Key message Glucose was a key substance as an energy source in the root growth promotion by Al, and ABA may relate to metabolism involved with its process. Generally, excess aluminum (Al) ions in soil solution are toxic to many cultivated plant species, but beneficial effects of Al for plant growth have been reported. Previously, we reported stimulation of root growth and nitrate reductase (NR) activity by Al. In this study, we focused on sugars, such as sucrose, glucose, and fructose, as energy sources and also signaling substances to regulate root growth. To understand the mechanism of root growth stimulation by Al, we investigated the change in concentration of sugars and phytohormones, and the activity of NR in roots using Quercus serrata seedlings. Ten-week-old Q. serrata seedlings were hydroponically cultured with nutrient solution containing 2.5 mM Al (pH 4.0) or 3.25 mM calcium (Ca) (pH 4.0) for 3 and 15 days. The growth of first lateral root and NR activity was stimulated for 3 and 15 days of Al treatment. The concentration of starch and sucrose decreased, while the concentration of glucose increased in the Al-treated roots. The concentration of abscisic acid (ABA) in Al-treated roots increased gradually throughout the experiment. From the present study, the mechanism of root growth promotion by Al involves a complex signaling network. We suggest that glucose is a key substance as an energy source and a signaling substance to promote root growth induced by Al and ABA may relate to nitrogen (N) and carbon (C) metabolism involved with the signaling network to promote root growth induced by Al.