In nature, plants live with a wide range of microbes that reside on the surface of or within plant tissues. Plants disregard or tolerate these nonpathogenic microbes, but mount inducible defenses when they encounter potentially infectious microbes. The danger hypothesis predicts that a plant can sense and respond to danger/damage-associated molecular patterns (DAMPs) associated with pathogen challenge in addition to the microbe-associated molecular patterns (MAMPs) that are largely shared by all microbes. Here we provide an overview of recent studies on DAMP sensing and signaling in plant immunity. We also introduce our studies pointing to the importance of layered crosstalk between MAMP and DAMP signaling pathways as a critical step in basal resistance and systemic acquired resistance.
The genus Rhizobium includes two plant pathogens, crown gall and hairy root bacteria. Among the existing species of the genus, the following six species have at least either of these two pathogens as a member: R. radiobacter species complex (including R. nepotum and R. pusense), R. rhizogenes, R. vitis, R. rubi, R. larrymoorei, and R. skierniewicense. Of these six species, four species (R. radiobacter species complex, R. rhizogenes, R. vitis, and R. rubi) were the focus of this study to develop a method for their simultaneous identification by means of a multiplex colony-direct PCR. Respective species-specific primer sets were designed based on sequences of the recA or pyrG genes. The universal primer set for 16S rRNA gene (16S rDNA), chosen as an internal control, was also added to the PCR mix to readily judge the success of the reaction and exclude false-negative reactions. Reliability of the method as an identification tool was then validated using the Rhizobium species containing pathogens and their relatives of 383 strains in total. As a result, two DNA fragments, one from species-specific amplifications (R. radiobacter species complex [245 bp], R. vitis [319 bp], R. rubi [420 bp], or R. rhizogenes [503 bp]) and the other (780–784 bp) from 16S rDNA as the internal control, were stably obtained from all the strains belonging to the targeted four species. When the other strains were used, only the fragment derived from 16S rDNA was amplified. Therefore, this method is useful for the rapid identification of the four Rhizobium species (R. radiobacter species complex, R. rhizogenes, R. vitis, and R. rubi), which will improve the efficiency in areas such as surveillance and diagnosis of crown gall and hairy root diseases and epidemiological and ecological studies.
No viruses or viroids other than Citrus vein enation virus (CVEV) were detected from Yuzu seedlings that had clear vein enation symptoms after aphid transmission. The results indicated that the vein enation symptom of Yuzu was due to a single infection of CVEV.