Vegetative compatibility in the entomopathogen Verticillium lecanii

Expression of exogenous dsRNA by Lecanicillium attenuatum enhances its virulence to Dialeurodes citri

BACKGROUND

Dialeurodes citri is an important pest in citrus-producing areas of the world. Lecanicillium attenuatum parasitizes D. citri and kills it, suggesting a potential approach for the biological control of pests. However, the low virulence of the fungus and its slow rate of killing have limited its commercial competitiveness. The objective reason for these disadvantages is immunological rejection by the host. Our strategy was to use fungi to express the double-stranded RNA (dsRNA) of the host immune genes. The fungal hyphae release siRNA at the time of infection, thus interfering with the expression of immune genes in the host and facilitating fungal invasion.

RESULTS

We selected prophenoloxidase (DcPPO), prophenoloxidase-activating factor (DcPPO-AF), and lysozyme (DcLZM) as target genes to construct intron-splicing hairpin RNA expression vectors and to successfully obtain transgenic fungi. Two days after infection, the immune genes of D. citri showed varying degrees of silencing compared with those in the positive control group. The median lethal concentration (LC₅₀ ; spores mL^-1 ) values of La::GFP, La::DcPPO, La::DcPPO-AF, and La::DcLZM were 9.63 × 10⁴ , 2.66 × 10⁴ , 1.21 × 10⁵ , and 3.31 × 10⁴ , respectively. The 50% lethal time (LT₅₀ ) values of these fungi were 5.15, 3.60, 5.34, and 4.04 days, respectively. The virulence of La::DcPPO and La::DcLZM increased 3.62- and 2.91-fold, respectively, and their LT₅₀ decreased by 30.10% and 21.55%, respectively.

CONCLUSIONS

The results indicate that this method, which uses tens of thousands of hyphae to inject dsRNA to improve the virulence of transgenic fungi, can play a greater role in the prevention and control of pests in the future. © 2018 Society of Chemical Industry.

Identification of Immunity-Related Genes in Dialeurodes citri against Entomopathogenic Fungus Lecanicillium attenuatum by RNA-Seq Analysis

Dialeurodes citri is a major pest in citrus producing areas, and large-scale outbreaks have occurred increasingly often in recent years. Lecanicillium attenuatum is an important entomopathogenic fungus that can parasitize and kill D. citri. We separated the fungus from corpses of D. citri larvae. However, the sound immune defense system of pests makes infection by an entomopathogenic fungus difficult. Here we used RNA sequencing technology (RNA-Seq) to build a transcriptome database for D. citri and performed digital gene expression profiling to screen genes that act in the immune defense of D. citri larvae infected with a pathogenic fungus. De novo assembly generated 84,733 unigenes with mean length of 772 nt. All unigenes were searched against GO, Nr, Swiss-Prot, COG, and KEGG databases and a total of 28,190 (33.3%) unigenes were annotated. We identified 129 immunity-related unigenes in transcriptome database that were related to pattern recognition receptors, information transduction factors and response factors. From the digital gene expression profile, we identified 441 unigenes that were differentially expressed in D. citri infected with L. attenuatum. Through calculated Log₂Ratio values, we identified genes for which fold changes in expression were obvious, including cuticle protein, vitellogenin, cathepsin, prophenoloxidase, clip-domain serine protease, lysozyme, and others. Subsequent quantitative real-time polymerase chain reaction analysis verified the results. The identified genes may serve as target genes for microbial control of D. citri.

Genetic, morphological, and virulence characterization of the entomopathogenic fungus Verticillium lecanii

In order to clarify relationships among genetic diversity, virulence, and other characteristics of conidia, 46 isolates of Verticillium lecanii from various hosts and geographical locations were examined. The internal transcribed spacer (ITS) and intergenic spacer (IGS) regions of ribosomal DNA (rDNA), mitochondrial small subunit rDNA (mt-SrDNA) and beta-tubulin were analyzed by PCR-RFLP. PCR-single stranded conformational polymorphism (SSCP) was performed on regions of the mitochondrial large subunit rDNA, mt-SrDNA, beta-tubulin and histone 4. There were no relationships among the results of RFLP, SSCP, isolation source, and location. However, amplified product size of IGS did have relationships with conidia size and sporulation. Six isolates with 4.0-kb IGS products had large conidia dimensions, and yielded low numbers of conidia compared with other isolates. Three out of the six isolates were high virulence (over 90%) against green peach aphids. Furthermore, double-stranded RNA (dsRNA) was detected in 22 out of 35 V. lecanii isolates and related with the amplicon sizes of IGS, though not with virulence or isolation location. Isolates containing dsRNA were divided into six distinct types based on banding pattern. These data demonstrate the level of genetic diversity of V. lecanii, and suggest relations among the genetic properties and conidial morphology.

Vegetative Compatibility Groups in Colletotrichum coccodes, the Causal Agent of Black Dot on Potato

ABSTRACT Black dot of potato, caused by Colletotrichum coccodes, is a disease of growing economic importance, but the degree of genetic diversity and pathogenic differentiation among isolates is unknown. Using nitrate auxotrophic (Nit) mutants, we characterized vegetative compatibility groups (VCG) diversity for C. coccodes for 110 isolates originating from Israel, The Netherlands, and France. We recovered frequencies of nit1 and NitM mutant classes at 38.5 and 7.2%, respectively, and selected 12 isolates as tester isolates. Using these testers, we defined four multimember VCGs at 7.3, 35.5, 20.0, and 10.0% frequency in this sample. Thirty isolates (27.3% of all tested isolates) could not be assigned to any of the major groups, and showed only self-compatibility. The frequency of recovery of Nit mutant sectors was highest in isolates from VCG4, with 50.9 and 13.6% recovery for nit1 and NitM, respectively. However, we did not detect differences in the frequency of mutant classes among the three countries of origin. In pathogenicity tests, isolates from VCG3 were the most aggressive to potato, as expressed by high stem colonization levels and sclerotia density on root and crown. These results suggest that there is significant VCG diversity in this species and that this VCG diversity may be correlated with pathogenic characteristics or specialization.

Association of Mycotoxin and Sclerotia Production with Compatibility Groups in Aspergillus flavus from Peanut in Argentina

Vegetative compatibility (VC) of Aspergillus flavus isolates from peanut seed was studied to evaluate preliminary diversity and its association with mycotoxin production and sclerotia production and number. A. parasiticus isolates also were included as a comparative group. Isolates were divided into five categories based on mycotoxin production combination. Five of the A. flavus isolates were considered atypical because they simultaneously produced aflatoxins B, G, and cyclopiazonic acid (CPA). Vegetative compatibility groups (VCGs) were determined through complementation tests between nitrate-nonutilizing mutants. Sclerotia diameters and the number of sclerotia produced per square centimeter were determined for each isolate. Out of 32 isolates of A. flavus, 25 combined in 13 VCGs, whereas the remaining could not be assigned to any particular group. Each VCG included isolates of the same mycotoxin category, with only one exception. Also, all isolates within the same VCG were characterized by their ability to produce or not produce sclerotia. Isolates between VCGs showed significant differences in number of sclerotia per square centimeter, but differences in sclerotia size were not evident. Atypical isolates simultaneously producing aflatoxins B, G, and CPA formed a single and exclusive VCG.

Vegetative Compatibility Groups of Verticillium dahliae in Israel: Their Distribution and Association with Pathogenicity

A collection of 565 isolates of Verticillium dahliae, recovered between 1992 and 1997 from 13 host plant species and soil at 47 sites in Israel, was tested for vegetative compatibility using nitrate-nonutilizing (nit) mutants. Three vegetative compatibility groups (VCGs) were found and identified as VCG2A (28 isolates), VCG2B (158 isolates), and VCG4B (378 isolates) by using international reference strains. One isolate was heterokaryon self-incompatible. Of the VCG2B isolates, 92% were recovered from the northern part of Israel and 90% of VCG4B isolates were recovered from the south, with some overlap in the central region. Isolates of the minor group VCG2A were geographically scattered among the two major VCGs. Isolates of the same VCG resembled one another more than isolates from different VCGs based on colony and microsclerotial morphology, temperature responses, and, partially, pathogenicity. Different pathotypes were defined among 60 isolates tested, using cotton (cv. Acala SJ-2) and eggplant (cv. Black Beauty) as differentials. All isolates in VCG2A and 86% of the isolates in VCG4B, irrespective of their origin, induced weak to moderate symptoms on cotton and moderate to severe symptoms on eggplant and were similar to the previously described cotton nondefoliating patho-type. In contrast, all cotton isolates in VCG2B caused severe foliar symptoms, stunting, and often death, but little or no defoliation of inoculated cotton plants. These were defined as a cotton defoliating-like pathotype and induced only weak to moderate symptoms on eggplant. We concluded that vegetative compatibility grouping of V. dahliae in Israel is closely associated with specific pathogenicity and other phenotypic traits.