Purification and Characterization of Germination Stimulants from Sesamum indicum L. and Their Activity on Striga hermonthica (Del.) Benth

Abstract

Root parasitic weeds, a serious problem in agriculture, severely reduced yield of several economically important crops in tropical and semi-tropical regions n Africa.  The Striga seeds of these weeds may remain viable in soil for many years until germination is stimulated by a chemical signal exuded by roots of host and some non-host plants. Sesame, a major crop in Striga endemic areas, plant residues, irrespective of parts, drying method and amount of powder induced considerable Striga germination and haustorium initiation. However, germilings resulting from seeds stimulated to germinate by sesame powder, irrespective of plant parts and drying method, reduced radicle extension compared to positive control. Root exudates collected from hydroponically grown plants, induced 60% germination of S. hermonthica seeds. Sesame aqueous extract applied during conditioning reduced Striga germination and radicle extension in response to subsequent GR24 treatments in a concentration dependent manner. Striga seeds previously conditioned in Sesame aqueous extract for 3-15 days and applied with GR24 stimulated a considerable number of haustoria (70 -100%). The germination activity of the crude root exudates on S. hermonthica seed was comparable to that of the synthetic germination stimulant GR24 at 0.1 ppm. A bioassay guided column and high-performance liquid chromatography (HPLC) confirmed the stimulatory activity of the root exudates on S. hermonthica germination. Further analysis using HPLC connected to tandem mass spectrometry (LC/MS/MS) showed that the active fraction from sesame has a mass spectrum different from those of the natural strigolactones (strigol, sorgolactone) and is probably not a strigolactone.