Effects of 5-aminolevulinic acid on growth and amylase activity in the radish taproot

5-Aminolevulinic acid (ALA) promotes the growth of plants by enhancing their photosynthetic activities, but there is little information on how ALA influences the metabolism of sugars produced by photosynthesis. Here, we report the effects of ALA on tissue growth, sugar content, and amylase activity in the radish taproot. 5-Aminolevulinic acid was applied with a foliar spray (5.3-13,500 mu M), and application at concentrations of 53, 530, and 2,700 mu M enhanced the fresh weight of the taproot. Glucose is a major soluble sugar of the radish taproot. 5-Aminolevulinic acid slightly increased the glucose content but did not influence the fructose, sucrose, or starch contents. Radishes have beta-amylase (RsBAMY1), which is expressed in the taproot. 5-Aminolevulinic acid enhanced both the amylase activity and the RsBAMY1 protein accumulation. These results suggest that ALA may control starch accumulation by increasing the RsBAMY1 expression in the radish taproot. The relationship between taproot growth and free sugar accumulation by ALA is also discussed.