Abstract
Several secreted and surface-associated conserved microbial molecules are recognized by the host to mount the defence response. One such evolutionarily well-conserved bacterial process is the production of cell-cell signalling molecules which regulate production of multiple virulence functions by a process known as quorum sensing. Here it is shown that a bacterial fatty acid cell-cell signalling molecule, DSF (diffusible signal factor), elicits innate immunity in plants. The DSF family of signalling molecules are highly conserved among many phytopathogenic bacteria belonging to the genus Xanthomonas as well as in opportunistic animal pathogens. Using Arabidopsis, Nicotiana benthamiana, and rice as model systems, it is shown that DSF induces a hypersensitivity reaction (HR)-like response, programmed cell death, the accumulation of autofluorescent compounds, hydrogen peroxide production, and the expression of the PATHOGENESIS-RELATED1 (PR-1) gene. Furthermore, production of the DSF signalling molecule in Pseudomonas syringae, a non-DSF-producing plant pathogen, induces the innate immune response in the N. benthamiana host plant and also affects pathogen growth. By pre- and co-inoculation of DSF, it was demonstrated that the DSF-induced plant defence reduces disease severity and pathogen growth in the host plant. In this study, it was further demonstrated that wild-type Xanthomonas campestris suppresses the DSF-induced innate immunity by secreting xanthan, the main component of extracellular polysaccharide. The results indicate that plants have evolved to recognize a widely conserved bacterial communication system and may have played a role in the co-evolution of host recognition of the pathogen and the communication machinery.
Highlights
Plants have evolved the ability to recognize the highly con- a defence response
Infiltration of N. benthamiana leaves with synthetic diffusible signal factor (DSF) induced browning of the infiltrated region, which appeared similar to hypersensitivity reaction (HR)-like symptoms, in a dose-dependent manner (Fig. 1D)
In order to examine the ability of DSF to induce callose deposition in plants, synthetic DSF was infiltrated in N. benthamiana, Arabidopsis, and rice leaves, at a concentration of 10–200 μM
Summary
Elicitors; examples are flagellin (Felix et al, 1999), lipopolysaccharide (Newman et al, 2002; Silipo et al, 2005), cold shock protein (Felix and Boller, 2003), and elongation factor Tu (Kunze et al, 2004). The recognition of these elicitors occurs primarily through receptors at the plant cell surface. Apart from MAMPs, plants have developed the ability to recognize endogenous molecules released upon damage caused due to microbial attack, known as damage-associated molecular patterns (DAMPs) (Darvill and Albersheim, 1984; Jha et al, 2007; Aparna et al, 2009)
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