vir-Gene-inducing activities of hydroxycinnamic acid amides in Agrobacterium tumefaciens

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

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.

Tapetum-specific location of a cation-dependent O-methyltransferase in Arabidopsis thaliana

Cation- and S-adenosyl-L-methionine (AdoMet)-dependent plant natural product methyltransferases are referred to as CCoAOMTs because of their preferred substrate, caffeoyl coenzyme A (CCoA). The enzymes are encoded by a small family of genes, some of which with a proven role in lignin monomer biosynthesis. In Arabidopsis thaliana individual members of this gene family are temporally and spatially regulated. The gene At1g67990 is specifically expressed in flower buds, and is not detected in any other organ, such as roots, leaves or stems. Several lines of evidence indicate that the At1g67990 transcript is located in the flower buds, whereas the corresponding CCoAOMT-like protein, termed AtTSM1, is located exclusively in the tapetum of developing stamen. Flowers of At1g67990 RNAi-suppressed plants are characterized by a distinct flower chemotype with severely reduced levels of the N ',N ''-bis-(5-hydroxyferuloyl)-N '''-sinapoylspermidine compensated for by N(1),N(5),N(10)-tris-(5-hydroxyferuloyl)spermidine derivative, which is characterized by the lack of a single methyl group in the sinapoyl moiety. This severe change is consistent with the observed product profile of AtTSM1 for aromatic phenylpropanoids. Heterologous expression of the recombinant protein shows the highest activity towards a series of caffeic acid esters, but 5-hydroxyferuloyl spermidine conjugates are also accepted substrates. The in vitro substrate specificity and the in vivo RNAi-mediated suppression data of the corresponding gene suggest a role of this cation-dependent CCoAOMT-like protein in the stamen/pollen development of A. thaliana.

Coniferyl alcohol, a lignin precursor, stimulates Rhizobium rhizogenes A4 virulence

Rhizobium rhizogenes, a soil bacterium, is the causative agent of the neoplastic disease hairy root. Upon incubation of Rhizobium rhizogenes A4 with coniferyl alcohol, a lignin precursor, bacterial virulence on cotton cotyledon slices was stimulated. This was observed both in numbers of root hairs produced and in their length. Stimulation was maximized after exposure of bacteria to 150 microg/mL of coniferyl alcohol for 4 h. This was shown to be at the early log phase of bacterial growth.