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, DiaaL and DiaaM, of strain Psn23 of P. savastanoi pv. nerii were produced. Pathogenicity tests using the host plant Nerium oleander showed that DiaaL and DiaaM 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 DiaaL and DiaaM, as well as growth of the non-pathogenic mutant DhrpA, 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.

Knots Untie: Molecular Determinants Involved in Knot Formation Induced by Pseudomonas savastanoi in Woody Hosts

“…A Psn PB213 iaaL mutant obtained by Tn 5 transposition displayed virulence attenuation in woody oleander plants in comparison with the wild-type strain (Glass and Kosuge, 1988). Conversely, a recent report showed that a knockout mutation of the iaaL gene in a different Psn isolate, Psn23, yielded a hypervirulent strain that resulted in a severe increase in knot size, higher systemic dispersion and a larger final population size than the wild-type strain in micropropagated (i.e., non-woody) oleander plants (Cerboneschi et al, 2016). Further studies are necessary to clarify the role of the iaaL gene in the virulence of P. savastanoi strains isolated from different hosts.…”

A MATE Transporter is Involved in Pathogenicity and IAA Homeostasis in the Hyperplastic Plant Pathogen Pseudomonas savastanoi pv. nerii

“…Plant pathogens have been demonstrated to produce IAA to hijack plant immunity, by subverting plant auxin signaling to increase host susceptibility to infection [6,[12][13][14]. In addition, microbial IAA is also essential as signal molecules within the producer populations, and in plant pathogenic bacteria, IAA was demonstrated to affect the expression of genes of their virulence network [7,[15][16][17].…”

“…nerii regulates free IAA levels in the infected tissues by its conversion to the conjugate IAA-Lysine (hereafter indicated as IAA-Lys), supposed to be less biologically active than the IAA free form. This reaction is mediated by the enzyme IAA-Lys synthase, encoded by the iaaL gene [17]. Interestingly, most of the P. syringae pathovars and strains possess the iaaL gene in their genomes, even if they do not cause hyperplastic symptoms, and this gene appears to be very well conserved and present independently from the genes for IAA biosynthesis [22,23].…”

“…In this frame, it is thus not surprising that in P. savastanoi pv. nerii both the expression of the operon for IAA biosynthesis and that of iaaL gene are also under the control of TTSS, in addition to being auxin-regulated, to further stress the involvement of bacterial IAA and IAA-Lys in the plant-pathogen dialogue since the very first steps of their interaction [17].…”

“…In the P. savastanoi pv. nerii strain Psn23 genome, a gene coding a putative multidrug and toxic compound extrusion (MATE) efflux transporter (hereafter named matE) was found upstream to the iaaL gene, and whose expression was demonstrated to be TTSS-regulated [17]. MATE transporters are widely distributed in Gram-positive and Gram-negative bacteria, where they are usually associated with the efflux of organic cations for multidrug resistance.…”