Identification and full genomic sequence of nerine yellow stripe virus.

This study reports the first complete genome sequence of nerine yellow stripe virus (NeYSV, GenBank MT396083). The genome consists of 10,165 nucleotides, excluding the 3'-terminal poly(A) tail. A single open reading frame encodes a large polyprotein of 3294 amino acids with typical potyvirus features. The nuclear inclusion b and coat protein region shares 95% identity with a previously reported partial NeYSV sequence (NC_043153.1). Phylogenetic analysis of the polyprotein amino acid sequence showed that NeYSV clustered with hippeastrum mosaic virus (HiMV YP_006382256.1).

Genetic dissection of basal resistance to Pseudomonas syringae pv. phaseolicola in accessions of Arabidopsis.

We have examined the genetics of nonhost resistance in Arabidopsis, using the bean pathogen Pseudomonas syringae pv. phaseolicola race 6 1448A to probe accessions for natural variation in basal defense. Symptoms rarely developed in leaves of Niedersenz (Nd), some yellowing and occasional necrosis developed in Columbia (Col), whereas tissue collapse was observed in Wassilewskija (Ws) after inoculation by infiltration. Analysis of F2 progeny and recombinant inbred lines (RIL) from a cross between Col and Nd revealed a pattern of continuous symptom increase, indicating the operation of quantitative determinants of resistance. By mapping quantitative trait loci (QTL), significant linkage was determined for resistance (low symptom score) to markers on chromosome 4. Segregation in the F2 cross from Nd × Ws indicated the operation of two dominant genes for resistance, one of which was FLS2 encoding the flagellin receptor. The requirement for FLS2 to confer resistance was confirmed by transgenic experiments, and we showed that the response to P. syringae pv. phaseolicola was affected by FLS2 gene dosage. Using RIL, the second locus was mapped as a QTL to a large interval on chromosome 1. Both FLS2 and the QTL on chromosome 1 were required for the highest level of resistance to bacterial colonization and symptom development in Nd.

A genome-wide functional investigation into the roles of receptor-like proteins in Arabidopsis.

Receptor-like proteins (RLPs) are cell surface receptors that typically consist of an extracellular leucine-rich repeat domain, a transmembrane domain, and a short cytoplasmatic tail. In several plant species, RLPs have been found to play a role in disease resistance, such as the tomato (Solanum lycopersicum) Cf and Ve proteins and the apple (Malus domestica) HcrVf2 protein that mediate resistance against the fungal pathogens Cladosporium fulvum, Verticillium spp., and Venturia inaequalis, respectively. In addition, RLPs play a role in plant development; Arabidopsis (Arabidopsis thaliana) TOO MANY MOUTHS (TMM) regulates stomatal distribution, while Arabidopsis CLAVATA2 (CLV2) and its functional maize (Zea mays) ortholog FASCINATED EAR2 regulate meristem maintenance. In total, 57 RLP genes have been identified in the Arabidopsis genome and a genome-wide collection of T-DNA insertion lines was assembled. This collection was functionally analyzed with respect to plant growth and development and sensitivity to various stress responses, including susceptibility toward pathogens. A number of novel developmental phenotypes were revealed for our CLV2 and TMM insertion mutants. In addition, one AtRLP gene was found to mediate abscisic acid sensitivity and another AtRLP gene was found to influence nonhost resistance toward Pseudomonas syringae pv phaseolicola. This genome-wide collection of Arabidopsis RLP gene T-DNA insertion mutants provides a tool for future investigations into the biological roles of RLPs.

Pseudomonas syringae effector AvrPtoB suppresses basal defence in Arabidopsis.

The virulence and avirulence activities of members of the Pseudomonas syringae HopAB family of effectors and AvrPto were examined in bean, tomato and Arabidopsis. Proteins were delivered by the RW60 strain of P. syringae pv. phaseolicola. RW60 causes a hypersensitive reaction (HR) in bean and tomato but is restricted without the HR in Arabidopsis. Dual avirulence and virulence functions in tomato and bean, respectively, were identified in virPphA homologues but only avrPtoB strongly enhanced virulence to Arabidopsis, overcoming basal defences operating against RW60. Virulence activity in both bean and Arabidopsis required regions of the C-terminus of the AvrPtoB protein, whereas elicitation of the rapid HR in tomato, with the matching Pto resistance gene, did not. The effect of AvrPtoB on Arabidopsis was accession-specific; most obvious in Wassilewskija (Ws-3), intermediate in Columbia and not detectable in Niedersenz (Nd-1) after inoculation with RW60 + avrPtoB. Analysis of crosses between Ws-3 and Nd-1 indicated co-segregation for the AvrPtoB virulence function with the absence of the Nd-1 FLS2 gene which mediates recognition of bacterial flagellin. In planta expression of AvrPtoB did not prevent the HR activated by P. syringae pv. tomato DC3000 + avrB, avrRpm1, avrRps4 or avrRpt2, but suppressed cell wall alterations, including callose deposition, characteristic of basal defence and was associated with reprogramming of the plant's transcriptional response. The success or failure of AvrPtoB in suppressing basal defences in Nd-1 depended on the timing of exposure of plant cells to the effector and the flagellin flg22 peptide.

Bidirectional electron transfer in photosystem I: electron transfer on the PsaA side is not essential for phototrophic growth in Chlamydomonas.

We have used pulsed electron paramagnetic resonance (EPR) measurements of the electron spin polarised (ESP) signals arising from the geminate radical pair P700(z.rad;+)/A(1)(z.rad;-) to detect electron transfer on both the PsaA and PsaB branches of redox cofactors in the photosystem I (PSI) reaction centre of Chlamydomonas reinhardtii. We have also used electron nuclear double resonance (ENDOR) spectroscopy to monitor the electronic structure of the bound phyllosemiquinones on both the PsaA and PsaB polypeptides. Both these spectroscopic assays have been used to analyse the effects of site-directed mutations to the axial ligands of the primary chlorophyll electron acceptor(s) A(0) and the conserved tryptophan in the PsaB phylloquinone (A(1)) binding pocket. Substitution of histidine for the axial ligand methionine on the PsaA branch (PsaA-M684H) blocks electron transfer to the PsaA-branch phylloquinone, and blocks photoaccumulation of the PsaA-branch phyllosemiquinone. However, this does not prevent photoautotrophic growth, indicating that electron transfer via the PsaB branch must take place and is alone sufficient to support growth. The corresponding substitution on the PsaB branch (PsaB-M664H) blocks kinetic electron transfer to the PsaB phylloquinone at 100 K, but does not block the photoaccumulation of the phyllosemiquinone. This transformant is unable to grow photoautotrophically although PsaA-branch electron transfer to and from the phyllosemiquinone is functional, indicating that the B branch of electron transfer may be essential for photoautotrophic growth. Mutation of the conserved tryptophan PsaB-W673 to leucine affects the electronic structure of the PsaB phyllosemiquinone, and also prevents photoautotrophic growth.