Investigation of Root Exudation and Phytotoxic Potential of Diverse Sorghum spp. through Regulation of Sorgoleone Biosynthesis

Sorgoleones, long chain benzoquinones, are exuded in large quantities by root hairs of Sorghum spp. The most abundant and phytoinhibitory hydroquinone is sorgoleone 358, although others are present in trace quantities. Sorgoleone and sorghum root exudates inhibit PQ synthesis, and electron transport in photosynthesis and respiration in sensitive seedlings. Research has focused solely on African Sorghum spp., including S. bicolor, S. halepense and S. sudanense. Australia however has 17 native Sorghum species which have not yet been examined with respect to photosynthesis, metabolism, root architecture and exudation. Therefore, a collection of 18 Sorghum species were assembled and evaluated for metabolism, and sorgoleone biosynthesis, with an emphasis on pathway flux and gene regulation. This collection represents most species in the genus, including 1, 4 and 13 species that are native to Asia, Africa and Australia, respectively. On average, Sorghum species of African, Asian or Australian origin produced variable amounts of root exudate (measured as dried exudate per root fresh weight), with African and Australian species producing up to 2.5 fold higher amounts than Asian species. Metabolic profiling of extracts enriched in exuded sorgoleones was performed using methanolic root dips of 10-day-old seedlings by UPLC/MS ESI QToF. Sorgoleones were detected by PDA detection at 280 nm and positively identified using accurate mass with known standards. Exudates differed qualitatively with Asian and African species yielding over 50 times greater abundance of sorgoleone compared to Australian species. Currently, flux analysis of the biosynthetic pathways present in African, Asian and Australian genotypes is underway with specific focus on genes regulating biosynthesis and root architecture, and photosynthetic efficiency. Study of Sorghum spp. native to three continents may be of importance in the selection for useful traits including those impacting root architecture, root exudation, and more competitive sorghum crops through enhanced weed suppression.