Rice root-knot nematode (Meloidogyne graminicola) infestation in rice

Rice (Oryza sativa) is an important staple food crop for majority of human population in the world in general and in Asia in particular. However, among various pests and diseases which constitute important constraints in the successful crop production, plant parasitic nematodes play an important role and account for yield losses to the extent of 90%. The major nematode pests associated with rice are Ditylenchus angustus, Aphelenchoides besseyi, Hirschmanniella spp., Heterodera oryzicola and Meloidogyne graminicola. However, rice root-knot nematode (M. graminicola) happens to be the most important pest and is prevalent in major rice producing countries of the world. In India, the distribution of M. graminicola in rice growing areas of different states has been documented in nematode distribution atlas prepared by All India Coordinated Research Project (Nematodes) and published by Directorate of Information and Publications of Agriculture, Indian Council of Agricultural Research, New Delhi, India during 2010. M. graminicola affected rice plants show stunting and chlorosis due to the characteristic terminal swellings/galls on the roots which ultimately result in severe reduction in growth and yield. Number of eco-friendly management technologies against M. graminicola have been developed and demonstrated, including the use of bioagents for minimising the losses due to rice root-knot nematode. This review is focused on collating information to understand the current scenario of rice root-knot nematodes with greater emphasis on its ecological requirements, damage symptoms, biology, morphology, host range and management strategies.     

Biological control of rice root-knot nematode,Meloidogyne graminicola through mixture of Pseudomonas fluorescens strains

The plant growth promoting rhizobacterium, Pseudomonas fluorescens strains PF1, TDK1, and PY15 were evaluated individually and in combinations for their efficacy against root-knot nematode, Meloidogyne graminicola, in rice plants under in vitro, glass house and field conditions. Culture filtrates of these strains either individually or as mixture inhibited egg hatching and caused mortality of juveniles of M. graminicola in vitro. The efficacy was more pronounced when filtrates of the strain were used as mixtures than as individual strains. Mixtures of P. fluorescens strains signficantly reduced M. graminicola infestation when applied as bacterial suspensions through seed treatment. The higher activity of peroxidase and chitinase enzymes was observed in plants treated with P. fluorescens mixtures than the plants treated with individual strains, two strain mixtures and untreated control. In field trials on rice, talc formulations of the P. fluorescens strains individually as well as mixtures were evaluated as seed treatment, soil treatment and combination of both. A mixture of the three strains was the most effective when applied either as seed + soil treatment or as seed treatment alone. The introduced P. fluorescens strains survived endophytically on rice roots. The application of the P. fluorescens mixture PF1 + TDK1 + PY15 in seed + soil treatment resulted in higher grain yield which provided a 27.3% increase over the control followed by P. fluorescens mixture PF1 + TDK1 + PY15 in seed treatment alone, which increased the grain yield of rice by 24.7% compared to the control.     

Efficacy of Pseudomonas fluorescens and Paecilomyces lilacinus against Meloidogyne graminicola infecting rice under system of rice intensification

The talc-based formulations of the plant growth promoting rhizobacterium, Pseudomonas fluorescens and egg parasitic fungi, Paecilomyces lilacinus, were evaluated as seed treatment, soil application and combination of both for the management of M. graminicola in fields of rice grown under system of rice intensification. Both the bioformulations significantly reduced the root invasion and soil populations of M. graminicola but P. fluorescens was most effective when applied as seed cum soil application and seed treatment alone. Effect of these treatments was comparable with the standard chemical carbofuran application. The introduced P. fluorescens survived significantly in rice roots when applied as seed cum soil application and seed application alone than as soil application. There was significant increase in phenol, peroxidase and chitinase accumulation in plants treated with P. fluorescens. Application of bioagents had positive influence on growth parameters such as plant height, root length, shoot weight, root weight and number of tillers per hill. Application of P. fluorescens as seed cum soil treatment resulted in higher grain yield, which was 20.6%–26.9% increase over control followed by P. fluorescens as seed treatment alone that increased grain yield of rice by 10.7%–11.2% than control. However, economic returns per investment was higher when P. fluorescens was applied as seed treatment alone (1:8.8–1:12.0 incremental cost benefit ratio) followed by the P. fluorescens as seed cum soil treatment (1:6.2–1:9.7 incremental cost benefit ratio).     

Physiological and genetic mapping study of tolerance to root-knot nematode in rice

The root-knot nematode Meloidogyne graminicola is an obligate biotrophic parasite and a major pest of rice (Oryza sativa) for which resistant varieties are not currently available. Quantitative trait loci (QTLs) for partial resistance to M. graminicola were identified using a mapping population based on two rice varieties, Bala x Azucena. Experiments were carried out to investigate the interactions between M. graminicola and these two varieties in terms of nematode establishment, reproduction and effect on rice yield. Nematode establishment was also assessed in the mapping population. Meloidogyne graminicola consistently caused more galling and had higher reproductive success in Azucena than in Bala. M. graminicola did not significantly reduce yield in Bala, but caused a yield reduction of almost half in Azucena, suggesting that the partial resistance to nematode establishment was related to nematode tolerance. A total of six significant or putative QTLs for nematode tolerance were detected. For two of the QTLs detected, Azucena was the donor of the tolerance alleles, suggesting it may be possible to breed plants with greater tolerance than Bala.     

Host response of rice genotypes to the rice root-knot nematode (Meloidogyne graminicola) under aerobic soil conditions

Aerobic rice is a production system where adapted rice varieties are established via direct seeding in non-puddled, non-flooded, non-saturated fields and grown under conditions similar to upland conditions. On land cultivated continuously with aerobic rice, a yield reduction has been observed. The rice root-knot nematode Meloidogyne graminicola is considered one of the possible causes of these yield reductions. Resistance to and tolerance for M. graminicola are essential traits for aerobic rice cultivars in alleviating this problem. In our study, the host response of nineteen aerobic, seven upland and four lowland rice genotypes which are either being used in the International Rice Research Institute’s aerobic rice breeding programme or already cultivated by farmers in Asia was evaluated under aerobic soil conditions in an outdoor raised-bed experiment. Our study showed a large variation in susceptibility and sensitivity to M. graminicola infection among the rice genotypes examined. Resistance comparable to the resistant reference genotypes included in the experiment (CG14, TOG5674, TOG7235) was not found but in terms of susceptibility, the upland genotype Morobereken may be an interesting less-susceptible genotype. Tolerance was found and in terms of sensitivity, the high yielding aerobic genotype IR78877-208-B-1-2 may be an interesting tolerant genotype. Our study also allowed the identification of rice genotypes that are either highly susceptible or sensitive to M. graminicola infection and of which the cultivation should be discouraged. On average, M. graminicola caused an almost 30% reduction in yield. Excluding the two susceptible and three resistant reference genotypes included in the experiment, most affected was dry-shoot biomass (23.6% reduction) followed by root length, which was more affected than fresh-root weight (19.8 vs. 8%) and grain filling (17.3%), while plant height and the number of spikelets/panicle were less affected (10.2 and 8.1%, respectively). Neither tillering nor the number of panicles/plant were affected.     

Host response of Oryza glaberrima and O. sativa rice genotypes to the rice root-knot nematode Meloidogyne graminicola in a hydroponic system under growth chamber

The host response of 25 rice genotypes belonging to Oryza glaberrima and Oryza sativa to Meloidogyne graminicola infection was examined in a hydroponic system. The M. graminicola can build up high population densities in a hydroponic system. Resistance to this nematode species was found in O. glaberrima genotypes which supported significantly lower nematode numbers per plant and per unit root than O. sativa genotypes. The M. graminicola-infected O. sativa genotypes showed a higher root galling index than the O. glaberrima genotypes. The hydroponic system is efficient and reliable method to examine the host response of rice genotypes to M. graminicola infection, and can be useful for the fast screening of high numbers of rice genotypes for the selection of M. graminicola-resistant rice germplasm for breeding purposes.     

Differential expression of root-knot nematode resistance genes in tomato and pepper: evidence with Meloidogyne incognita virulent and avirulent near-isogenic lineages

Reproduction of artificially selected near isogenic Meloidogyne incognita lineages virulent and avirulent against the Mi resistance gene of tomato was assessed on host and resistant lines and cultivars of pepper. Egg mass production following inoculation of individual potted seedlings with second-stage juveniles was studied in experiments conducted in controlled environment. Artificially selected Mi-virulent nematode populations were unable to develop on resistant pepper lines PM 217 and PM 687. This suggests that the genetic systems governing resistance to root-knot nematodes are differently expressed in tomato and pepper, in spite of the very close phylogenetic relationships and structural genomic homologies occurring between these two vegetable crops. Moreover, these artificially selected nematode populations were also found unable to develop on the susceptible pepper cultivars California Wonder and Doux Long des Landes, while their pathogenicity was not significantly affected on susceptible tomatoes. Due to the existence of naturally virulent Meloidogyne populations, these results enhance the need for a better understanding of the mechanisms involved, in order to develop new forms of management of plant resistance to root-knot nematodes.     

Induced systemic resistance (ISR) against pathogens in the context of induced plant defences

Induced systemic resistance (ISR) of plants against pathogens is a widespread phenomenon that has been intensively investigated with respect to the underlying signalling pathways as well as to its potential use in plant protection. Elicited by a local infection, plants respond with a salicylic-dependent signalling cascade that leads to the systemic expression of a broad spectrum and long-lasting disease resistance that is efficient against fungi, bacteria and viruses. Changes in cell wall composition, de novo production of pathogenesis-related-proteins such as chitinases and glucanases, and synthesis of phytoalexins are associated with resistance, although further defensive compounds are likely to exist but remain to be identified. In this Botanical Briefing we focus on interactions between ISR and induced resistance against herbivores that is mediated by jasmonic acid as a central signalling molecule. While many studies report cross-resistance, others have found trade-offs, i.e. inhibition of one resistance pathway by the other. Here we propose a framework that explains many of the thus far contradictory results. We regard elicitation separately from signalling and from production, i.e. the synthesis of defensive compounds. Interactions on all three levels can act independently from each other.     

Herbivore-induced rice resistance against rice blast mediated by salicylic acid

In agro-ecosystems,plants are important mediators of interactions between their associated herbivorous insects and microbes,and any change in plants induced by one species may lead to cascading effects on interactions with other species.Often,such effects are regulated by phytohormones such as jasmonic acid(JA)and salicylic acid(SA).Here,we investigated the tripartite interactions among rice plants,three insect herbivores(Chilo suppressalis,Cnaphalocrocis medinalis or Nilapai-vata lugens),and the causal agent of rice blast disease,the fungus Magnaporthe oryzae.We found that pre-infestation of rice by C.suppressalis or N.lugens but not by C.medinalis conferred resistance to M.oryzae.For C.suppressalis and N.lugens,insect infestation without fungal inoculation induced the accumulation of both JA and SA in rice leaves.In contrast,infestation by C.medinalis increased JA levels but reduced SA levels.The exogenous application of SA but not of JA conferred resistance against M.oryzae.These results suggest that preinfestation by C suppressalis or N.lugens conferred resistance against M.oryzae by increasing SA accumulation.These findings enhance our understanding of the interactions among rice plant,insects and pathogens,and provide valuable information for developing an ecologically sound strategy for controlling rice blast.     

RAPD linkage map of the genomic region encompassing the root-knot nematode (Meloidogyne javanica) resistance locus in carrot

Inheritance studies have indicated that resistance to the root-knot nematode (Meloidogyne javanica) in carrot inbred line ’Brasilia-1252’ is controlled by the action of one or two (duplicated) dominant gene(s) located at a single genomic region (designated the Mj-1 locus). A systematic search for randomly amplified polymorphic DNA (RAPD) markers linked to Mj-1 was carried out using bulked segregant analysis (BSA). Altogether 1000 ten-mer primers were screened with 69.1% displaying scorable amplicons. A total of approximately 2400 RAPD bands were examined. Four reproducible markers (OP-C2₁₇₀₀, OP-Q6₅₀₀, OP-U12₇₀₀, and OP-AL15₅₀₀) were identified, in coupling-phase linkage, flanking the Mj-1 region. The genetic distances between RAPD markers and the Mj-1 locus, estimated using an F₂ progeny of 412 individuals from ’Brasilia 1252’×’B6274’, ranged from 0.8 to 5.7 cM . The two closest flanking markers (OP-Q6₅₀₀ and OP-AL15₅₀₀) encompassed a region of 2.7 cM . The frequency of these RAPD loci was evaluated in 121 accessions of a broad-based carrot germplasm collection. Only five entries (all resistant to M. javanica and genetically related to ’Brasilia 1252’) exhibited the simultaneous presence of all four markers. An advanced line derived from the same cross, susceptible to M. javanica but relatively resistant to another root-knot nematode species (M. incognita), did not share three of the closest markers. These results suggest that at least some genes controlling resistance to M. incognita and M. javanica in ’Brasilia 1252’ reside at distinct loci. The low number of markers suggests a reduced amount of genetic divergence between the parental lines at the region surrounding the target locus. Nevertheless, the low rate of recombination indicated these markers could be useful landmarks for positional cloning of the resistance gene(s). These RAPD markers could also be used to increase the Mj-1 frequency during recurrent selection cycles and in backcrossing programs to minimize ’linkage drag’ in elite lines employed for the development of resistant F₁ hybrids. Received: 22 June 1999 / Accepted: 6 July 1999     

Jasmonic acid and salicylic acid activate a common defense system in rice

Jasmonic acid (JA) and salicylic acid (SA) play important roles in plant defense systems. JA and SA signaling pathways interact antagonistically in dicotyledonous plants, but, the status of crosstalk between JA and SA signaling is unknown in monocots. Our rice microarray analysis showed that more than half of the genes upregulated by the SA analog BTH are also upregulated by JA, suggesting that a major portion of the SA-upregulated genes are regulated by JA-dependent signaling in rice. A common defense system that is activated by both JA and SA is thus proposed which plays an important role in pathogen defense responses in rice.     

Contribution of salicylic acid glucosyltransferase, OsSGT1, to chemically induced disease resistance in rice plants

Systemic acquired resistance (SAR), a natural disease response in plants, can be induced chemically. Salicylic acid (SA) acts as a key endogenous signaling molecule that mediates SAR in dicotyledonous plants. However, the role of SA in monocotyledonous plants has yet to be elucidated. In this study, the mode of action of the agrochemical protectant chemical probenazole was assessed by microarray-based determination of gene expression. Cloning and characterization of the most highly activated probenazole-responsive gene revealed that it encodes UDP-glucose:SA glucosyltransferase (OsSGT1) , which catalyzes the conversion of free SA into SA O- β-glucoside (SAG). We found that SAG accumulated in rice leaf tissue following treatment with probenazole or 2,6-dichloroisonicotinic acid. A putative OsSGT1 gene from the rice cultivar Akitakomachi was cloned and the gene product expressed in Escherichia coli was characterized, and the results suggested that probenazole-responsive OsSGT1 is involved in the production of SAG. Furthermore, RNAi-mediated silencing of the OsSGT1 gene significantly reduced the probenazole-dependent development of resistance against blast disease, further supporting the suggestion that OsSGT1 is a key mediator of development of chemically induced disease resistance. The OsSGT1 gene may contribute to the SA signaling mechanism by inducing up-regulation of SAG in rice plants.     

Role of cyanide production by Pseudomonas fluorescens CHA0 in the suppression of root-knot nematode, Meloidogyne javanica in tomato

Summary Pseudomonas fluorescens strain CHA0 produces hydrogen cyanide (HCN), a secondary metabolite that accounts largely for the biocontrol ability of this strain. In this study, we examined the role of HCN production by CHA0 as an antagonistic factor that contributes to biocontrol of Meloidogyne javanica, the root-knot nematode, in situ. Culture filtrate of CHA0, resulting from 1/10-strength nutrient broth yeast extract medium amended with glycine, inhibited egg hatch and caused mortality of M. javanica juveniles in vitro. The bacterium cultured under high oxygen-tension conditions exhibited better inhibitory effects towards nematodes, compared to its cultivation under excess oxygen situation. Growth medium amended with 0.50 or 1.0 mM FeEDDHA further improved hatch inhibition and nematicidal activity of the strain CHA0. Strain CHA77, an HCN-negative mutant, failed to exert such toxic effects, and in this strain, antinematode activity was not influenced by culture conditions. Exogenous cyanide also inhibited egg hatch and caused mortality of M. javanica juveniles in vitro. Strains CHA0 or CHA77 applied in unsterilized sandy-loam soil as drench, caused marked suppression of root-knot disease development incited by M. javanica in tomato seedlings. However, efficacy of CHA77 was noticeably lower compared to its wild type counterpart CHA0. An increased bioavailability of iron following EDTA application in soil substantially improved nematode biocontrol potential of CHA0 but not that of CHA77. Soil infestation with M. javanica eggs resulted in significantly lower nematode population densities and root-knot disease compared to the juveniles used as root-knot disease-inducing agents. Strain CHA0 significantly suppressed nematode populations and inhibited galling in tomato roots grown in soil inoculated with eggs or juveniles and treated with or without EDTA. Strain CHA0 exhibited greater biocontrol potential in soil inoculated with eggs and treated with EDTA. To demonstrate that HCN synthesis by the strain CHA0 acts as the inducing agent of systemic resistance in tomato, efficacy of the strain CHA0 was compared with CHA77 in a split root trial. The split-root experiment, guaranteeing a spatial separation of the inducing agent and the challenging pathogen, showed that HCN production by CHA0 is not crucial in the induction of systemic resistance in tomato against M. javanica, because the HCN-negative-mutant CHA77 induced the same level of resistance as the wild type but exogenous cyanide in the form of KCN failed to trigger the resistance reaction. In the root section where both nematode and the bacterium were present, strain CHA0 reduced nematode penetration to a greater extent than CHA77, suggesting that for effective control of M. javanica, a direct contact between HCN-producing CHA0 and the nematode is essential.     

Green leaf volatiles and jasmonic acid enhance susceptibility to anthracnose diseases caused by Colletotrichum graminicola in maize

Colletotrichum graminicola is a hemibiotrophic fungus that causes anthracnose leaf blight (ALB) and anthracnose stalk rot (ASR) in maize. Despite substantial economic losses caused by these diseases, the defence mechanisms against this pathogen remain poorly understood. Several hormones are suggested to aid in defence against C. graminicola, such as jasmonic acid (JA) and salicylic acid (SA), but supporting genetic evidence was not reported. Green leaf volatiles (GLVs) are a group of well-characterized volatiles that induce JA biosynthesis in maize and are known to function in defence against necrotrophic pathogens. Information regarding the role of GLVs and JA in interactions with (hemi)biotrophic pathogens remains limited. To functionally elucidate GLVs and JA in defence against a hemibiotrophic pathogen, we tested GLV- and JA-deficient mutants, lox10 and opr7 opr8, respectively, for resistance to ASR and ALB and profiled jasmonates and SA in their stalks and leaves throughout infection. Both mutants were resistant and generally displayed elevated levels of SA and low amounts of jasmonates, especially at early stages of infection. Pretreatment with GLVs restored susceptibility of lox10 mutants, but not opr7 opr8 mutants, which coincided with complete rescue of JA levels. Exogenous methyl jasmonate restored susceptibility in both mutants when applied before inoculation, whereas methyl salicylate did not induce further resistance in either of the mutants, but did induce mutant-like resistance in the wild type. Collectively, this study reveals that GLVs and JA contribute to maize susceptibility to C. graminicola due to suppression of SA-related defences.     

镍处理对水稻叶片H_2O_2 积累和抗病的诱导效应

2 .2 0mmol/L的硝酸镍处理水稻幼苗后第 3天用稻白叶枯菌 (Xanthomonasoryzaepv .oryzae)挑战接种 ,硝酸镍处理的稻苗病情比对照明显减轻 ,并且叶片中过氧化物酶 (POD)活性上升 ,过氧化氢酶 (CAT)和抗坏血酸过氧化物酶 (APX)活性明显下降 ,H2 O2 和丙二醛 (MDA)含量显著增加。这些结果表明 ,H2 O2 积累与镍诱导的抗病作用密切有关     

水稻抗稻瘟病天然免疫机制及抗病育种新策略

稻瘟病是水稻最严重的病害之一,由子囊菌(Magnaporthe oryzae)引起。利用抗病品种是防治稻瘟病最经济、最有效的措施。近年来,稻瘟病已发展为研究植物与病原真菌分子互作机制的模式系统,在水稻与稻瘟菌互作和寄主抗性分子生物学、基因组学和蛋白组学等领域取得了一系列重要的研究成果。文章综述了近年来水稻抗稻瘟病两种天然免疫机制,即病原菌相关分子模式诱导和效应蛋白诱导的抗病机制研究的最新进展,讨论了GWAS、TALLEN、CRISPR和HIGS等基因组研究新方法和新技术在水稻抗病育种中的应用,并对目前稻瘟病抗性机制研究和抗病育种中的问题和挑战进行了探讨和展望。