Stimulation of Root Growth Induced by Aluminum in <i>Quercus serrata</i> Thunb. Is Related to Activity of Nitrate Reductase and Maintenance of IAA Concentration in Roots

Abstract

Aluminum (Al) is the most abundant metal in the earth’s crust. Excess Al3+ released by soil acidification in soil solution is thought to be a growth limiting factor to many cultivated plant species, but it has been reported to stimulate plant growth in some crop and tree species in certain concentration of Al3+. Previously, we had reported that Al treatment enhanced root development, uptake from growth media and in vivo nitrate reductase (NR) activity of roots. NR is one of the key enzymes in nitrogen metabolism and acts at the first step of nitrate assimilation in plants. In this study, we investigated the process of Al-induced root development in an early stage, focusing on the change in in vitro NR activity, and indole-3-acetic acid (IAA) and cytokinins concentration in roots of Quercus serrata seedlings, which were treated for 1 h with Al or Ca. In Al-treated roots, NR activity increased and IAA concentration was maintained at the same level as pretreatment, and indole-3-acetyl-L-aspartic acid (IA-Asp), which is a metabolic intermediate of IAA degradation, was not detected in roots. In Ca-treated roots, NR activity increased, but IAA concentration decreased as IA-Asp concentration increased. Thus, the maintenance of IAA concentration in Al-treated roots seems to result from suppression in the process of IAA decomposition. Al treatment increased the length and number of second lateral roots but Ca treatment did not. We concluded that root development induced by Al in the early stage was related to NR activity and maintenance of IAA concentration.