Study of plant R genes is one of the key subjects of plant pathology. Cloning and characterization of R genes facilitates the development of research on mechanism of interaction between host and pathogen. This review focuses on the isolation, structure and function, signal transduction pathway and evolution of R genes from recent information about molecular analysis of R genes.

Twenty-four and fifty-six mono ascospore progenies were obtained from two crosses of the strains of rice blast fungus, S1522 with P131 and S159. The strain S1522 belonging to mating type a is avirulent, and P131 and S159 belonging to mating type A are virulent to rice cultivar Tsuyuake. Pathogenicity test showed that ratio of avirulent progenies to virulent ones was approximately 1:1, indicating that a single locus confers the avirulence in S1522 to Tsuyuake. RAPD technique was used to compare difference in DNA polymorphism between the avirulent and virulent progenies. One RAPD marker with about 1 000 bp was found closely linked with the avirulence locus, and genetic distance between the marker and the avirulence locus is about 3.7 cM.

Genes for resistance to leaf rust were detected in 40 wheat cultivars originated from National Adaptability Test of Wheat Varieties in 1998~1999 using 11 isolates of Puccinia recondita f.sp. tritici wihch previously characterized for avirulence or virulence gene combination, respectively. Ten of the 39 test genes (or gene combination) were postulated to be present in 24 cultivars, and unknown resistance gene (or genes) in 11 cultivars. No genes for leaf rust resistance in the seedling stage were found in 5 cultivars. Stability of leaf rust resistance was tested in 40 wheat cultivars using BBB, DHS, PGT and PHT Puccinia triticina pathotypes under 5/10℃, 15/20℃ and 25/30℃ (dark/illumination) temperature ranges, separately. As the results indicated, 15 cultivars of 40 test cultivars displayed stable or basically stable infection type, high temperature resistance was detected in 3 cultivars, low temperature resistance was detected in 2 cultivars, the interactions of pathogen pathotype, wheat cultivars and temperature were detected in 20 rest cultivars. Adult plant resistance of 40 wheat cultivars was identified with 6 Puccinia triticina pathotypes in the field. Twenty one cultivars displayed good resistance in the adult plant stage, among them, slow rusting resistance was displayed in at least 6 cultivars and obvious adult plant resistance was shown in 4 cultivars. It was found that the leaf tip necrosis (LTN) symptoms were shown in 4 cultivars, which probably possessed adult plant resistant gene Lr34. Resistance features of several important Lr genes and their applicability were also discussed in the paper.

Effect of triadimefon seed coating formulation (TriSCF) on the changes of the amount of silicon and other elements in infected and non-infected sites by Erysiphe graminis on epidermal cell of wheat leaves was tested by means of SEM-energy dispersive analysis of X-ray. The results showed that in the epidermal cell of leaves of the wheat seedling coated with TriSCF 20-36 h after inoculation, the amount of silicon in the infection and non-infection sites of Erysiphe graminis increased from 5.204% to 42.064%, 5.821% to 12.342%, respectively. In the untreated seedlings with the same inoculation time, the amount of silicon in the infection site increased from non-testable level to 17.388%, the amount of silicon in non-infection site was 19.460% 36 h after inoculation. The percentage of P, S, Cl, and other elements tested appeared differently in infected and non infected sites of the seedling from above treated and untreated seeds.

A set of common wheat (Triticum aestivum) alien addition and substitution lines involving different Haynaldia villosa chromosomes and five 6VS/6AL translocation lines deve loped by Institute of Cytogenetics, Nanjing Agricultural University, were identified for wheat yellow rust resistance. The resistance test was made by inoculating new races of Puccinia striiformis for three successive years, 1997, 1998 and 1999 in Shaanxi, Beijing and Sichuan. The results indicated that Triticum aestivum-Haynaldia villosa alien addition line 6V, substitution lines 6V(6A) and translocation lines 6VS/6AL were high resistant to new races:CY29, CY31, Hybrid Ⅲ, Shuiyuan 11-2, Shuiyuan 11-5 and Shuiyuan 11-13 with strong virulence. The resistance of Triticum durum-Haynaldia villosa amphiploid involving all 7 chromosomes of V genome excluded the race Shuiyuan 11-13. This might mean that the yellow rust resistance in the T.aestivum-Haynaldia villosa addition line 6V, substitution lines 6V(6A) and translocation lines 6VS/6AL probably related with gene actions of different sources including wheat pa rents involved.

A new bacterial disease in tomato was first found in Jilin, Liaoning,Heilongjiang Provinces in 1998-1999. 23 strains were isolated from stems and leaves of tomato. Inoculation on tomato seedlings with the strains produced the same symptom as naturally infected plants. All 23 isolates were identified as Pseudomonas syringae pv. tomato (Okabe) Young, Dye & Wilkie by pathogenicity, stain reaction, morphological characterization, culturing pattern, physiological and biochemical reactions, and G+C mol%. The new disease was identified as bacterial speck of tomato. When the plants were inoculated, the speck symptoms only occured on tomato, pepper, eggplant, Jimsonweed, and Solanum nigrum, but not on potato and Franchet groungcherry.

Tobacco plants (var.K₃₂₆) were inoculated with three isolates of cucumber mosaic virus, and the changes in the defendant enzyme activity and the membrane permeability of tobacco cells were detected after infection. For PAL, isolate Xb caused the increase of the enzyme activity at first, followed by a decrease. Isolate M caused an increased activity of PAL, while isolate PE resulted in a decrease of the enzyme activity. For POD, isolates PE and M increased POD activity whereas isolate Xb had no effect. For SOD, isolate Xb and M first caused a higher activity of SOD, then a lower one than the control. PE had a reversed result. For PPO, isolate Xb increased the activity of PPO rapidly first, then decreased it, which was exa ctly opposite to the results brought by isolate M. Isolate PE caused a higher activity of PPO all through the infection period. These three isolates all caused the decrease of permeability of cell membranes, but with distinctive patterns. In addition, the concentrations of the soluble proteins in the infected cells were also affected by the infection of the three isolates.

Since the symptoms of the banana bunchy top virus (BBTV) occurred in Guangdong are so similar, it is impossible to identify any existing different strains of BBTV by symptom. One way to solve the problem is to analyze diseased samples collected randomly from different locations in banana-growing regions in the province. Eleven to fifteen samples from each locations of 8 banana-growing regions and a total of 111 samples were collected in the province. BBTV in these diseased samples was each artificially transmitted to healthy banana (Musa nana) tissue culture seedlings via banana black aphids (Pentalonia nigronervosa) in greenhouse. One isolate was selected from each region as the representative isolate. The result of the host range tests showed that the 8 representative isolates could be divided into NSP strain that could infect banana, plantain (M. sapientum) and pisang awake (M. pisang-awake), and NS strain that could infect only the first two but not the later. By means of the EcoRI digestion test, the DNA component 1 of the 87 out of the 111 isolates could not be cleaved. These isolates could be grouped into NSP strain, while 24 out of the 111 isolates (15 isolates from Gaozhou region and 9 out of the 14 isolates from Xinyi) could be cleaved. These isolates could be grouped into NS strain. Between the two strains, the differences were significant on the latent periods in the plantain. They differred, among the four banana varieties. They also differred, in the Guangdong 2 banana, in the rate of virus multiplication and virus translocation and in the time needed to reach the highest concentration in the host.

The effects of low temperature ranging from 20℃ to -4℃ on mycelial growth, the conidial germination of Botrytis cinerea in culture and its infectivity to stored chicory (Cichorium intybus L.) were investigated. Although temperatures below 0℃ could inhibit the mycelial growth and delay the conidial germination, B. cinerea was able to grow and germinate on PDA and to infect chicory even at -4℃. Germination rate reached 100% after 14 days and mycelial diameter was 10 mm after 24 weeks at such temperature. No decay was found in chicory kept at -4℃ before 8 weeks and there was very low disease incidence in chicory stored at -2℃ in 6 weeks of storage. After 12 weeks in storage, disease incidence reached 77% and 71% with a disease severity score of 37 and 31, respectively, which may be related partly to freezing injury of chicory stored at such temperatures.

The effects of the cellulase, pectinase and toxin produced by Cladosporium cucumerinum on the leaf cell of cucumber were studied by TEM and SEM. The results indicated that the three pathogenic factors played independent and symplastic roles. The cellulase and pectinase played leading roles respectively in decomposition of cell wall and plasma membrane. The three factors could all result in plasmolysis and damage vacuoles and endoplasmic reticulum. Chloroplast envelope and lamellae structure were mainly digested by cellulase, whiles, mitochondrion envelope was chiefly digested by the pectinase. Each of the three factors could cause vacuolation in chloroplast and mitochondria. During the process of breach up the structure of epidermis of leaf, the cellulase acted on reticulate region, then the pectinase dissolved pectic layer. Following this, palisade tissue was damaged by the cellulase, pectinase and toxin.Finally, parenchyma was destroyed by cellulase and toxin,successively.

Greenhouse experiments were conducted to explore the interaction between eight potato varieties and two races of Phytophthora infestans (Montague) de Bary. The relationship between horizontal resistance and aggressiveness in the parasitic fitness of interaction between host cultivar and pathogen race was studied. It was found that in the case when no vertical resistance was involved or vertical resistance had been overcomed by virulence genes correspon dingly, the product of relative horizontal resistance coefficient of cultivar and relative aggressiveness value of the race turned to be parasitic fitness.Their relative formula was as following:relative parasitic fitness=horizontal resistance×aggressiveness.

Fifty sexual hybrids were obtained from 3760 single oospore isolates from pairing of two Phytophthora cactorum isolates (AP14M^r-1-52 with resistance to metalaxyl and sensitivity to chloroneb, PK9Cn^r-2-24 with resistance to chloroneb and sensitivity to metalaxyl) which were incubated on SL media for 36 days. Biological characteristics of randomly selected 32 hybrids were tested in growth rate, sexual and asexual reproduction capacity, oospore and zoospore germination rate,pathogenicity, and tolerance towards high temperature. Results were as follow, the 32 hybrids tested grew well on lima bean agar (LBA), 24 of which were the intermediate between their parents in growth rate; most of hybrids tested produced a great number of oospores on LBA or SL,37% of hybrids produced much more oospores than their parents on SL; most of hybrids produced a great number of zoosporangia in vitro, 15 hybrids were the intermediate between their parents in production of zoosporangia; oospores or zoospores of hybrids tested may germinate and create effective single oospore or zoospore isolates; the germination rate of oospores from 22 hybrids were more than 50%; the tested hybrids inoculated on apple fruits were of virulent, 16 of which were of significantly stronger pathogenicity as compared with their parents. These results show that sexual hybrids from homothallic P. cactorum were of strong fitness at population level in vitro, indicating that sexual recombination of different isolates might play a important role in enhancing the diversity of P. cactorum.

To overcome limitations in the morphological identification of Bursaphelenchus species, a molecular approach was used. The internal transcribed spacer 1 (ITS1) of the nuclear ribosomal DNA (rDNA) region was amplified and sequenced. A pairwise comparison of this sequence data revealed that the differences in the ITS1 region (approximately 308 bp) between four B. xylophilus isolates were slight (only 1 bp), but there are marked differences (up to 7 bp) in this region between the B. mucronatus isolates from China (Guangdong) and France. The data also revealed that the differences in the ITS1 region between the two Bursaphelenchus species range from 32 39 bp. Based on these data, the method of extracting the genomic DNA of the individual pinewood nematode and PCR SSCP (single strand conformation polymorphism) analysis were employed for the unequivocal differentiation of the Bursaphelenchus variant from B. xylophilus and B. mucronatus. These methods should provide a valuable tool for sensitive and accurate identification of individual worm of B. xylophilus.

Thirty seven representative isolates of Fusarium oxysporum f.sp. vasinfectum from 24 counties in Xinjiang were all identified as typical race 7 by their pathogenic reactions on the differential hosts. The random amplified polymorphic DNA(RAPD) analysis also showed that these 37 isolates were all extremely similar to 3 isolates known as race 7 in RAPD fingerprints and very close to race 7 in genetic distance, but far different from the known isolates of race 3 and race 8. The results above indicated that physiological race 7 was the most predominant race in Xinjiang at present,but race 3 once distributing in Xinjiang before was not collected in this research.The pathogenicity test,which was carried out on 4 differential cotton hosts added 8 complement cotton hosts,indicated that among race 7 population existed further differentiation of aggressiveness,and that Fusarium oxysporum f.sp. vasinfectum has strong variability and adaptability.

Seeds of cotton (Gossypium hirsutum), either resistant or susceptible cultivar to Fusarium wilt, were sown in soil treated with trifluralin at 2 μg a.i./g soil. The 10 day old seedlings were inoculated with Fusarium oxysporum f.sp. vasinfectum (Fov). Active oxygen species and lipid peroxidation levels in the seedling stems/roots were determined to evaluate their relationship to cotton resistance against the disease. The levels of malondialdehyde (MDA) and the content of unsaturated fatty acid in membrane significantly increased after the inoculation in the seedlings of resistant cultivar, especially in the trifluralin-treated seedlings, showing the enhanced lipid peroxidation. Moreover, the generation rate of superoxide anion (O₂^·), the level of hydrogen peroxide (H₂O₂) and the activity of lipoxygenase (LOX) were also increased markedly in the seedlings mentioned above. But no significant increases in the activities of superoxide dismutase (SOD) and catalase (CAT) were observed in those seedlings. In contrast, the MDA level,the production rate of O₂^·,the content of H₂O₂,and LOX activity in cotton seedlings of susceptible cultivar increased slightly. The content of unsaturated fatty acid in membrane decreased slightly after infection by Fov,while the activities of SOD and CAT increased significantly. The trifluralin treatment, showing effect on inducing cotton resistance to Fusarium wilt,increased the LOX activity,the contents of O₂^· and H₂O₂,and MDA level. The results suggested that the accumulation of active oxygen species (O₂^· and H₂O₂) and the increased activity of LOX may play a role in the resistance of cotton seedlings to Fusarium wilt by initiation of lipid peroxidation.

Kinetics of Xanthomonas oryzae pv. oryzae multiplication in rice suspension cell and effect of various extracellular substances in rice suspension cell on X.oryzae pv. oryzae multiplication was studied. The results indicate that quantity of X.oryzae pv. oryzae multiplication was obviously less in the suspension cell of the incompatible IRBB4 than it was in suspension cells of the compatible IRBB3 or the compatible IR24. X.oryzae pv. oryzae multiplication was not depressed by extracellular crude protein of rice suspension cell. Bihydric phenol of rice suspension cell increased remarkably in incompatible interaction but unremarkably in compatible interaction. The transcription and the translation of PO and PAL gene were detected in incompatible interaction. The result indicated depression of X. oryzae pv. oryzae multiplication in suspension cells of IRBB4 was related to start of rice defence reaction.

Xa21 transgenic rice lines derived from 8 cultivars with different genetic background through the Agrobacterium-mediated transformation system and their generations T 0 and T 1 were inoculated with 12 international differential races and 7 Chinese pathotypes of Xanthomonas oryzae pv. oryzae. Resistance analysis showed that Xa21 gene expressed dominantly, and inherited stably in transgenic progenies with a broad resistance spectrum. The results also revealed that different genetic background showed certain effect on the resistance expression level of Xa21.

The newly occurred corn disease, top rot of corn,caused by Fusarium moniliforme var. subglutinans Wr. & Reink was studied. The pathogen could infect corn, sorghum, sudan grass, columbus grass, foxtail millet, wheat, rice, pearl millet, water grass and crabgrass. The optimum temperature for the pathogen growth in culture was 25-30℃ and was 25-30℃for germination of both microconidia and macroconidia. The fungus grew well when xylose, sucrose, lactose, galactose and inulin was used as carbon sources. Many mediums such as Richard's, PDA, PSA, cornmeal agar, and oatmeal agar were good for the pathogen growth. The incidence of disease was different under different soil conditions and more plants were infected in lowland and garden soil.

Maize dwarf mosaic virus (MDMV) helper component-proteinase (HC-Pro) was purified and the specific antiserum to it was prepared in this experiment. The molecular weight of MDMV HC-Pro was ca. 57 kD and its yield was 0.3 mg/100 g diseased leaves. The titer of antiserum was 1/16 in agar gel double diffusion test and 1/1024 in microprecipitation test. The antiserum reacted strongly with MDMV HC-Pro, but did not with MDMV and healthy maize extracts. It could inhibit transmission efficiency of virus by aphids specifically.

Spore killer gene (SK^k), a genetically abnormal variant in Fusarium section Liseola, was found in this study. More than 95 percent of asci produced by sexual cross of strains YM97 from corn and MH50 from cotton contained 4 spores. Typical 8 spore asci (98%) were got when the cultures from the F 1 progenies backcrossed to strain MH50. It was confirmed that strain MH50 contains a spore killer gene (SK^k).

Plant tissues and their extracts from garlic,onion,celery,clove,wild pepper,rhubarb et al.twenty four species (at Chinese medicinal herbs decocted in distilled water and the other sixteen plants soaked in sterile distilled water) were used to test their effect on encysted zoospore germination (EZG), appressorium formation (AF), penetration hypha formation (PHF) of Phytophthora infestans by cellulose membrane technique. And the inducing effects of the extracts on the formation of penetration structure of P.infestanse were also carried out.On the basis of these tests,the effects of immunizing treatment of seed tubers,inducing the resistance of manure plants,increasing the yield of tuber were primarily discussed.Results showed that, the rates of EZG on the surfaces of twelve species of plant tissues were remarkably lower than those in control, appressorium and penetration hypha did not appear on the surfaces of six and ten species of plant tissues, respectively. In plant extracts of thirteen species, the rates of EZG were notable lower than those in control, the appressorium and penetration hypha could not be formed in six and seven species of plant extracts, respectively. Eight species of plant extracts were selected as the inducers to induce resistance of potato tuber slices and seed tubers against P.infestans. The immunization treatment of tuber slices with eight kinds of inducers could not only keep the tuber slices fresh for some time, but strengthen the ability of tuber slices to inhibit the formation of penetration structure of P.infestans. The rates of EZG, AF, and PHF of P.infestans on the surfaces of tuber slices treated were remarkably lower than those in control. The seed tubers treated with eight kinds of plant extracts were much better than control in budding, early growth, disease resistance, and tuber yield. And the disease protection was up to 54%, the yield increase was up to 31%.

The dynamic changes in content of malonaldehyde (MDA), superoxide dismutase (SOD) and catalase (CAT) caused by infection Fusarium oxysporum Schlecht.in root cells of watermelon at seedling stage were studied. After infection, Kelunsheng, a resistant cultivar, showed a lower increasing rate of MDA content in cells of seedling than Zaohua, a susceptible cultivar. The resistant cultivar showed a lower decrease rate of SOD activity than the susceptible. Smaller change in CAT activity occurred in the resistant than the susceptible. The resistant cultivar, Kelunsheng, resumed basic normal metabolisms of MDA, SOD and CAT within a shorter time (48 hr). It demonstrated that Kelunsheng possessed a significantly larger self regulating ability than Zaohua.

A papaya (Carica papaya L.) mutant plant (M₁) with tolerance to papaya ringspot potyvirus (PRSV) was selected from the first mutated generation population after ⁶⁰Coγ ray irradiating seeds. A part of plants (VM₁) propagated vegetatively from the lateral shoots of M₁ plant still showed tolerance to PRSV. Back cross was performed using VM₁ as maternal parent and the seedlings (BM₂) were developed from the seeds of individual fruit. The resistance of BM₂ plants was evaluated by artificial inoculation with PRSV Ys, Vb and Sm strains. The BM₂ populations showed that their resistance and susceptibility to PRSV Ys and Vb strains were in the ratio of one to one. But none was found to be resistant to Sm strain in tested BM₂ populations. The segregation ratio in BM₃ population derived from the back cross of BM₂ resistant plants as maternal parent fit the theoretical value (1:1). The results suggested that γ ray irradiation might has induced a single dominant gene resisting to PRSV Ys and Vb, named Rys. Because the segregation ratio of resistant and susceptible plants to PRSV Ys in M 3 population derived from BM₂ resistant plant self pollination did not fit the theoretical value (3:1) of single dominant gene character, a part of male gametes with mutated gene were thought to be excluded during their development and fertilization. Using bulked segregant analysis (BSA), a RAPD marker significantly related with the resistance was identified and will be of great use for aiding resistance screening in papaya breeding. Rys is the first PRSV resistant gene found in papaya cultivar.

Genetic diversities of 15 isolates of Botryosphaeria berengeriana and one of Macrophoma were analyzed with random amplified polymorphic DNA (RAPD). Two hundred and twenty loci were obtained with 14 primers. The result of cluster analysis indicated that the 16 isolates could be divided into 3 groups. The first group included 8 isolates causing apple ring rot and pear ring rot. The second group included 7 isolates causing apple Botryosphaeria canker, peach gummosis and hawthorn ring rot. The third group included the isolate of Macrophoma. The genetic relationships between the pathogen of apple ring rot and that of apple Botryosphaeria canker were very close, but marked difference also existed at molecular level.

Studies showed that extracellular secretions of Xanthomonas citri had certain abilities to regulate the interaction between the bacterium and Seedwax, a fungicide. The secretions, especially extracellular polysaccharide (EPS), could strongly antagonize the inhibition of Seedwax. Output of electrolyte, EPS, extracellular protein, and bioactivity of extracellular hydrolytic enzymes are regularized to vary with the concentration of Seedwax. The results indicated that the pathogenic bacteria possess an active echo to the adverse environment by chan ging the attributions of extracellular secretions when surrounded by fungicides.

We introduce the idea of using Chinese traditional herbal medicines as alternatives to the nematicides currently used in agriculture for the management of plant parasitic nematodes.We tested 58 of the approximately 500 herbal remedies documented to kill worms in humans,or insects, for their effects on Meloidogyne javanica and Pratylenchus vulnus.Twenty two of the remedies killed M.javanica juveniles.Twenty six of the remedies strongly affected P.vulnus, and nine were toxic.In many cases the herbal medicines had a narcotic effect and the nematodes recovered activity when placed in water.The narcotic effect varied with medicines and increased with time of exposure.

Meloidogyne graminicola was described and illustrated from roots of Allium tistulosum in Sanya, Hainan Province of China.This root knot nematode was characterized by the followings:Female perineal pattern dorsoventrally ovoid to rounded, striae usually fine and smooth, occationally broken by short and irregular striae, phasmids small but distinct, closely spaced. Male head region low and wide, smooth or with two indistinct annules. Second stage juvenile tail with a long and narrow hyaline terminus. The terminus clavate. M.graminicola was a new record species in China. Its host plant Allium tistulosum belongs to Liliaceae that is first reported into the host plant families of M. graminicola.

The bacterial spot on Nai plum caused by Xanthomonas arboricola pv. pruni is an important disease in Hunan Province, causing a considerable yield loss. Studies were carried out on this disease, such as field investigation on occurrence and development and integrated control. In this paper we report the fine structure study of the diseased tissues.

Rice blast, caused by Magnaporthe grisea, is one of the most important crop diseases in China. Resistance break down often happens, especially firstly occurs in those fields supplied with a high dose of nitrogen fertilizer. In the simulation study on race dynamics and cultivar race interaction on population level, the influence of nitrogen application on the relative parasitic fitness of cultivar race combinations must be realized as quantitatively as possible.
Greenhouse experiment pertaining to seedlings of 6 cultivars(cv)×3 races×7 levels of nitrogen(N) applications×3 replications and field experiment pertaining to 4 cv×3 races×3 doses of N applications (corresponding to 0,105, and 210 kg of pure N per hectare)×3 replications were conducted in 1998. In greenhouse experiment, 10 days after inoculation by spraying equal volume of spore suspension, number of lesions on unit leaf area, infection probability and area of sporulation per lesion were recorded. In field experiment, equal amount of spore suspension was inoculated onto the 10 leaves in the center point of each plot(1m×1m, one cv) and disease index was recorded for each plot every 5 days after disease apparition.
Two main points were showed by the results. Firstly, the pattern of response of disease intensity to the amount of N application appeared similar to a parabola, disease intensity increasing with the increasing N from very low to medium high but decreasing with further increasing N. Secondly, the influence of amount of N application on relative parasitic fitness mightbeveryslightorevendinkyinmanycultivar racecombinations.Basedontheresults mentioned above, it is obvious that N application must be comprised in the construction of the simulation model for rice blast because it is an important factor influencing the rate of pathogen multiplication no less than meteorological ones. Furthermore, at present the estimation of relative parasitic fitness of cultivar race combinations would be better undertaken under moderate to high amount of N supply in order to reveal the highest range of F value that may leads to cultivar resistance break down. As for how and why different cultivar race combinations react to N application differently in manifesting parasitic fitness, further studies are needed.

16 strains of hrp-mutants were obtained from wild type JXOⅢ of Xanthomonas oryzae pv. oryzae (Xoo) by using diethyl sulfate (DES) as a mutagenic chemical. All the mutants lost their pathogenicity on susceptible rice, IR26 and Shanyou 63, and were unable to trigger hypersensitive response (HR) on nonhost plant, tobacco. The obtained hrp-mutants did not show xanthomonadin deficiency. Extracellular enzyme (amylase,pectase lyase,protease,cellulase and lipase) activities of all the hrp-mutants were similar to those of the wild type.The composition analyse of the pathogenic bacteria demonstrated that lipopolysaccharide (LPS) is not the signal substance to induce HR on tobacco. Four hrp-mutants, JCM2092,JCM2211,JCM2252 and JCM2457, were due to mutagenesis in the type Ⅲ secretion pathway while other 12 hrp-mutants were not since the substance triggering HR on tobacco was not detected in their sonicated cells and supernatants. 1.1 kb SacI KpnI fragment from hrp gene clone, pUHRX245, from JXOⅢ gene library can restore the ability of HR elicitation on tobacco to hrp-mutants, JCM0066 and JCM2482, but not the pathogenicity to hrp-mutants.

Preinoculation of roots and foot of rice seedling,cultivar IR26 with a nonpathogenic binucleate Rhizoctonia (BNR) species isolate(232 CG)induced resistance to rice sheath blight caused by R.solani. A significant reduction of disease severity was observed on plant with BNR treatment compared with the non treat seedlings.Treatment with BNR at least 24 h prior to pathogen challenge resulted in significant protection of rice seedlings from R.solani infection.Protected rice seedlings showed enhanced resistance to bacterial blight caused by Xanthomonas oryzae pv. oryzae (OS 14) and bacterial streak caused by X.oryzae pv.oryzicola (RS 105) as well.Remarkable increases in activities of phenylalanine ammonia lyase(PAL) and peroxidases(POX),the key enzymes in plant defense responses,were observed in BNR treated rice seedlings.The elevated levels of PAL and POX were positively correlated with the increase in disease resistance in rice induced by BNR.

Bioactivity and stability of purified 90 kD extracellular elicitor protein from Phytophthora boehmeriae were studied. The elicitor could induce hypersensitive responses (HR) on tobacco leaves at different doses ranging from 0.5 nmol/L to 100 nmol/L but not at 100 pmol/L, which indicated that the minimal efficient dosage of inducing HR on tobacco was between 100 pmol/L and 0.5 nmol/L. The elicitor could also induce HR on other cultivars of Nicotiana tabacum butnotonotherSolanaceae (eggplant and pepper)oroncotton(Gossypium arboreum), which suggested that this 90 kD elicitor was host-specific. When the 90 kD elicitor was infiltrated into the tobacco leaves, it could induce the systemic acquired resistance (10%~68%) to P. nicotianae that was inoculated on the tobacco stem. The highest level of induced resistance was acquired when tobacco stems were inoculated immediately after its leaves being treated with elicitor at different pH (pH2 pH14) for 30 minutes or at different temperature (4℃, 25℃, 60℃, 100℃) for 5 minutes, the elicitor could maintain its activity of inducing HR. However,the treatment with proteinase K could cause the loss of its ability of inducing HR in tobacco. Results showed that the elicitor was not sensitive to acid, alkali or temperature, but was sensitive to proteinase K.

The CP gene of a Chinese isolate of barley yellow dwarf virus (BYDV) PAV was cloned by using RT PCR. Sequence analysis shows that the CP gene contains 600 nucleotides encoding 199 amino acids. Nucleotide similarity with other reported PAV isolates is about 81%, higher than that with other strains. Deduced amino acid similarity is also higher than other strains. Compared with other PAV isolates, the difference is mainly at 46th to 60th amino acid.