Role of Salicylic Acid in Pseudomonas aeruginosa Strain IE-6S+-Mediated Induction of Systemic Resistance against Meloidogyne javanica in Tomato

Abstract

Root colonization by certain non-pathogenic bacteria can induce systemic resistance to pathogen infections
in plants. In a split-root assay with tomato plants, we investigated which determinants of the rhizobacterium
Pseudomonas aeruginosa IE-6S+ were important for induction of resistance to the root-knot nematode Meloidogyne
javanica. P. aeruginosa IE-6S+ produced 3.9±1.1 µg ml-1 salicylic acid (SA) in a liquid casamino acid medium under
laboratory conditions. The bacterial inoculant induced resistance equivalent to the application of 10 mM synthetic
SA. However, SA at this concentration did not produce significant mortality of M. javanica juveniles in vitro. Soil iron
(2.4 mM FeCl3·6H2O) did not markedly alter the resistance that P. aeruginosa IE-6S+ induced in tomato roots, which
suggested that P. aeruginosa IE-6S+ activity was not iron-regulated. However, the resistance reaction was greatly
enhanced when IE-6S+ and SA were co-inoculated with 0.5% Tween-20. While IE-6S+ colonized the tomato rhizosphere
at 6.38 log cfu per g fresh weight of root during the first 3 days after inoculation, the bacterial populations declined
steadily, reaching a mean population density of 4.73 log cfu g-1 fresh weight of root at 21 days. The bacterium was not
isolated from the unbacterized half of the split root system.