Transgenic plants expressing Bacillus thuringiensis (Bt) toxins are currently being deployed for insect control. In response to concerns about Bt resistance, we investigated a toxin secreted by a different bacterium Photorhabdus luminescens, which lives in the gut of entomophagous nematodes. In insects infected by the nematode, the bacteria are released into the insect hemocoel; the insect dies and the nematodes and bacteria replicate in the cadaver. The toxin consists of a series of four native complexes encoded by toxin complex loci tca, tcb, tcc, and tcd. Both tca and tcd encode complexes with high oral toxicity to Manduca sexta and therefore they represent potential alternatives to Bt for transgenic deployment.
Background: Halyomorpha halys (Stål), the brown marmorated stink bug, is a highly invasive insect species due in part to its exceptionally high levels of polyphagy. This species is also a nuisance due to overwintering in humanmade structures. It has caused significant agricultural losses in recent years along the Atlantic seaboard of North America and in continental Europe. Genomic resources will assist with determining the molecular basis for this species' feeding and habitat traits, defining potential targets for pest management strategies.
Incubation with cucumber phloem exudate in vitro results in a dramatic decrease in the electrophoretic mobility of Hop stunt viroid. UV cross-linking and a combination of size exclusion and ion exchange chromatography indicate that this phenomenon reflects a previously unsuspected ability of phloem protein 2, a dimeric lectin and the most abundant component of phloem exudate, to interact with RNA. In light of its demonstrated ability to move from cell to cell via plasmodesmata as well as long distances in the phloem, our results suggest that phloem protein 2 may facilitate the systemic movement of viroids and, possibly, other RNAs in vivo.
Strain PRAA4-1T, a motile, Gram-negative, violet-pigmented bacterium, was isolated from Maryland forest soil and found to be orally toxic to Colorado potato beetle larvae and other insects. Morphological, biological, biochemical and molecular characterization revealed that this strain was most similar to Chromobacterium violaceum, the type species and only currently recognized member of the genus Chromobacterium. DNA–DNA hybridization with C. violaceum ATCC 12472T was 27 %. Phylogenetic analysis of 16S rRNA gene sequences revealed that strain PRAA4-1T and Chromobacterium violaceum form a monophyletic clade, with the closest ancestral taxon Vogesella indigofera within the Betaproteobacteria. On the basis of phenotypic, genotypic and phylogenetic analyses, strain PRAA4-1T (=NRRL B-30655T=DSM 17043T) is proposed as the type strain of a novel species of the genus Chromobacterium, Chromobacterium subtsugae sp. nov.
The nematode Heterorhabditis bacteriophora transmits a monoculture of Photorhabdus luminescens bacteria to insect hosts, where it requires the bacteria for efficient insect pathogenicity and as a substrate for growth and reproduction. Siderophore production was implicated as being involved in the symbiosis because an ngrA mutant inadequate for supporting nematode growth and reproduction was also deficient in producing siderophore activity and ngrA is homologous to a siderophore biosynthetic gene, entD. The role of the siderophore in the symbiosis with the nematode was determined by isolating and characterizing a mini-Tn5-induced mutant, NS414, producing no detectable siderophore activity. This mutant, being defective for growth in iron-depleted medium, was normal in supporting nematode growth and reproduction, in transmission by the dauer juvenile nematode, and in insect pathogenicity. The mini-Tn5 transposon was inserted into phbH; whose protein product is a putative peptidyl carrier protein homologous to the nonribosomal peptide synthetase VibF of Vibrio cholerae. Other putative siderophore biosynthetic and transport genes flanking phbH were characterized. The catecholate siderophore was purified, its structure was determined to be 2-(2,3-dihydroxyphenyl)-5-methyl-4,5-dihydro-oxazole-4-carboxylic acid [4-(2,3-dihydroxybenzoylamino)-butyl]-amide, and it was given the generic name photobactin. Antibiotic activity was detected with purified photobactin, indicating that the siderophore may contribute to antibiosis of the insect cadaver. These results eliminate the lack of siderophore activity as the cause for the inadequacy of the ngrA mutant in supporting nematode growth and reproduction.Photorhabdus luminescens (Enterobacteriaceae) is an insect pathogen mutually associated with and transmitted by the entomopathogenic nematode Heterorhabditis bacteriophora (29, 42; for reviews, see references 21 and 22). The specialized dauer juvenile (DJ) stage of the nematode contains a pure culture of P. luminescens cells in the anterior region of the gut mucosa (14,19). The DJ nematodes seek insect larvae and enter the hemocoel through natural openings or by penetrating the cuticle and then regurgitate their charge of P. luminescens (14, 35, 42). The bacteria alone are highly lethal to insect larvae when injected into the hemocoel, with less than 30 P. luminescens cells causing 50% mortality to insect larvae (LD 50 ), but are not pathogenic when ingested by insect larvae (29, 35). The nematode requires P. luminescens for growth and reproduction in insect larvae and on artificial medium (2, 18). The bacteria produce antibiotics and bacteriocins that appear to inhibit other saprophytic microorganisms in the infected insect cadaver (1,40,44). Before nutrients in the insect are depleted, the nematodes again differentiate to the DJ stage, each colonized by P. luminescens in the intestine and disperse from the cadaver in search of another insect victim.Iron is essential to most bacteria and is often found at limiting concentrations...
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