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.
BACKGROUND
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. 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.


RESULTS
A 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.


CONCLUSIONS
Our results provide new evidence for the roles of non-type III effector genes belonging to the Pto DC3000 HrpL regulon in virulence.
Área de Genética, Universidad de Málaga, Instituto de Hortofruticultura Subtropical y Mediterránea "La Mayora", Universidad de Málaga-CSIC (IHSM-UMA-CSIC), Campus de Teatinos, 29071, Málaga, Spain.|German Center for Neurodegenerative Diseases, DZNE, Otfried-Müller-Straße, 27, 72076, Tübingen, Germany.
Melissa G
MG
Castillo-Lizardo
Área de Genética, Universidad de Málaga, Instituto de Hortofruticultura Subtropical y Mediterránea "La Mayora", Universidad de Málaga-CSIC (IHSM-UMA-CSIC), Campus de Teatinos, 29071, Málaga, Spain.
Isabel M
IM
Aragón
Estación Experimental del Zaidín, CSIC (EEZ-CSIC), Profesor Albareda 1, 18008, Granada, Spain.
Vivian
V
Carvajal
Área de Genética, Universidad de Málaga, Instituto de Hortofruticultura Subtropical y Mediterránea "La Mayora", Universidad de Málaga-CSIC (IHSM-UMA-CSIC), Campus de Teatinos, 29071, Málaga, Spain.|Departamento de Producción Agraria, Universidad Pública de Navarra, Pamplona, Navarra, Spain.
Isabel M
IM
Matas
Estación Experimental del Zaidín, CSIC (EEZ-CSIC), Profesor Albareda 1, 18008, Granada, Spain.
María Luisa
ML
Pérez-Bueno
Estación Experimental del Zaidín, CSIC (EEZ-CSIC), Profesor Albareda 1, 18008, Granada, Spain.
María-Trinidad
MT
Gallegos
Estación Experimental del Zaidín, CSIC (EEZ-CSIC), Profesor Albareda 1, 18008, Granada, Spain.
Matilde
M
Barón
Área de Genética, Universidad de Málaga, Instituto de Hortofruticultura Subtropical y Mediterránea "La Mayora", Universidad de Málaga-CSIC (IHSM-UMA-CSIC), Campus de Teatinos, 29071, Málaga, Spain. crr@uma.es.
Cayo
C
Ramos
D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D018551:Solanum lycopersicum / Q000382:microbiology*; D010935:Plant Diseases / Q000382:microbiology*; D044224:Pseudomonas syringae / Q000235:genetics / Q000472:pathogenicity*; D018085:Regulon; D012808:Sigma Factor / Q000235:genetics / Q000378:metabolism*; D014774:Virulence; D037521:Virulence Factors / Q000235:genetics / Q000378:metabolism*
D001426:Bacterial Proteins; D004268:DNA-Binding Proteins; C085855:HrpL protein, Pseudomonas syringae; D012808:Sigma Factor; D037521:Virulence Factors