P. van West

Resistance of Nicotiana benthamiana to Phytophthora infestans Is Mediated by the Recognition of the Elicitor Protein INF1

Phytophthora infestans, the agent of potato and tomato late blight disease, produces a 10-kD extracellular protein, INF1 elicitin. INF1 induces a hypersensitive response in a restricted number of plants, particularly those of the genus Nicotiana. In virulence assays with different P. infestans isolates, five Nicotiana species displayed resistance responses. In all of the interactions, after inoculation with P. infestans zoospores, penetration of an epidermal cell was observed, followed by localized necrosis typical of a hypersensitive response. To determine whether INF1 functions as an avirulence factor in these interactions, we adopted a gene-silencing strategy to inhibit INF1 production. Several transformants deficient in inf1 mRNA and INF1 protein were obtained. These strains remained pathogenic on host plants. However, in contrast to the wild-type and control transformant strains, INF1-deficient strains induced disease lesions when inoculated on N. benthamiana. These results demonstrate that the elicitin INF1 functions as an avirulence factor in the interaction between N. benthamiana and P. infestans.

A gene encoding a protein elicitor of Phytophthora infestans is down-regulated during infection of potato.

Most species of the genus Phytophthora produce 10-kDa extracellular protein elicitors, collectively termed elicitins. Elicitins induce hypersensitive response in a restricted number of plants, particularly in the genus Nicotiana within the Solanaceae family. A cDNA encoding INF1, the major secreted elicitin of Phytophthora infestans, a pathogen of solanaceous plants, was isolated and characterized. The expression of the corresponding inf1 gene during the disease cycle of P. infestans was analyzed. inf1 was shown to be expressed in mycelium grown in various culture media, whereas it was not expressed in sporangiospores, zoospores, cysts, and germinating cysts. In planta, during infection of potato, particularly during the biotrophic stage, expression of inf1 was down-regulated compared to in vitro. The highest levels of expression of inf1 were observed in in vitro grown mycelium and in late stages of infection when profuse sporulation and leaf necrosis occur. The potential role of INF1 as an elicitor in interactions between P. infestans and Solanum species was investigated. Nineteen lines, representing nine solanaceous species with various levels of resistance to P. infestans, were tested for response to an Escherichia coli expressed INF1. Within the genus Solanum, resistance to P. infestans did not appear to be mediated by a defense response elicited by INF1. However, INF1 recognition could be a component of nonhost resistance of tobacco to P. infestans.

Oomycete Plant Pathogens Use Electric Fields to Target Roots

Plant roots generate electrical currents and associated electrical fields as a consequence of electrogenic ion transport at the root surface. Here we demonstrate that the attraction of swimming zoospores of oomycete plant pathogens to plant roots is mediated in part by electrotaxis in natural root-generated electric fields. The zones of accumulation of anode- or cathode-seeking zoospores adjacent to intact and wounded root surfaces correlated with their in vitro electrotactic behavior. Manipulation of the root electrical field was reflected in changes in the pattern of zoospore accumulation and imposed focal electrical fields were capable of overriding endogenous signals at the root surface. The overall pattern of zoospore accumulation around roots was not affected by the presence of amino acids at concentrations expected within the rhizosphere, although higher concentrations induced encystment and reduced root targeting. The data suggest that electrical signals can augment or override chemical ones in mediating short-range tactic responses of oomycete zoospores at root surfaces.

tef1, a Phytophthora infestans gene encoding translation elongation factor 1α ☆

From a set of Phytophthora infestans cDNA clones randomly selected from a potato-P. infestans interaction cDNA library, three out of 22 appeared to correspond to a gene encoding translation elongation factor 1alpha. The gene, called tef1, is a single copy gene in P. infestans. During the life cycle of P. infestans, tef1 is expressed in all developmental stages. Alignment and phylogeny analysis based on EF-1alpha proteins from several taxonomic groups, including fungi, slime molds, algae, higher plants and archeabacteria, support the view that oomycetes evolved completely independently from the true fungi. In the phylogenetic tree, P. infestans EF-1alpha forms one branch with EF-1alpha from the unicellular alga Cyanophora paradoxa, an organism belonging to a taxonomic group that occupies a key position in the evolution of plastids.

Ric1 ,a Phytophthora infestans gene with homology to stress-induced genes

Abstract From a set of Phytophthora infestans cDNA clones that were randomly selected from a potato- P. infestans interaction cDNA library, a relatively high proportion (5 out of 22) appeared to be derived from the same gene. The gene was designated ric1. P. infestans contains two copies of ric1 which share 98% homology at the nucleotide-sequence level and 100% at the amino-acid level. The nucleotide sequence predicts an open reading frame of 171 bp encoding a 57 amino-acid hydrophobic-peptide with two potential membrane-spanning domains. The predicted peptide shows high homology to a peptide encoded by plant genes whose expression is specifically induced during stress conditions. Southern-blot analysis of genomic DNA of several Phytophthora species indicated that most species contain ric1 homologues. During the life cycle of P. infestans, ric1 was expressed in all developmental stages but the level of expression varied. Sporangia and germinating cysts appeared to contain only very little ric1 mRNA whereas in the mycelium and during in planta growth higher levels were detected. Subjecting the mycelium to osmotic stress or to a high pH resulted in increased ric1 expression.