Quorum sensing genes rpfF and xanB2 are not essential for albicidin production nor sugarcane colonization by Xanthomonas albilineans

Xanthomonas albilineans (Xa) produces albicidin, a unique and specific toxin that causes foliar symptoms of sugarcane leaf scald disease. In X. campestris pv. campestris, a cluster of rpf (for regulation of pathogenicity factors) genes and xanB2 are involved in control of various cellular processes. rpfF and xanB2 encode DSF (diffusible signal factor) and DF (diffusible factor), respectively, which are two quorum sensing signalling molecules. Both quorum sensing systems appear to be used by Xa, since mutation of rpfF in Florida strain XaFL07-1 resulted in reduced protease production, and mutation of xanB2 resulted in loss of xanthomonadin (yellow pigment) production. Mutations of rpfF and xanB2 were verified by PCR analyses. Mutations of rpfF and complementation in trans were also verified by use of an X. campestris DSF reporter strain. Sugarcane cultivar CP80-1743, moderately susceptible to leaf scald, exhibited pencil line symptoms indicative of albicidin production on emerging leaves and colonization of leaf vessels after inoculation of stalks by the decapitation method with all mutants, including separate deletion mutations of rpfG and rpfC (encoding two sensor components of the DSF system). Preliminary experiments indicated that several rpfF and xanB2 mutants colonized sugarcane stalks as efficiently, both spatially and in intensity, as wild type Xa. Additional inoculation experiments are in progress to assess disease severity caused by rpf mutants, including deletion of the entire rpfGCF region. However, our preliminary data showed that neither DSF nor DF is essential for albicidin production or sugarcane colonization by Xa. Therefore, albicidin production and sugarcane colonization by Xa may not be controlled by quorum sensing or may involve another system. (Texte intégral

Identification of pathogenicity factors in the xylem-invading pathogen Xanthomonas albilineans by Transposon Mutagenesis

Xanthomonas albilineans is a systemic, xylem-invading pathogen that causes sugarcane leaf scald. Leaf symptoms vary from a single, white, narrow, sharply defined stripe to complete wilting and necrosis of infected leaves, leading to plant death. X albilineans produces the toxin albicidin that blocks chloroplast differentiation, resulting in disease symptoms. Albicidin is the only previously known pathogenicity factor in X albilineans, yet albicidin-deficient mutant strains are still able to efficiently colon\ze sugarcane. We used TnS (transposome) mutagenesis in an attempt to identifY additional X albilineans pathogenicity factors. Sugarcane cultivar CP80-1743, moderately susceptible to leaf scald, was inoculated by the decapitation method with 1,216 independently derived TnS insertions in Florida strain XaFL07-1. Leaf scald symptoms were recorded on emerging leaves one month after inoculation, and stalk colonization by the pathogen was determined two months after inoculation. In addition to the previously identified albicidin biosynthesis gene ciuster mutations, 33 new loci were identified in which insertions were correlated with reduced pathogenicity. These insertions affected genes predicted to encode proteins involved in a variety of functions, inciuding exopolysaccharide and lipopolysaccharide biosynthesis, fatty acid biosynthesis, regulatory and cell signaling, and secretion systems. Several of these have been associated with virulence in other bacterial plant pathogens that invade the xylem. However, sorne loci were identified that are predicted to encode previously unrecognized and apparently essential pathogenicity factors, at least for sugarcane leaf scald, inciuding an OmpA family outer membrane protein. Five independent TnS insertions in OmpA locus XALc_0557 of X albilineans strain XaFL07-1 produced no or very few leaf symptoms. These mutants produced albicidin in vitro and were able to multiply in sugarcane leaf tissue to levels similar to the wild-type strain, but did not efficiently colonize the sugarcane stalk. These ompA mutants were also affected in growth rate, motility and biofilm formation in vitro. (Résumé d'auteur

Metagenomic screening of the sugarcane virome in Florida. [P.36]

Viral metagenomics has revolutionized the way pathologists decipher viral diseases. While the impact of this new approach is still debatable in plant virus diagnostics, viral metagenomics has already produced key advances in viral ecology and has the potential to become a central approach for viral surveillance at the ecosystem scale. A viral metagenomics study of the sugarcane virome in Florida was carried out in 2013/2014. One hundred and eighty sugarcane leaf samples were collected from different commercial sugarcane (Saccharum interspecific hybrids) fields in Florida and from other Saccharum and related species taken from two local germplasm collections. Sequence-independent next generation sequencing (NGS) of virion-associated nucleic acids (VANA) was used for detection and identification of viruses present within the collected leaf samples. All four previously reported sugarcane viruses occuring in Florida were detected: Sugarcane yellow leaf virus (149 infected samples out of 180), Sugarcane mosaic virus (2/180), Sugarcane mild mosaic virus (10/180) and Sugarcane bacilliform virus (51/180). Interestingly, this viral metagenomics approach also resulted in the detection of potential new viruses of sugarcane, including Chrysovirus, Mastrevirus, and Umbravirus. This study provided a snapshot vision of the SCYLV genetic diversity in 2013/2014 in Florida where several genotypes of this virus are present. It also allowed us to assemble the whole genome of at least one new mastrevirus species. (Résumé d'auteur

Genome sequencing of the sugarcane orange rust pathogen Puccinia kuehnii

Puccinia kuehnii, the causal agent of orange rust, is a fungal pathogen that causes damaging losses to the sugarcane crop in Florida since 2007. During plant infection, rust fungi deliver effector proteins into host tissues using a specialized feeding structure called a haustorium, to manipulate plant functions and promote parasitic growth. Identification of these effectors is therefore an essential step to understand host-pathogen interactions and to develop new control methods. In this study, two isolates of P. kuehnii from Florida were used for genome sequencing and effector discovery based on sequence features. Genomic DNA was extracted from spores of P. kuehnii 1040 and P. kuehnii 2143 that were collected from sugarcane cultivars CL 85-1040 and CP 89-2143, respectively. Short Illumina DNAseq and long read Pacbio sequencing was performed and a hybrid assembly was obtained for the genome of each isolate of P. kuehnii. These assemblies will be used as genomic resources for gene calling and proteome prediction

Inelastic quantum transport in superlattices: success and failure of the Boltzmann equation

Electrical transport in semiconductor superlattices is studied within a fully self-consistent quantum transport model based on nonequilibrium Green functions, including phonon and impurity scattering. We compute both the drift velocity-field relation and the momentum distribution function covering the whole field range from linear response to negative differential conductivity. The quantum results are compared with the respective results obtained from a Monte Carlo solution of the Boltzmann equation. Our analysis thus sets the limits of validity for the semiclassical theory in a nonlinear transport situation in the presence of inelastic scattering.Comment: final version with minor changes, to appear in Physical Review Letters, sceduled tentatively for July, 26 (1999

Effects of impurity scattering on electron-phonon resonances in semiconductor superlattice high-field transport

A non-equilibrium Green's function method is applied to model high-field quantum transport and electron-phonon resonances in semiconductor superlattices. The field-dependent density of states for elastic (impurity) scattering is found non-perturbatively in an approach which can be applied to both high and low electric fields. I-V curves, and specifically electron-phonon resonances, are calculated by treating the inelastic (LO phonon) scattering perturbatively. Calculations show how strong impurity scattering suppresses the electron-phonon resonance peaks in I-V curves, and their detailed sensitivity to the size, strength and concentration of impurities.Comment: 7 figures, 1 tabl