Document: TON-3-31-7

Supply-side economics of phenolic induction in plants: DAHPS and PAL coordinate the production pipeline.

SCHAEFFER, T.M*, A.BWALTON and J.CSCHULTZ

Penn State University, University Park, PA 16802 USA ¹

Abstract:
De novo production of phenolic compounds is a common response to insect herbivory, wounding, and disease in many plants. Phenolics are biosynthetic products of the shikimate (SP) and phenylpropanoid (PP) pathways and can comprise a substantial fraction of the plant's carbon budget. Understanding how this commitment is regulated is central to plant ecology. Jones and Hartley (1999, Oikos 86:27-44) propose that phenolic synthesis is limited by the deamination of phenylalanine (PHE) by phenylalanine-ammonia lyase (PAL), which marks the start of the PP and the end of the SP. PAL activity thus comprises a point of potential biosynthetic competition between proteins and phenolics. There are two problems with this view. First, many phenolics are synthesized by the SP, before PAL (e.g. gallic acid, chlorogenic acids, hydrolyzable tannins). Second, while herbivory, wounding, and disease increase the activity of PAL, the PHE supply to meet PAL's increased demand needs to be considered. Mechanical wounding and pathogen infection also have been shown to induce 3-deoxy-arabino heptulosonate 7-phosphate synthase (DAHPS), the first committed step in the SP. We hypothesized that coordinate regulation of both DAHPS and PAL is required to increase the carbon supply for enhanced phenolic production. In a completely randomized, bi-factorial design, we subjected five-week old tobacco plants to four levels of a wound signal, jasmonic acid (JA), and two levels of light over a one-week period. As predicted, DAHPS activity was induced by the application of JA. Enzyme activities for both DAHPS and PAL were higher in full light treatments than in shaded treatments, and phenolic production could be predicted based the activities of both DAHPS and PAL. Thus, either DAHPS or PAL can act as bottlenecks in phenolic production, and theories of phenolic production based on biochemical regulation need to focus on supply as well as demand.

Keywords: insect herbivory, wounding, jasmonic acid, JA, induced defenses, phenolics, phenylalanine-ammonia lyase, PAL, DAHPS, shikimate pathway, phenylpropanoid pathway, carbon allocation, Nicotiana tabacum

This abstract is being presented at: 3:30 PM in session:
HERBIVORE EFFECTS ON PLANTS