Insecticidal activity of a Moroccan strain of Streptomyces phaeochromogenes LD-37 on larvae, pupae and adults of the Mediterranean fruit fly, Ceratitis capitata (Diptera: Tephritidae).

The Mediterranean fruit fly (medfly), Ceratitis capitata, is considered the most important fruit pest worldwide. Its management is mainly based on the use of chemical insecticides. Although these conventional pesticides are effective at high doses, they cause considerable human health and environment problems. Thus, the aim of this study was to assess insecticidal activity of Moroccan actinobacteria against C. capitata. A total of 12 preselected actinobacteria isolated from various Moroccan habitats were screened for their insecticidal activity against larvae, pupae and adults of C. capitata. Four actinobacteria isolates were significantly active against the first-instar larvae, and nine were active against the medfly adult, while no significant mortality was obtained against the third-instar larval and pupal stages. Among the selected isolates, the biological screening revealed that strain Streptomyces LD-37, which showed 99.4% similarity with Streptomyces phaeochromogenes, exhibited the maximal corrected larval mortality of 98%. Moreover, the isolates AS1 and LD-37 showed the maximum significant corrected mortality against adults of 32.5 and 28.2%, respectively. The crude extract obtained from a fermented culture of strain S. phaeochromogenes LD-37 was separated into six fractions by thin layer chromatography. Fractions F3 and F4 caused a significant corrected larval mortality of 66.7 and 53.3%, respectively; whereas the maximum reduction in adult emergence was obtained with fraction F4. This finding could be useful for utilizing S. phaeochromogenes LD-37 as an alternative to chemical insecticides in pest management of C. capitata.

Control of potato soft rot caused by Pectobacterium carotovorum and Pectobacterium atrosepticum by Moroccan actinobacteria isolates.

Pectobacterium carotovorum and Pectobacterium atrosepticum are dreadful causal agents of potato soft rot. Actually, there are no efficient bactericides used to protect potato against Pectobacterium spp. Biological control using actinobacteria could be an interesting approach to manage this disease. Thus, two hundred actinobacteria isolated from Moroccan habitats were tested for their ability to inhibit in vitro 4 environmental Pectobacterium strains and the two reference strains (P. carotovorum CFBP 5890 and P. atrosepticum CFBP 5889). Eight percent of these isolates were active against at least one of the tested pathogens and only 2% exhibited an antimicrobial activity against all tested Pectobacterium strains. Four bioactive isolates having the greatest pathogen inhibitory capabilities and classified as belonging to the genus Streptomyces species through 16S rDNA analysis were subsequently tested for their ability to reduce in vivo soft rot symptoms on potato slices of Bintje, Yukon Gold, Russet and Norland cultivars caused by the two pathogens P. carotovorum and P. atrosepticum. This test was carried out by using biomass inoculums and culture filtrate of the isolates as treatment. Among these, strain Streptomyces sp. OE7, reduced by 65-94% symptom severity caused by the two pathogens on potato slices. Streptomyces OE7 showed a potential for controlling soft rot on potato slices and could be useful in an integrated control program against potato soft rot pathogens in the objective to reduce treatments with chemical compounds.

Calcium- and ROS-mediated defence responses in BY2 tobacco cells by nonpathogenic Streptomyces sp.

AIMS: The early molecular events underlying the elicitation of plant defence reactions by Gram-positive bacteria are relatively unknown. In plants, calcium and reactive oxygen species are commonly involved as cellular messengers of a wide range of biotic stimuli from pathogenic to symbiotic bacteria. In the present work, we checked whether nonpathogenic Streptomyces sp. strains could induce early signalling events leading to defence responses in BY2 tobacco cell suspensions. METHODS AND RESULTS: We have demonstrated that nonpathogenic Streptomyces sp. OE7 strain induced a cytosolic Ca(2+) increase and a biphasic oxidative burst in the upstream signalling events, leading to defence responses in BY2 tobacco cell suspensions. Streptomyces sp. OE7 also elicited delayed intracellular free scopoletin production and programmed cell death. In agreement with scopoletin production, OE7 induced accumulation of PAL transcripts and increased accumulation of transcripts of EREBP1 and AOX genes that are known to be regulated by the jasmonate/ethylene pathway. Transcript levels of PR1b and NIMIN2α, both salicylic acid pathway-linked genes, were not modified. Moreover, Streptomyces sp. OE7 culture filtrates could reduce Pectobacterium carotovorum- and Pectobacterium atrosepticum-induced death of BY2 cells and soft rot on potato slices. CONCLUSIONS: New insights are thus provided into the interaction mechanisms between Streptomyces sp. and plants; Streptomyces sp. could be sensed by plant cells, and through cytosolic Ca(2+) changes and the generation of reactive oxygen species, defence responses were induced. SIGNIFICANCE AND IMPACT OF THE STUDY: These induced defence responses appeared to participate in attenuating Pectobacterium-induced diseases in plants. Thus, Streptomyces sp. OE7 could be a biocontrol agent against Pectobacterium sp.