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Rice sheath blight, which is caused by Rhizoctonia solani, is a devastating disease in many areas of the world where rice is cultivated. The role of polygalacturonase (PG) in the infection process of R. solani was investigated in the present study by cloning and investigating the function of a new PG-encoding gene, which was designated RsPG2. Bioinformatics showed that RsPG2 consisted of a 1,633 bp open reading frame (ORF) with six introns; the deduced protein contained 436 amino acids with a predicted mass of 45.88 kDa after cleavage of the predicted 17-amino acid signal sequence. Culture supernatants obtained from Pichia pastoris producing recombinant RsPG2 degraded polygalacturonic acid in vitro, thus confirming that RsPG2 encodes a PG gene. Purified recombinant RsPG2 also degraded rice tissue and elicited obvious necrotic symptoms 48 h after inoculation. Real-time qRT-PCR indicated that the expression of RsPG2 was strongly induced during the infection of rice by R. solani. These data unequivocally demonstrate that RsPG2 plays an important role in R. solani infection and provide further insights for elucidating the function of pg genes in the virulence of R. solani.

Citrus mandarin (Citrus reticulata) cultivar Shatangju has great economic importance in southern China because of its delicious fruit and medicinal peel. During surveys conducted in 2012 and 2013, stem rot and plant death were observed on 2- to 3-year-old Shatangju grafted seedlings in 21 nurseries located in three regions (Yangjiang City, Sihui City and Deqing County) of Guangdong Province, China. The aims of this study were to characterize the symptoms of seedling stem rot and death; evaluate the incidence and loss; identify the causal agent and evaluate its pathogenicity. The disease was present in all surveyed nurseries but incidence varied with location. A Pythium-like oomycete was consistently obtained from symptomatic plants collected in the nurseries surveyed. On the basis of morphological and cultural characteristics and combined phylogenetic analysis of three DNA sequences including the internal transcribed spacer region (ITS), mitochondrial CoxI and CoxII genes, 21 of the isolates obtained were identified as belonging to the species Phytopythium helicoides. The isolates were pathogenic to the scions but not rootstocks of Shatangju mandarin seedlings cultivated in a controlled greenhouse and symptoms identical to that observed in the field were reproduced. The pathogen was re-isolated consistently from the inoculated plants, confirming Koch’s postulates. Seedling stem rot and eventual death caused by P. helicoides represents a serious threat to Shatangju mandarin nursery productivity and longevity in southern China.

Peptides and small molecules produced by both the plant pathogen Phytophthora and host plants in the apoplastic space mediate the relationship between the interplaying organisms. Various Phytophthora apoplastic effectors, including small cysteine-rich (SCR) secretory proteins, have been identified, but their roles during interaction remain to be determined. Here, we identified an SCR effector encoded by scr96, one of three novel genes encoding SCR proteins in P. cactorum with similarity to the P. cactorum phytotoxic protein PcF. Together with the other two genes, scr96 was transcriptionally induced throughout the developmental and infection stages of the pathogen. These genes triggered plant cell death (PCD) in the Solanaceae, including Nicotiana benthamiana and tomato. The scr96 gene did not show single nucleotide polymorphisms in a collection of P. cactorum isolates from different countries and host plants, suggesting that its role is essential and non-redundant during infection. Homologues of SCR96 were identified only in oomycetes, but not in fungi and other organisms. A stable protoplast transformation protocol was adapted for P. cactorum using green fluorescent protein as a marker. The silencing of scr96 in P. cactorum caused gene-silenced transformants to lose their pathogenicity on host plants and these transformants were significantly more sensitive to oxidative stress. Transient expression of scr96 partially recovered the virulence of gene-silenced transformants on plants. Overall, our results indicate that the P. cactorum scr96 gene encodes an important virulence factor that not only causes PCD in host plants, but is also important for pathogenicity and oxidative stress tolerance.

Bacillus sp. 6256 is a good biocontrol agent against Botrytis cinerea which caused tomato gray mold disease. Strain 6256 was identified as B. amyloliquefaciens by analysis of its partial gyrB gene sequence. To identify and characterize the antimycotic peptides from the culture broth of the bacterium, the antimicrobial substances produced by B. amyloliquefaciens 6256 were isolated by ammonium sulfate precipitation, Superdex 200 gel filtration chromatography and DEAE anion exchange chromatography. The purified compound was designated as P657. The biological activity of P657 was stable at as high as 100 °C for 20 min and in pH value ranged from 5 to 10. The antimycotic compound was resistant to trypsin and proteinase K, and could completely inhibit spore germination of Botrytis cinerea in vitro. MALDI-TOF-MS analysis results showed the presence of fengycins A (C16–C17) and fengycins B (C15–C17) isoforms in P657.

Antifungal metabolites produced by Bacillus sp. W10, which was previously isolated from the tomato rhizosphere, were investigated. Strain W10 was identified as Bacillus amyloliquefaciens by analysis of its 16S rDNA and gyrB gene partial sequences. PCR analysis showed the presence of fenB, sfp, and ituD genes, coding for fengycin, surfactin, and iturin, respectively. A novel small antifungal peptide, designated 5240, produced by this strain was isolated by ammonium sulfate precipitation and Superdex 200 gel filtration chromatography. The 5240 peptide was stable at 100 °C for 20 min and remained active throughout a wide pH range (4–10). The antagonistic activity was not affected by protease K and trypsin. The purified 5240 peptide exhibited a broad inhibitory spectrum against various plant pathogenic fungi and was identified as iturin A (C14-C16). Moreover, matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry indicated the presence of fengycin A (C14-C15), fengycin B (C16-C17), and surfactin (C13-C16) isoforms in supernatants from strain W10. These results suggest that B. amyloliquefaciens W10 has significant potential as a biocontrol agent.