Contribution of the non-effector members of the HrpL regulon, iaaL and matE, to the virulence of Pseudomonas syringae pv. tomato DC3000 in tomato plants.

Melissa G Castillo-Lizardo,María-Trinidad Gallegos,Matilde Barón,Isabel M Matas,Cayo Ramos,Isabel M Aragón,Vivian Carvajal,María Luisa Pérez-Bueno

doi:10.1186/s12866-015-0503-8

Abstract

BackgroundThe phytohormone indole-3-acetic acid (IAA) is widely distributed among plant-associated bacteria. Certain strains of the Pseudomonas syringae complex can further metabolize IAA into a less biologically active amino acid conjugate, 3-indole-acetyl-ε-L-lysine, through the action of the iaaL gene. In P. syringae and Pseudomonas savastanoi strains, the iaaL gene is found in synteny with an upstream gene, here called matE, encoding a putative MATE family transporter. In P. syringae pv. tomato (Pto) DC3000, a pathogen of tomato and Arabidopsis plants, the HrpL sigma factor controls the expression of a suite of virulence-associated genes via binding to hrp box promoters, including that of the iaaL gene. However, the significance of HrpL activation of the iaaL gene in the virulence of Pto DC3000 is still unclear.ResultsA conserved hrp box motif is found upstream of the iaaL gene in the genomes of P. syringae strains. However, although the promoter region of matE is only conserved in genomospecies 3 of this bacterial group, we showed that this gene also belongs to the Pto DC3000 HrpL regulon. We also demonstrated that the iaaL gene is transcribed both independently and as part of an operon with matE in this pathogen. Deletion of either the iaaL or the matE gene resulted in reduced fitness and virulence of Pto DC3000 in tomato plants. In addition, we used multicolor fluorescence imaging to visualize the responses of tomato plants to wild-type Pto DC3000 and to its ΔmatE and ΔiaaL mutants. Activation of secondary metabolism prior to the development of visual symptoms was observed in tomato leaves after bacterial challenges with all strains. However, the observed changes were strongest in plants challenged by the wild-type strain, indicating lower activation of secondary metabolism in plants infected with the ΔmatE or ΔiaaL mutants.ConclusionsOur results provide new evidence for the roles of non-type III effector genes belonging to the Pto DC3000 HrpL regulon in virulence.Electronic supplementary materialThe online version of this article (doi:10.1186/s12866-015-0503-8) contains supplementary material, which is available to authorized users.

Highlights

The phytohormone indole-3-acetic acid (IAA) is widely distributed among plant-associated bacteria
Genomic context analysis of the iaaL and matE loci in P. syringae and P. savastanoi Gene iaaL is widely distributed within the P. syringae complex, mainly within genomospecies 2, 3 and 4 corresponding to phylogenetic Multi-Locus Sequence Typing (MLST) groups 3, 1 and 4, respectively [22, 39]
Genomic context analysis of the iaaL gene in a selection of sequenced P. syringae and P. savastanoi strains belonging to genomospecies 2, 3 and 4 revealed that this gene is under synteny with the matE gene in all of these strains (Fig. 1a)

Summary

Introduction

The phytohormone indole-3-acetic acid (IAA) is widely distributed among plant-associated bacteria. Certain strains of the Pseudomonas syringae complex can further metabolize IAA into a less biologically active amino acid conjugate, 3-indole-acetyl-ε-L-lysine, through the action of the iaaL gene. Nerii, the causal agent of oleander (Nerium oleander) knot disease, converts IAA to indole-acetyl-ε-L-lysine (IAALys), a less biologically active compound as estimated in a coleoptile elongation assay [4,5,6]. This conversion involves the enzyme IAA-Lys synthase, encoded by the iaaL gene [7,8,9]. Inactivation of the iaaL gene by transposon mutagenesis in P. savastanoi pv. nerii resulted in the accumulation of IAA in the culture medium; this mutant did not cause typical knot symptoms, probably due to its inability to multiply within host tissues [8]

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