Pseudomonas savastanoi pv. savastanoi: some like it knot

Abstract

Pseudomonas savastanoi pv. savastanoi is the causal agent of olive (Olea europaea) knot disease and an unorthodox member of the P. syringae complex, causing aerial tumours instead of the foliar necroses and cankers characteristic of most members of this complex. Olive knot is present wherever olive is grown; although losses are difficult to assess, it is assumed that olive knot is one of the most important diseases of the olive crop. The last century has witnessed a good deal of scientific articles describing the biology, epidemiology and control of this pathogen. However, most P. savastanoi pv. savastanoi strains are highly recalcitrant to genetic manipulation, which has effectively left the pathogen out of the scientific progress in molecular biology that has elevated the foliar pathogens of the P. syringae complex to supermodels. A series of studies in the last years have made significant advances in the biology, ecology and genetics of P. savastanoi pv. savastanoi, paving the way for the molecular dissection of its interaction with other non-pathogenic bacteria and their woody hosts. The selection of a genetically pliable model strain was soon followed by the development of rapid methods for virulence assessment with micropropagated olive plants and the analysis of cellular interactions with the plant host. The generation of a draft genome of strain NCPPB 3335 and the closed sequence of its three native plasmids has allowed for functional and comparative genomic analyses for the identification of its pathogenicity gene complement. This includes 34 putative type III effector genes and genomic regions, shared with other pathogens of woody hosts, that encode metabolic pathways associated with the degradation of lignin-derived compounds. Now, the time is right to explore the molecular basis of the P. savastanoi pv. savastanoi-olive interaction and to get insights into why some pathovars like it necrotic and why some like it knot.

Synonyms: Pseudomonas syringae pv. savastanoi Taxonomy: Kingdom Bacteria; Phylum Proteobacteria; Class Gammaproteobacteria; Family Pseudomonadaceae; Genus Pseudomonas; included in genomospecies 2 together with at least P. amygdali, P. ficuserectae, P. meliae and 16 other pathovars from the P. syringae complex (aesculi, ciccaronei, dendropanacis, eriobotryae, glycinea, hibisci, mellea, mori, myricae, phaseolicola, photiniae, sesami, tabaci, ulmi, and certain strains of lachrymans and morsprunorum); when a formal proposal is made for the unification of these bacteria, the species name P. amygdali would take priority over P. savastanoi. Microbiological properties: Gram-negative rods, 0.4-0.8 by 1.0-3.0 µm, aerobic. Motile by one to four polar flagella, rather slow growing, optimal temperatures for growth of 25–30 °C, oxidase negative, arginine dihydrolase negative, elicits the hypersensitive response on tobacco, most isolates are fluorescent and levan negative although some isolates are non-fluorescent and levan positive. Host range: P. savastanoi pv. savastanoi causes tumours in cultivated and wild olive and ash (Fraxinus excelsior). Although strains from olive were reported to infect oleander (Nerium oleander), this is generally not the case; however, strains of P. savastanoi pv. nerii can infect olive. Pathovars fraxini and nerii differentiate from pv. savastanoi mostly in their host range, and were not formally recognized until 1996. Literature previous to about 1996 generally name strains of the three pathovars as P. syringae subsp. savastanoi or P. savastanoi subsp. savastanoi, contributing to confusion about host range and biological properties. Disease symptoms: Symptoms of infected trees include hyperplastic growths (tumorous galls or knots) on the stems and branches of the host plant and, occasionally, on leaves and fruits. Epidemiology: The pathogen can survive and multiply on aerial plant surfaces, as well as in knots, from where it can be dispersed by rain, wind, insects and human activities, entering the plant through wounds. Populations are very unevenly distributed in the plant, and suffer drastic fluctuations throughout the year, with maximum numbers of bacteria occurring during rainy and warm months. Populations of P. savastanoi pv. savastanoi are normally associated to non-pathogenic bacteria, both epiphytically and endophytically, and were demonstrated to form mutualistic consortia with Erwinia toletana and Pantoea agglomerans that could result in increased bacterial populations and disease symptoms. Disease control: Based on preventive measures, mostly sanitary and cultural practices. Integrated control programs benefit from regular applications of copper formulations, which should be maintained at least a few years for maximum benefit. Olive cultivars vary in their susceptibility to olive knot, but there are no known cultivars with full resistance to the pathogen. Useful websites: http://www.pseudomonas-syringae.org/; http://genome.ppws.vt.edu/cgi-bin/MLST/home.pl; ASAP access to the P. savastanoi pv. savastanoi NCPPB 3335 genome sequence https://asap.ahabs.wisc.edu/asap/logon.php.