The jasmonate-inducible AP2/ERF-domain transcription factor ORCA3 activates gene expression via interaction with a jasmonate-responsive promoter element

The AP2/ERF-domain transcription factor ORCA3 is a master regulator of prim ary and secondary metabolism in Catharanthus roseus (periwinkle). Here we d emonstrate that ORCA3 specifically binds to and activates gene expression v ia a previously characterized jasmonate- and elicitor-responsive element (J ERE) in the promoter of the terpenoid indole alkaloid biosynthetic gene Str ictosidine synthase (Str). Functional characterization of different domains in the ORCA3 protein in yeast and plant cells revealed the presence of an N-terminal acidic activation domain and a serine-rich C-terminal domain wit h a negative regulatory function. Orca3 mRNA accumulation was rapidly induc ed by the plant stress hormone methyljasmonate with biphasic kinetics. A pr ecursor and an intermediate of the jasmonate biosynthetic pathway also indu ced Orca3 gene expression, further substantiating the role for ORCA3 in jas monate signaling. The protein synthesis inhibitor cycloheximide did not inh ibit jasmonate-responsive expression of Orca3, nor of its target genes Str and Tryptophan decarboxylase (Tdc). In conclusion, ORCA3 regulates jasmonat e-responsive expression of the Str gene via direct interaction with the JER E. The activating activities of ORCA proteins do not seem to depend on jasm onate-induced de novo protein synthesis, but presumably occur via modificat ion of pre-existing ORCA protein.