Far beyond HopQ1 in the interaction between Nicotiana spp. and Pseudomonas syringae sensu lato

HopQ1 is an effector secreted by some strains of P. syringae sensu lato through the Type Three Secretion System, and which is suggested to be involved in host range determination (Ferrante et al., 2009). To definitely unveil its role in pathogenesis, data from increasing P. syringae sensu lato genomes is pivotal, as well as to have several model plants, whose genomic data are accessible or easily achievable, and amenable to molecular techniques (i.e. transformation, targeted mutagenesis and gene-silencing). Besides Arabidopsis thaliana, the most popular model plants for many P. syringae bacteria also included Nicotiana benthamiana, N. tabacum and Tomato. The development of similar model pathosystems is particularly useful to speed up studies on P. syringae diseases of woody plants, which are often less prone than herbaceous species to genetic manipulation and in vitro culture. In this work N. langsdorffii was used for the first time as a model species, challenged by P. savastanoi pv. nerii Psn23, a strain missing the gene coding for the effector HopQ1. This feature was here demonstrated to be mainly associated with pv. nerii strains rather than with those from savastanoi or fraxini pvs. Typical hyperplastic symptoms were induced by Psn23 wild type on N. langsdorffi, on which the bacterium multiplied comparably to its host Nerium oleander. Similarly, when overexpressed by Psn23, HopQ1 was demonstrated to be a virulence factor in both N. oleander and N. langsdorffii. Conversely on N. glauca, which has an opposite hormonal profile compared to N. lansgdorffi (Fuoco et al., 2013), no symptoms and bacterial growth were ever observed after inoculation of both Psn23 wild type or overexpressing hopQ1. When N. langsdorffii hormone profile was altered by stable expression of GR and rolC genes (Giannarelli et al. 2010), its master role in the fate of HopQ1 interaction with Psn23 was confirmed.