of the strains usedanti relative species has been developed. Based on the conserved sequence determined bythe alignment rDNA sequence of all the strains used in the study, one pair of specificprimers (WP-1R/WP-2F) designed from the sequence of ribosomal DNA-ITS forRhizoctonia oryzae, and the other one (SP-1F/ITS1) for Sclerotium hydrophilum. Thespecific bands from Rhizoctonia oryzae and Sclerotium hydrophilum could be amplifiedwith the primer pairs WP-1R/WP-2F and SP-1F/ITS1, respectively, but not from otherfungal species associated with rice sheath blight, and the primer pairs showed the sameresults in detecting Rhizoctonia oryzae and Sclerotium hydrophilum from the infected leafof rice, which indicated that they could be used to develop a rapid PCR-based diagnosistechnique for this disease at early stage, and could be applied for quarantine and forecast.4. All the 145 R. solani AGI-IA isolates could induce typical lesions in the four ricecultivars screened, and the pathogenicity parameters showed high variability of virulenceamong the isolates with the disease index from 37.3 to 78.3, and most of the isolates showedhigh virulence, while few of them show lower virulence. The PCR products of 28S rDNAshowed 3 patterns among R. solani AG1-IA isolates, and the patterns showed certaincorrelation with geographical origins and ecological habitats of the isolates, which wasproposed preliminarily that there were different karyotypes among R. solani AG1-IAisolates. These results suggested a high genetic variation potential among R. solani AG1-IAisolates.5. A UPGMA phylogenetic tree of Sclerotium hydrophilum was established based onthe sequence analysis of ITS and 28S rDNA. The results indicated that Sclerotium hydrophilum showed high homology with many Rhizoctonia isolates in the phylogeneticrDNA. Based on these results, it is suggested that Sclerotium hydrophilum should belong toRhizoctonia but not Sclerotium in its taxonomy.6. The genetic variation in R. solani AG1-IA strains after host-pathogen interactionwas studied in terms of vegetative compatibility, protein and DNA. Compared with theparental isolate, the adapted isolates showed differences in certain extent at physiologicaland biochemical levels, and some of the adapted isolates even showed incompatibility withthe parental isolate in the vegetative compatibility test, suggesting that some geneticdifferences had happened among these isolates, and this was further demonstrated byAFLP analysis. These results indicated that the directional selection of rice was theimportant cause to the evolution of the pathogen, and suggested that maybe host-typeincompatible VCGs among the isolates exist under field conditions. It should be useful forcontrolling the disease through reasonable cultivating measures.