Responses of Mikania micrantha, an Invasive Weed to Elevated CO2: Induction of β-Caryophyllene Synthase, Changes in Emission Capability and Allelopathic Potential of β-Caryophyllene

To better understand the effect of predicted elevated levels of carbon dioxide (CO2) on an invasive weed Mikania micrantha, we constructed a suppressive subtractive hybridization (SSH) library from the leaves of M. micrantha exposed to CO2 at 350 and 750 ppm for 6 d, and isolated a novel gene named beta-caryophyllene synthase. beta-Caryophyllene synthase catalyses the conversion of farnesyl diphosphate to beta-caryophyllene, a volatile sesquiterpene with allelopathic potential. Real-time PCR analysis revealed that gene expression of beta-caryophyllene synthase in M. micrantha leaves was strongly induced in response to elevated CO2. Gas chromatography-mass spectrometry (GC-MS) and gas chromatography (GC) analyses showed that emission levels of beta-caryophyllene from leaves of M. micrantha increased when exposed to 750 ppm CO2. Bioassays showed that phytotoxicity of beta-caryophyllene against Raphanus sativus, Brassica campestris, Lactuca sativa, and M. micrantha was dose-dependent and varied with the receptor plants and concentrations of CO2. beta-Caryophyllene displayed higher phytotoxic effects at 750 ppm than those at 350 ppm CO2, especially on R. sativus. These results suggest that elevated atmospheric CO2 levels may enhance biosynthesis and phytotoxicity of allelochemicals in M. micrantha, one of the worst invasive weeds in the world, which in turn might enhance its potential allelopathic effect on neighboring native plants if released in bioactive concentrations. Further investigations are required to determine the adaptive responses of both invasive and native plants to a gradual increase of atmospheric CO2 to 750 ppm predicted over a 100 year period.