Developmental regulation and physical interaction among enzymes involved in sorgoleone biosynthesis

Ammonium in the soil is converted into nitrate by nitrifying bacteria or archaea. While nitrate is readily available for plants, it is prone to leaching and contributes to eutrophication. In addition, when the soil conditions become anaerobic, nitrate can be reduced to nitrous oxide, a powerful greenhouse gas. Therefore, slowing nitrification in agricultural soil offers some benefits by reducing nitrogen loss and decreasing water and air pollution. Since nitrogen is a limiting nutrient for most ecological niches, many plants have evolved specialized compounds that reduce nitrification. One such compound, sorgoleone, which is secreted from the root hair of sorghum, has been relatively well studied due to its allelopathic function, with most enzymes involved in its biosynthesis elucidated. However, the secretion mechanisms remain unknown. Previous studies reported numerous lipidic vesicles in the sorghum root hair and speculated that they are involved in sorgoleone storage or secretion, but their roles remain unclear. Also, the subcellular organelles that are involved in sorgoleone synthesis have not been identified. In the present study, we found that the expression of sorgoleone biosynthesis enzymes is induced in a specific root zone, indicating that the secretion is developmentally regulated. The accumulation of internal vesicles preceded the peak of sorgoleone biosynthesis and secretion, indicating that the vesicles play a role in precursor storage rather than secretion. Moreover, our data suggest that enzymes that catalyze the first three steps, SbDES2, SbDES3, and SbARS1, interact with each other to form a multi-enzyme complex on the endoplasmic reticulum surface.