Recent research has illustrated that signaling networks, after the plant host perceives diverse pathogen-derived signals, facilitate defense responses through mitogen-activated protein kinase (MAPK) cascades, calcium-dependent protein kinase (CDPK), or receptor-like cytoplasmic kinase (RLCK). The pathogen-induced reactive oxygen species (ROS) burst is mainly caused by activation of an NADPH oxidase, designated as the respiratory burst oxidase homolog (RBOH). Emerging evidence emphasizes that NbRBOHB could be activated by CDPK- or RLCK-dependent phosphorylation in association with the pattern-triggered immunity (PTI)–ROS burst. On the other hand, the effector-triggered immunity (ETI)–ROS burst appears to be regulated by upregulation of NbRBOHB via the MAPK–WRKY pathway, followed by the activation of NbRBOHB through CDPK-mediated phosphorylation.
To verify the relation between disease severity by Chinese cabbage yellows (Verticillium wilt, caused by Verticillium dahliae, V. longisporum and disease-promoting nematode Pratylenchus penetrans) and soil microbiota in fields, bulked soils were periodically sampled in fields with different severities in Gunma Prefecture. Shannon-Wiener diversity index (H') for bacteria, fungi and nematodes, as well as presence of the causal pathogens, were determined based on fingerprint patterns generated by polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE). Consequently, although the diversity indices of soil bacteria and fungi sampled after harvesting were not significantly correlated with the disease severities in fields the next year, H' for nematodes in the soil was highly negatively correlated with disease severity. Furthermore, the relative intensities of DGGE bands that represented the causal agents in the soils sampled after harvesting were positively correlated with disease severity, and those for P. penetrans were negatively correlated with H' of all nematodes in the soil microbiota. Thus, H' of the all nematodes and relative intensities of DGGE bands of the causal pathogens in the soils after harvesting in the previous year could serve as soil microbiological predictors correlating with disease severities in the fields in the succeeding year.
Common scab, a serious disease of potatoes and other root crops, affects crop quality and market value. The disease is caused by Gram positive bacteria in the genus Streptomyces. We surveyed species in Streptomyces isolates causing potato common scab in Kagoshima and Nagasaki Prefectures, major potato-production areas in Japan. Isolates were identified as S. scabiei, S. turgidiscabies or S. acidiscabies by polymerase chain reaction (PCR) based on species-specific primers in the 16S-23S rRNA internal transcribed spacer gene (ITS) and characteristics of the pathogenicity island (PAI). About 42% of the isolates from Kagoshima Prefecture belonged to S. scabiei and about 52% to S. turgidiscabies. However, about 83% of the isolates from Nagasaki Prefecture belonged to S. scabiei. A few isolates from both prefectures belonged to S. acidiscabies. All pathogenic isolates on potato belonged to S. scabiei, S. turgidiscabies or S. acidiscabies and had genes for biosynthesis of the pathogenicity determinant thaxtomin. S. acidiscabies from Nagasaki Prefecture and Saga Prefecture lacked two genes (tomA and nec1) characteristic of the PAI. Pathogenic S. scabiei isolates belonged to two genotypes (type T or JK) based on the sequence of variable regions in the ITS. Some isolates had both genotypes (type B). In particular, all strains of S. scabiei that did not produce melanin belonged to type T. The identification using primers specific to the three pathogenic Streptomyces species agreed with morphological characteristics and physiological properties; they were almost identical to the American Type Culture Collection strains and published reference strains.
Common scab of potato tuber caused by pathogenic Streptomyces species reduces vegetable quality and marketability, causing significant economic loss. In previous studies, quantitative real-time polymerase chain reactions (PCR) using primers designed based on the DNA sequence of pathogenicity island genes (e.g., txtAB, nec1) of pathogenic Streptomyces were developed. However, Streptomyces species vary in character, especially in their pH tolerance, so determining and quantifying the species is critical for developing effective control measures of scab. Here we developed a species-specific method to quantify Streptomyces spp. with an assay detection limit of about 200 fg of the target DNA. Cycle threshold (Ct) values were linearly correlated with the concentration of the target DNA (S. scabiei JK type: R2=0.9985, S. turgidiscabies: R2=0.9988) and were not affected by the presence of DNA from either species in plant tissues and soil. In this study, we developed new primer sets for real-time PCR to distinguish S. scabiei and S. turgidiscabies. By using these new primers and primers for txtAB, we also investigated the population dynamics of these species at different soil pHs (4.4, 4.7, 4.9, or 5.2 at planting), using SYBR Green quantitative real-time PCR. Potatoes were planted in bottomless frame pots with soil that had been amended with S. scabiei, S. turgidiscabies or both, then grown in the greenhouse. S. turgidiscabies tolerates lower soil pH than S. scabiei and grows well with a pH greater than 4.7. In all pots inoculated with the mix of S. scabiei and S. turgidiscabies, S. turgidiscabies always grew better and caused more severe root and scab lesions.
Rotting of roots and stem bases and wilting of entire plants of gentian (Gentiana triflora Pall.) were found in Tochigi Prefecture in August 2011. A fungus, isolated repeatedly from the diseased plants, was identified as a species belonging to clade 3 of the Fusarium solani species complex based on morphological and molecular analyses. The isolates reproduced the symptoms on gentian plants in an inoculation test and were reisolated from the inoculated plants. In this report, we propose the Japanese name “tachigare-byo” for this root rot disease of gentian caused by a Fusarium sp. belonging to the F. solani species complex.