Characterization of three novel genetic loci encoding bacteriocins associated with Xanthomonas perforans.

Bacterial spot is a destructive disease of tomato in Florida that prior to the early 1990s was caused by Xanthomonas euvesicatoria. X. perforans was first identified in Florida in 1991 and by 2006 was the only xanthomonad associated with bacterial spot disease in tomato. The ability of an X. perforans strain to outcompete X. euvesicatoria both in vitro and in vivo was at least in part associated with the production of three bacteriocins designated Bcn-A, Bcn-B, and Bcn-C. The objective of this study was to characterize the genetic determinants of these bacteriocins. Bcn-A activity was confined to one locus consisting of five ORFs of which three (ORFA, ORF2 and ORF4) were required for bacteriocin activity. The fifth ORF is predicted to encode an immunity protein to Bcn-A based on in vitro and in vivo assays. The first ORF encodes Bcn-A, a 1,398 amino acid protein, which bioinformatic analysis predicts to be a member of the RHS family of toxins. Based on results of homology modeling, we hypothesize that the amino terminus of Bcn-A interacts with a protein in the outer membrane of X. euvesicatoria. The carboxy terminus of the protein may interact with an as yet unknown protein(s) and puncture the X. euvesicatoria membrane, thereby delivering the accessory proteins into the target and causing cell death. Bcn-A appears to be activated upon secretion based on cell fractionation assays. The other two loci were each shown to be single ORFs encoding Bcn-B and Bcn-C. Both gene products possess homology toward known proteases. Proteinase activity for both Bcn-B and Bcn-C was confirmed using a milk agar assay. Bcn-B is predicted to be an ArgC-like serine protease, which was confirmed by PMSF inhibition of proteolytic activity, whereas Bcn-C has greater than 50% amino acid sequence identity to two zinc metalloproteases.

Survey of endosymbionts in the Diaphorina citri metagenome and assembly of a Wolbachia wDi draft genome.

Diaphorina citri (Hemiptera: Psyllidae), the Asian citrus psyllid, is the insect vector of Ca. Liberibacter asiaticus, the causal agent of citrus greening disease. Sequencing of the D. citri metagenome has been initiated to gain better understanding of the biology of this organism and the potential roles of its bacterial endosymbionts. To corroborate candidate endosymbionts previously identified by rDNA amplification, raw reads from the D. citri metagenome sequence were mapped to reference genome sequences. Results of the read mapping provided the most support for Wolbachia and an enteric bacterium most similar to Salmonella. Wolbachia-derived reads were extracted using the complete genome sequences for four Wolbachia strains. Reads were assembled into a draft genome sequence, and the annotation assessed for the presence of features potentially involved in host interaction. Genome alignment with the complete sequences reveals membership of Wolbachia wDi in supergroup B, further supported by phylogenetic analysis of FtsZ. FtsZ and Wsp phylogenies additionally indicate that the Wolbachia strain in the Florida D. citri isolate falls into a sub-clade of supergroup B, distinct from Wolbachia present in Chinese D. citri isolates, supporting the hypothesis that the D. citri introduced into Florida did not originate from China.

A dark incubation period is important for Agrobacterium-mediated transformation of mature internode explants of sweet orange, grapefruit, citron, and a citrange rootstock.

BACKGROUND: Citrus has an extended juvenile phase and trees can take 2-20 years to transition to the adult reproductive phase and produce fruit. For citrus variety development this substantially prolongs the time before adult traits, such as fruit yield and quality, can be evaluated. Methods to transform tissue from mature citrus trees would shorten the evaluation period via the direct production of adult phase transgenic citrus trees. METHODOLOGY/PRINCIPAL FINDINGS: Factors important for promoting shoot regeneration from internode explants from adult phase citrus trees were identified and included a dark incubation period and the use of the cytokinin zeatin riboside. Transgenic trees were produced from four citrus types including sweet orange, citron, grapefruit, and a trifoliate hybrid using the identified factors and factor settings. SIGNIFICANCE: The critical importance of a dark incubation period for shoot regeneration was established. These results confirm previous reports on the feasibility of transforming mature tissue from sweet orange and are the first to document the transformation of mature tissue from grapefruit, citron, and a trifoliate hybrid.

Establishment of Asian citrus psyllid (Diaphorina citri) primary cultures.

The Asian citrus psyllid (AsCP), Diaphorina citri Kuwayama (Hemiptera: Psyllidae), is a highly competent vector of the phloem-inhabiting bacterium Candidatus Liberibacter asiaticus associated with the citrus disease huanglongbing (HLB). Commonly referred to as citrus greening disease in the USA, HLB causes reduced fruit yields, quality, and ultimately tree death and is considered the most serious citrus disease. HLB has become a major limiting factor to the production of citrus worldwide. Studies of HLB have been impeded by the fact that C. Liberibacter has not yet been cultured on artificial nutrient media. After being acquired by a psyllid, C. Liberibacter asiaticus is reported to replicate within the psyllid and is retained by the psyllid throughout its life span. We therefore hypothesized that C. Liberibacter asiaticus could be cultured in vitro using psyllid cell cultures as the medium and investigated the establishment of a pure culture for AsCP cells. Several commercially available insect cell culture media along with some media we developed were screened for viability to culture cells from AsCP embryos. Cells from psyllid tissues adhered to the plate and migration was observed within 24 h. Cells were maintained at 20 degrees C. We successfully established primary psyllid cell cultures, referred to as DcHH-1, for D. citri Hert-Hunter-1, with a new media, Hert-Hunter-70.