title: Indole-3-acetic acid in plant-pathogen interactions: a key molecule for in planta bacterial virulence and fitness.
abstract: The plant pathogenic bacterium Pseudomonas savastanoi, the causal agent of olive and oleander knot disease, uses the so-called "indole-3-acetamide pathway" to convert tryptophan to indole-3-acetic acid (IAA) via a two-step pathway catalyzed by enzymes encoded by the genes in the iaaM/iaaH operon. Moreover, pathovar nerii of P. savastanoi is able to conjugate IAA to lysine to generate the less biologically active compound IAA-Lys via the enzyme IAA-lysine synthase encoded by the iaaL gene. Interestingly, iaaL is now known to be widespread in many Pseudomonas syringae pathovars, even in the absence of the iaaM and iaaH genes for IAA biosynthesis. Here, two knockout mutants, ΔiaaL and ΔiaaM, of strain Psn23 of P. savastanoi pv. nerii were produced. Pathogenicity tests using the host plant Nerium oleander showed that ΔiaaL and ΔiaaM were hypervirulent and hypovirulent, respectively and these features appeared to be related to their differential production of free IAA. Using the Phenotype Microarray approach, the chemical sensitivity of these mutants was shown to be comparable to that of wild-type Psn23. The main exception was 8 hydroxyquinoline, a toxic compound that is naturally present in plant exudates and is used as a biocide, which severely impaired the growth of ΔiaaL and ΔiaaM, as well as growth of the non-pathogenic mutant ΔhrpA, which lacks a functional Type Three Secretion System (TTSS). According to bioinformatics analysis of the Psn23 genome, a gene encoding a putative Multidrug and Toxic compound Extrusion (MATE) transporter, was found upstream of iaaL. Similarly to iaaL and iaaM, its expression appeared to be TTSS-dependent. Moreover, auxin-responsive elements were identified for the first time in the modular promoters of both the iaaL gene and the iaaM/iaaH operon of P. savastanoi, suggesting their IAA-inducible transcription. Gene expression analysis of several genes related to TTSS, IAA metabolism and drug resistance confirmed the presence of a concerted regulatory network in this phytopathogen among virulence, fitness and drug efflux.
authors.affiliation: Dipartimento di Scienze delle Produzioni Agroalimentari e dell'Ambiente, Laboratorio di Patologia Vegetale Molecolare, Università degli Studi di Firenze, Via della Lastruccia 10, 50019 Sesto Fiorentino (Firenze), Italy.
authors.forename: Matteo
authors.initials: M
authors.lastname: Cerboneschi
authors.affiliation: Dipartimento di Scienze Produzioni Agroalimentari e dell'Ambiente, Genexpress, Università degli Studi di Firenze, Via della Lastruccia 14, 50019 Sesto Fiorentino (Firenze), Italy.
authors.forename: Francesca
authors.initials: F
authors.lastname: Decorosi
authors.affiliation: Dipartimento di Scienze delle Produzioni Agroalimentari e dell'Ambiente, Laboratorio di Patologia Vegetale Molecolare, Università degli Studi di Firenze, Via della Lastruccia 10, 50019 Sesto Fiorentino (Firenze), Italy.
authors.forename: Carola
authors.initials: C
authors.lastname: Biancalani
authors.affiliation: Dipartimento di Scienze delle Produzioni Agroalimentari e dell'Ambiente, Laboratorio di Patologia Vegetale Molecolare, Università degli Studi di Firenze, Via della Lastruccia 10, 50019 Sesto Fiorentino (Firenze), Italy.
authors.forename: Maria Vittoria
authors.initials: MV
authors.lastname: Ortenzi
authors.affiliation: Dipartimento di Scienze delle Produzioni Agroalimentari e dell'Ambiente, Laboratorio di Patologia Vegetale Molecolare, Università degli Studi di Firenze, Via della Lastruccia 10, 50019 Sesto Fiorentino (Firenze), Italy.
authors.forename: Sofia
authors.initials: S
authors.lastname: Macconi
authors.affiliation: Dipartimento di Scienze Produzioni Agroalimentari e dell'Ambiente, Genexpress, Università degli Studi di Firenze, Via della Lastruccia 14, 50019 Sesto Fiorentino (Firenze), Italy.
authors.forename: Luciana
authors.initials: L
authors.lastname: Giovannetti
authors.affiliation: Dipartimento di Scienze Produzioni Agroalimentari e dell'Ambiente, Genexpress, Università degli Studi di Firenze, Via della Lastruccia 14, 50019 Sesto Fiorentino (Firenze), Italy.
authors.forename: Carlo
authors.initials: C
authors.lastname: Viti
authors.affiliation: ICCOM CNR, Via G. Moruzzi 1, 56124 Pisa, Italy.
authors.forename: Beatrice
authors.initials: B
authors.lastname: Campanella
authors.affiliation: ICCOM CNR, Via G. Moruzzi 1, 56124 Pisa, Italy.
authors.forename: Massimo
authors.initials: M
authors.lastname: Onor
authors.affiliation: ICCOM CNR, Via G. Moruzzi 1, 56124 Pisa, Italy.
authors.forename: Emilia
authors.initials: E
authors.lastname: Bramanti
authors.affiliation: Dipartimento di Scienze delle Produzioni Agroalimentari e dell'Ambiente, Laboratorio di Patologia Vegetale Molecolare, Università degli Studi di Firenze, Via della Lastruccia 10, 50019 Sesto Fiorentino (Firenze), Italy. Electronic address: stefania.tegli@unifi.it.
authors.forename: Stefania
authors.initials: S
authors.lastname: Tegli
mesh_terms: D017353:Gene Deletion; D015964:Gene Expression Regulation, Bacterial; D053263:Gene Regulatory Networks; D054884:Host-Pathogen Interactions; D007210:Indoleacetic Acids / Q000378:metabolism*; D053858:Metabolic Networks and Pathways / Q000235:genetics; D030007:Nerium / Q000382:microbiology*; D010935:Plant Diseases / Q000382:microbiology; D010937:Plant Growth Regulators / Q000378:metabolism*; D011549:Pseudomonas / Q000235:genetics / Q000378:metabolism*; D014158:Transcription, Genetic; D014774:Virulence
chemical_list: D007210:Indoleacetic Acids; D010937:Plant Growth Regulators; C030737:indoleacetic acid
keywords: IAA-lysine synthase; Indole-3-acetic acid; Multidrug and toxic compound extrusion (MATE); Phenotype microarray (PM); Pseudomonas savastanoi; Type three secretion system