The N-acyl homoserine-lactone depleted Rhizobium radiobacter mutant RrF4NM13 shows reduced growth-promoting and resistance-inducing activities in mono- and dicotyledonous plants
Ibrahim Alabid, Martin Hardt, Jafargholi Imani, Anton Hartmann, Michael Rothballer, Dan Li, Jenny Uhl, Philippe Schmitt‐Kopplin, Stefanie P. Glaeser, Karl‐Heinz Kogel
Abstract
Abstract The Alphaproteobacterium Rhizobium radiobacter (syn. Agrobacterium tumefaciens , “ A. fa brum ”) can live in close association with the sebacinoid fungus Serendipita (syn. Piriformospora ) indica that forms a mutualistic Sebacinalean symbiosis with a wide range of host plants. The endobacterial strain R. radiobacter F4 ( Rr F4), which was originally isolated from the fungu s , has plant growth promotion and resistance-inducing activities resembling the beneficial activities known from the endobacteria-containing S. indica . The mechanism by which free endobacterial cells influence growth and disease resistance of colonized host plants is not fully understood. Here, we show that Rr F4 produces a spectrum of quorum sensing-mediating N -acyl-homoserine lactones (AHLs) with acyl chains of C8, C10, and C12 as well as hydroxyl- or oxo-substitutions at the C3 position. In addition, and in line with previous findings showing that AHLs increase plant biomass and induce systemic resistance, the AHL-depleted lactonase-overexpressing transconjugant Rr F4NM13 was partially compromised in promoting growth and inducing resistance against bacterial pathogens in both Arabidopsis thaliana and wheat ( Triticum aestivum ). Scanning and transmission electron microscopy proved that Rr F4NM13, in contrast to Rr F4, does not form cellulose-like fiber scaffolds for efficient root surface attachment. Moreover, Rr F4NM13 does not penetrate into the intercellular space of the cortical tissue, which in contrast is strongly colonized by Rr F4. We discuss the possibility that AHLs contribute to the outcome of the Sebacinalean symbiosis.