Caffeoyl‐coenzyme A O‐methyltransferase cDNA was cloned from dark‐grown Ammi majus L. (Apiaceae) cells treated with a crude fungal elicitor and the open reading frame was expressed in Escherichia coli . The translated polypeptide of 27.1‐kDa shared significant identity to other members of this highly conserved class of proteins and was 98.8% identical to the corresponding O‐methyltransferase from parsley. For biochemical characterization, the recombinant enzyme could be purified to apparent homogeneity by metal‐affinity chromatography, although the recombinant enzyme did not contain any affinity tag. Based on sequence analysis and substrate specificity, the enzyme classifies as a cation‐dependent O‐methyltransferase with pronounced preference for caffeoyl coenzyme A, when assayed in the presence of Mg2+‐ions. Surprisingly, however, the substrate specificity changed dramatically, when Mg2+ was replaced by Mn2+ or Co2+ in the assays. This effect could point to yet unknown functions and substrate specificities in situ and suggests promiscuous roles for the lignin specific cluster of plant O‐methyltransferases.

Viroids are small (246–401 nucleotides), non‐coding, circular RNAs able to replicate autonomously in certain plants. Viroids are classified into the families Pospiviroidae and Avsunviroidae , whose members replicate in the nucleus and chloroplast, respectively. Replication occurs by an RNA‐based rolling‐circle mechanism in three steps: (1) synthesis of longer‐than‐unit strands catalyzed by host DNA‐dependent RNA polymerases forced to transcribe RNA templates, (2) processing to unit‐length, which in family Avsunviroidae is mediated by hammerhead ribozymes, and (3) circularization either through an RNA ligase or autocatalytically. Disease induction might result from the accumulation of viroid‐specific small interfering RNAs that, via RNA silencing, could interfere with normal developmental pathways.

Jasmonic acid (JA) and its derivatives are well‐characterized signaling molecules in plant defense and development, but the site of their localization within plant tissue is entirely unknown. To address the question whether applied JA accumulates extracellularly or intracellularly, leaves of tomato and barley were fed with 14C‐labeled JA and the label was localized in cryofixed and lyophilized leaf tissues by microautoradiography. In tomato the radioactivity was detectable within the apoplast, but no label was found within the mesophyll cells. By contrast, in barley leaf tissues, radioactivity was detected within the mesophyll cells suggesting a cellular uptake of exogenously applied JA. JA, applied to leaves of both plants as in the labeling experiments, led in all leaf cells to the expression of JA‐inducible genes indicating that the perception is completed by JA signal transduction.

Coronalon, a synthetic 6‐ethyl indanoyl isoleucine conjugate, has been designed as a highly active mimic of octadecanoid phytohormones that are involved in insect and disease resistance. The spectrum of biological activities that is affected by coronalon was investigated in nine different plant systems specifically responding to jasmonates and/or 12‐oxo‐phytodienoic acid. In all bioassays analyzed, coronalon demonstrated a general strong activity at low micromolar concentrations. The results obtained showed the induction of (i) defense‐related secondary metabolite accumulation in both cell cultures and plant tissues, (ii) specific abiotic and biotic stress‐related gene expression, and (iii) root growth retardation. The general activity of coronalon in the induction of plant stress responses together with its simple and efficient synthesis suggests that this compound might serve as a valuable tool in the examination of various aspects in plant stress physiology. Moreover, coronalon might become employed in agriculture to elicit plant resistance against various aggressors.

A recently discovered, S‐adenosyl‐L ‐methionine and bivalent cation‐dependent O‐methyltransferase from the ice plant, Mesembryanthemum crystallinum , is involved in the methylation of various flavonoid and phenylpropanoid conjugates. Differences in regiospecificity as well as altered kinetic properties of the recombinant as compared to the native plant O‐methyltransferase can be attributed to differences in the N‐terminal part of the protein. Upon cleavage of the first 11 amino acids, the recombinant protein displays essentially the same substrate specificity as observed earlier for the native plant enzyme. Product formation of the newly designed, truncated recombinant enzyme is consistent with light‐induced accumulation of methylated flavonoid conjugates in the ice plant. Therefore, substrate affinity and regiospecificity of an O‐methyltransferase in vivo and in vitro can be controlled by cleavage of an N‐terminal domain.