O-methyltransferase(s)-suppressed plants produce lower amounts of phenolic vir inducers and are less susceptible to Agrobacterium tumefaciens infection

Abstract

The first step of Agrobacterium tumefaciens/plant interaction corresponds to the activation of a transduction pathway of the bacterium by plant exudate. Phenolic compounds rapidly secreted by wounded plant cells induce the expression of bacterial virulence (vir) genes; however, little is known about their biosynthesis in plant. Here we show that inoculation of an Agrobacterium tumefaciens virulent strain on orthodiphenol-O-methyltransferases-suppressed tobacco plants leads to significantly smaller tumors compared to control plants. These transgenic plants are inhibited for caffeic acid O-methyltransferase class I or II (OMT; EC 2.1.1.6) and/or caffeoyl-coenzyme A O-methyltransferase (CCoAOMT; EC 2.1.1.104) that are involved in monolignol biosynthesis. The significant decrease of tumor size could be suppressed by the pre-activation of bacterial virulence, before inoculation, using acetosyringone a known vir inducer. Total soluble phenolic amounts and cell wall composition analyzed by FT-IR analysis did not show significant differences between transgenic and control plants. The potential of phenolic extracts from control and OMT-suppressed plants to induce virulence was evaluated using an Agrobacterium tumefaciens reporter strain carrying a vir::LacZ gene fusion plasmid. Lower vir-inducing activities were recorded for plants that show inhibition to caffeic acid O-methyltransferase activity. HPLC analysis confirmed that the levels of several phenolic compounds were differently affected by wounding and/or by bacterial inoculation. Statistical correlations were established between tumor sizes, vir-inducing activities, O-methyltransferases proteins accumulations and the levels of various soluble phenolic compounds such as acetosyringone. These results demonstrate the role of the O-methyltransferases of the phenylpropanoid pathway in the early production of soluble Agrobacterium tumefaciens vir inducers.

The role of the secondary cell wall in plant resistance to pathogens

“…Phenolic compounds are also important in plant-pathogen recognition. For example, the expression of Agrobacterium tumefaciens Virulence ( Vir ) genes, needed for infection, is induced by phenolic compounds and infection of the host plant cannot take place in the absence of these compounds (Lee et al, 1992, 1995; Maury et al, 2010). …”

“…nicotianae (Way et al, 2002, 2011; Shadle et al, 2003). Notably, COMT and CCoAOMT antisense tobacco lines were more resistant to Agrobacterium tumefaciens infection and showed a reduced tumor area and mass relative to wild-type plants (Maury et al, 2010). The phenolic compounds secreted by these antisense plants upon wounding did not induce the expression of the bacterial Vir genes equally well as those secreted from wounded wild-type plants (Maury et al, 2010).…”

The Family Rhizobiaceae

“…The release and transfer of T-DNA into the plant genome occurs through a diverse class of virulence genes (vir). Plant phenolic compounds such as p-hydroxybenzoic acid and vanillin induce the expression of the vir genes (Maury et al 2010;Wise et al 2010).…”

The role of CCoAOMT1 and COMT1 in Arabidopsis anthers

“…CCoAOMT1 contributes to the formation of soluble sinapoyl conjugates in Arabidopsis leaves (Do et al 2007) and is essential for the biosynthesis of the coumarin scopoletin in Arabidopsis roots (Kai et al 2008). Its role in the biosynthesis of acetosyringone, an early inducer of the Agrobacterium tumefaciens-derived vir-genes has also been proposed (Maury et al 2010). Therefore, ccoaomt1 but also comt1 plants appear less susceptible to this pathogen.…”