Induction and Activation of Plant Secondary Metabolism by External Stimuli

Abstract

It is widely recognized that plant cells are potentially rich sources of commercially important secondary metabolites. The production of secondary metabolites in plants would be mainly controlled by transcriptional activities of a series of genes which encode the specific enzymes in the biosynthetic pathway of desired products. It is likely, however, that the catalytic activities of these enzymes are sometimes repressed or maintained at very low levels, and the mechanism involved in 'switch on' of the genetic information on secondary metabolism in plants is, at present, only very poorly understood. The response of plant cells to ‘elicitors’ was first studied from the phytopathological point of view to elucidate the regulation mechanism of phytoalexin production. However, extensive investigations clearly indicated (Zhao et al., 2005) that the treatment of plant cells with elicitor-active substances sometimes results in a rapid accumulation of secondary products other than defense-related compounds. Recently, it has been also demonstrated (Gundlach et al., 1992) that the function of elicitors can be sometimes replaced by jasmonic acid and its methyl ester, methyl jasmonate, the plant specific messenger molecules derived from arachidonic acid. Effective application of elicitors and/or jasmonates to the production of useful metabolites in plant cells requires the elucidation of the basic biochemical mechanisms by which these external stimuli regulate the genetic information involved in the biosynthesis of the natural products. Several questions are raised against the external stimuli-induced activation of the secondary metabolites production in plants. 1) How are the external signals recognized by plant cells? 2) How are the signals transduced in the cells? 3) How do the signals alter the expression of biosynthesis-related genes? 4) How are the enzyme activities controlled to produce secondary metabolites? Effective use of elicitors and jasmonates in producing useful metabolites in plants requires the elucidation of these biochemical mechanisms by which these stimuli regulate the genetic information. To answer these questions, we first attempted to elucidate the possible participation of second messengers in the secondary metabolism activation stimulated by exogenous signals.