Transcriptional and post-translational regulation of S-adenosyl-L-methionine : salicylic acid carboxyl methyltransferase (SAMT) during Stephanotis floribunda flower development

Abstract

Methyl salicylate (MeSA) and a number of other volatiles are primarily emitted in the evening/night by Stephanotis floribunda leading to attraction of night active pollinators. A second minor emission peak for MeSA occurs in the morning/day. To understand these emission patterns, we have studied in detail the temporal regulation of the last step of the biosynthetic pathway of MeSA, the convertion of salicylic acid (SA) to MeSA catalysed by S-adenosyl-L-methionine: salicylic acid carboxyl methyltransferase (SAMT). We observed that in young flowers a maximum in SAMT activity occurs in the night, and that in flowers which were open longer than 4 days, two SAMT activity maxima occurred per day. These maxima correlated well with dawn and dusk and the previously detected MeSA emission peaks. The SAMT mRNA levels, however, have a broad maximum during the dark phase, while the SAMT protein levels continuously increase during floral development without showing daily rhythms. Furthermore, under continuous illumination (LL) the SAMT mRNA levels and activity patterns oscillate, suggesting the involvement of a circadian clock in the regulation network. Taken together, this analysis clearly demonstrates that regulation of MeSA emission occurs both at the transcriptional and post-translational levels, indicating that control at more than one level is necessary to guarantee the precise timing of volatile emission in flowers of S. floribunda.