Arabidopsis thaliana (L.) Heynh. genotypes limited in their ability to mount either octadecanoid-dependent induced resistance (IR–) or systemic acquired resistance (SAR–) were used to characterize the roles of these pathways in plant–herbivore interactions. Molecular and biochemical markers of IR were analysed in plants subject to herbivory by caterpillars of the beet armyworm, Spodoptera exigua Hübner, which had either intact or impaired salivary secretions since salivary enzymes, such as glucose oxidase, have been implicated in the ability of caterpillars to circumvent induced plant defences. Transcript expression of genes encoding laccase-like multicopper oxidase [AtLMCO4 (polyphenol oxidase)] and defensin (AtPDF1.2) showed salivary-specific patterns which were disrupted in the SAR– mutant plants. The activity of octadecanoid-associated anti-nutritive proteins, such as LMCO and trypsin inhibitor, showed similar patterns. Gene and protein changes parallel plant hormone levels where elevated jasmonic acid was observed in wild-type plants fed upon by caterpillars with impaired salivary secretions compared with plants subject to herbivory by normal caterpillars. This salivary-specific difference in jasmonic acid levels was alleviated in SAR– mutants. These results support the model that caterpillar saliva interferes with jasmonate-dependent plant defences by activating the SAR pathway.
The genes conferring pathogenicity in Streptomyces turgidiscabies, a pathogen causing common scab of potato, are grouped together on a pathogenicity island (PAI), which has been found to be mobile and appears to transfer and disseminate like an integrative and conjugative element (ICE). However, in Streptomyces scabiei, another common scab-inducing species, the pathogenicity genes are clustered in two regions: the toxicogenic region (TR) and the colonization region. The S. scabiei 87.22 genome was analysed to investigate the potential mobility of the TR. Attachment sites (att), short homologous sequences that delineate ICEs, were identified at both extremities of the TR. An internal att site was also found, suggesting that the TR has a composite structure (TR1 and TR2). Thaxtomin biosynthetic genes, essential for pathogenicity, were found in TR1, whereas candidate genes with known functions in recombination, replication and conjugal transfer were found in TR2. Excision of the TR1 or TR2 subregions alone, or of the entire TR region, was observed, although the excision frequency of TR was low. However, the excision frequency was considerably increased in the presence of either mitomycin C or Streptomyces coelicolor cells. A composite TR structure was not observed in all S. scabiei and Streptomyces acidiscabies strains tested. Of the ten strains analysed, seven lacked TR2 and no TR excision event could be detected in these strains, thus suggesting the implication of TR2 in the mobilization of S. scabiei TR.
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