Pseudomonas savastanoi pv. mandevillae pv. nov., a Clonal Pathogen Causing an Emerging, Devastating Disease of the Ornamental Plant Mandevilla spp.

Abstract

Commercial production of the ornamental plant dipladenia (Mandevilla spp.) is threatened by dipladenia leaf and stem spot disease, caused by the bacterium Pseudomonas savastanoi. P. savastanoi includes four pathovars of woody hosts differentiated by a characteristic host range in olive, oleander, ash and broom plants. However, isolates from dipladenia have not been ascribed to any particular lineage or P. savastanoi pathovar. Here we report that isolates from dipladenia represent a distinct, clonal lineage. First, dipladenia isolates display very similar plasmid profiles, including a plasmid encoding the iaaM gene for biosynthesis of indole-3-acetic acid. Second, multilocus sequence analysis and core-genome single-nucleotide-polymorphisms phylogenies showed a monophyletic origin for dipladenia isolates, which cluster with isolates from oleander (pathovar nerii) in a distinct clade well separated from other P. savastanoi strains. Metabolic profiling and cross-pathogenicity tests in olive, oleander, ash, broom and dipladenia clearly distinguished dipladenia isolates from the four P. savastanoi pathovars. Comparative genomics of the draft genome sequence of the dipladenia strain Ph3 with the other four pathovars showed that Ph3 encodes very few strain-specific genes, and a similar set of virulence genes to pv. nerii, including its repertoire of type III secretion system effectors. However, hierarchical clustering based on the catalogue of effectors and their allelic variants clearly separated Ph3 from pv. nerii strains. Based on their distinctive pathogenicity profile, we propose a de novo pathovar for P. savastanoi isolates from dipladenia, P. savastanoi pv. mandevillae pv. nov., for which strain Ph3 (CFBP 8832PT) has been designated as the pathotype strain.

HrpL Regulon of Bacterial Pathogen of Woody Host Pseudomonas savastanoi pv. savastanoi NCPPB 3335

“…During the last decade, it has been reported that bacterial species of the P. syringae complex can naturally lack the hrp / hrc cluster, making them unable to induce HR in non-host plants or compromising their ability to cause disease in their respective hosts [ 19 , 20 , 21 ]. Genomic analysis carried out for Psv NCPPB 3335, Psn Psn23 , Psm Ph3, Psf NCPPB 1006, and Psr CECT 4861 confirmed that all of these strains contain a canonical hrp / hrc cluster [ 39 , 44 ], which allows them to cause disease symptoms in their respective woody hosts ( Figure 2 ). In the P. syringae complex, the role of HrpL as a global regulator of virulence has been mainly studied in pathogenic bacteria affecting herbaceous plants [ 48 , 49 , 79 , 80 , 81 ].…”

“…It is well known that the repertoire of bacterial T3Es, as well as the repertoire of plant resistance genes, can vary among bacterial strains or cultivars, thus conditioning the induction of a characteristic HR [ 76 , 77 ]. Furthermore, the characterization of strains Psn Psn23 and Psm Ph3 shows that they contain a truncated version of avrPto1 , which encodes a shorter version of the effector with a C-terminal deletion [ 39 , 44 ]. In this sense, it has been described that point mutations occurring at the C-terminal of AvrPto1 prevent HR formation in tobacco but not in tomato leaves, indicating that this part of the effector could participate in its recognition by the plant resistance protein [ 78 ].…”

“…retacarpa (Psr, isolated from broom), and pv. mandevillae (Psm, isolated from dipladenia) [ 37 , 39 ]. In the Psv and Psn strains, the functionality of the T3SS has been shown to be essential for knot formation in the respective hosts and the induction of a characteristic HR in resistant hosts [ 37 , 40 , 41 , 42 , 43 ].…”

“…Recent comparative genomics analysis of strains belonging to these five pathovars identified the codification of highly conserved canonical T3SSs in strains of Psf, Psm, and Psr. However, their functionality and roles in pathogenesis in these three pathovars have not yet been established [ 39 , 44 , 45 ]. Furthermore, and as previously reported for Psv NCPPB 3335 [ 46 ], an additional T3SS resembling that found in Rhizobiaceae was also found in the other pathovars [ 39 , 44 ].…”

“…However, their functionality and roles in pathogenesis in these three pathovars have not yet been established [ 39 , 44 , 45 ]. Furthermore, and as previously reported for Psv NCPPB 3335 [ 46 ], an additional T3SS resembling that found in Rhizobiaceae was also found in the other pathovars [ 39 , 44 ]. The relevance of T3SS regulation by HrpL in P. savastanoi is evidenced by the inability to induce knot formation in olive plants and to induce HR in tobacco plants by using a hrpL mutant of the model Psv strain NCPPB 3335 [ 47 ].…”