SOR1, a gene associated with bioherbicide production in sorghum root hairs

Sorghum [Sorghum bicolor (L.) Moench] roots exude a potent bioherbicide known as sorgoleone, which is produced in living root hairs and is phytotoxic to broadleaf and grass weeds at concentrations as low as 10 μM. Differential gene expression was studied in sorghum (S. bicolorx S. sudanense) cv. SX17 between roots with abundant root hairs and those without root hairs using a modified differential display approach. A differentially expressed gene, named SOR1, was cloned by using Rapid Amplification of the 5′ ends of cDNA (5′-RACE). Real-time PCR analysis of multiple tissues of sorghum SX17 revealed that the SOR1 transcript level in root hairs was more than 1000 times higher than that of other tissues evaluated, including immature leaf, mature leaf, mature stem, panicle, and roots with hairs removed. Semi-quantitative RT-PCR revealed that SOR1 was expressed in the sorgoleone-producing roots of sorghum SX17, shattercane [S. bicolor (L.) Moench], and johnsongrass [S. halepense (L.) Pers.], but not in the shoots of sorghum or in the roots of sweet corn (Zea mays L.) 'Summer Flavor 64Y', in which sorgoleone production was not detected by HPLC analysis. Similarity searches indicated that SOR1 probably encodes a novel desaturase, which might be involved in the formation of a unique and specific double bonding pattern within the long hydrocarbon tail of sorgoleone.