Anatomy of sorgoleone-secreting root hairs of sorghum species

Johnsongrass (Sorghum halepense [L.] Pers.) and SX-17 (Sorghum bicolor × Sorghum sudanese) were investigated microscopically to identify specifically the location of root exudate production. Light, cryoscanning electron, and transmission electron microscopy were used to determine the area of exudate secretion. Light micrographs indicated that the exudate is solely produced by the root hairs. Scanning electron microscopy supported this conclusion. Transmission electron microscopy studies of root hairs support the hypothesis that root exudates are manufactured in the cytoplasmically dense root hair cell in association with smooth endoplasmic reticulum and possibly Golgi bodies. Ultrastructure studies indicated that small globules of cytoplasmic exudate are deposited between the cell wall and the plasma membrane, where they coalesce into larger globules that pass through the cell wall to form droplets near the tip of root hairs.