Mode of Action, Localization of Production, Chemical Nature, and Activity of Sorgoleone: A Potent PSII Inhibitor in Sorghum spp. Root Exudates¹

Abstract

The root exudates produced by sorghums contain a biologically active constituent known as sorgoleone. Seven sorghum accessions were evaluated for their exudate components. Except for johnsongrass, which yielded 14.8 mg root exudate/g fresh root wt, sorghum accessions consistently yielded approximately 2 mg root exudate/g fresh root wt. Exudates contained four to six major components, with sorgoleone being the major component (> 85%). Three-dimensional structure analysis was performed to further characterize sorgoleone's mode of action. These studies indicated that sorgoleone required about half the amount of free energy (493.8 kcal/mol) compared to plastoquinone (895.3 kcal/mol) to dock into the QB-binding site of the photosystem II complex of higher plants. Light, cryo-scanning, and transmission electron microscopy were utilized in an attempt to identify the cellular location of root exudate production. From the ultrastructure analysis, it is clear that exudate is being produced in the root hairs and being deposited between the plasmalemma and cell wall. The exact manufacturing and transport mechanism of the root exudate is still unclear. Studies were also conducted on sorgoleone's soil persistence and soil activity. Soil impregnated with sorgoleone had activity against a number of plant species. Recovery rates of sorgoleone-impregnated soil ranged from 85% after 1 h to 45% after 24 h. Growth reduction of 9 14-d-old weed species was observed with foliar applications of sorgoleone.Nomenclature: Sorgoleone (2-hydroxy-5-methoxy-3-[(8′Z,11′Z)-8′,11′,14′-pentadecatriene]-p-hydroquinone); common purslane, Portulaca oleracea L. POROL; common ragweed, Ambrosia artemisiifolia L. AMBEL; cress, Lepidium sativum L. ;ns3 LEPSA; giant foxtail, Setaria faberi Herrm. SETFA; johnsongrass, Sorghum halepense (L.) Pers. SORHA; lambsquarters, Chenopodium album L. CHEAL; large crabgrass, Digitaria sanguinalis (L.) Scop. DIGSA; lettuce, Lactuca sativa L.; nightshade, Solanum spp.; purple photosynthetic bacterium, Rhodopseudomonas viridis; redroot pigweed, Amaranthus retroflexus L. AMARE; sicklepod, Cassia obtusifolia L. CASOB; spinach, Spinacea oleracea; shattercane, Sorghum bicolor (L.) Moensch SORVU; sorghum, S. bicolor (L.) Moensch SORVU; sudex, S. bicolor × Sorghum sudanense; sweet sorghum, S. bicolor ‘Della’; SX-15 and SX-17, S. bicolor × S. sudanense; 8446 and 855-F, S. bicolor (L.) Moensch SORVU; tomato, Lycopersicon esculentum L.; velvetleaf, Abutilon theophrasti Medicus ABUTH.Additional index words: Sorgoleone, root hairs, SORVU, SORHA.Abbreviations: ER, endoplasmic reticulum; HPLC, high-pressure liquid chromatography; PSII, photosystem II; QB, quinone binding; SEM, scanning electron microscopy; TEM, transmission electron microscopy; TLC, thin-layer chromatography; 3D, three dimensional; UV, ultraviolet.