Three plant-pathogenic isolates of Streptomyces spp., isolated from potatoes with common scab disease lesions in Korea, are described as novel species. Morphological and physiological properties of these isolates were distinct from those of previously described Streptomyces species. Strain S63^T has yellow-white, smooth, cylindrical spores that are borne in monoverticillus flexuous spore-chains. Strain S77^T has purple-red, spiny spores that are borne in simple rectus flexuous spore-chains. Strain S78^T has white, smooth, cylindrical spores that are borne in simple rectus flexuous spore-chains. These three isolates differed from known pathogenic strains by analysis of 16S rRNA gene sequences in a previous study. Furthermore, genetic uniqueness of our isolates was confirmed by sequencing of the 16S–23S internal transcribed spacer (ITS) region, which indicated that isolates S63^T and S78^T belong to the genus Streptomyces and have low homology to other Streptomyces species (less than 71·2 and 75·7 %, respectively). The 16S–23S ITS region of strain S77^T was not amplified by these primer sets. DNA–DNA hybridization results for all three isolates show distant relationships to previously described Streptomyces species; therefore, on the basis of polyphasic evidence, the names Streptomyces luridiscabiei sp. nov. for strain S63^T (=LMG 21390^T=KACC 20252^T), Streptomyces puniciscabiei sp. nov. for strain S77^T (=LMG 21391^T=KACC 20253^T) and Streptomyces niveiscabiei sp. nov. for strain S78^T (=LMG 21392^T=KACC 20254^T) are proposed.

Thaxtomin A, a key phytotoxin produced by plant pathogenic Streptomyces sp., is implicit in common scab disease expression in potato. Primary targets and modes of action of thaxtomin A toxicity in plant cells are not well understood. In this work, early signalling events associated with thaxtomin A toxicity were studied using the ion-selective microelectrode ion flux estimation (MIFE) technique. Thaxtomin A-induced changes in net ion fluxes were measured across the plasma membrane (PM) of root and pollen tube tissue in Arabidopsis thaliana and tomato. Within a minute after toxin application, a rapid and short-lived Ca2+ influx was observed. Well ahead of the marked inhibition of root growth, a significant shift towards net H+ efflux across the PM occurred in all tissues. Similar to root tissues, thaxtomin A significantly modified ion flux profiles from growing pollen tubes. Thaxtomin A was more effective in young, physiologically active tissues (root elongation zone or pollen tube apex), suggesting a higher density of thaxtomin A-binding sites in these regions. Overall, our data provide the first evidence that thaxtomin A triggers an early signalling cascade, which may be crucial in plant-pathogen interactions. It also suggests a possible interaction between thaxtomin A and PM auxin receptors, as revealed from experiments on the auxin-sensitive ucu2-2/gi2 A. thaliana mutant.