4.2. Hormones in root elongation
A functional root apical meristem ensures proper root elongation. The root meristem structure consists of three specific zones: the proximal meristem zone, including the quiescent centre responsible for maintaining the stem cell population in the root apical meristem, the transition zone and the elongation–differentiation zone. The root growth rate is highly correlated to the size of the root meristem, the latter being determined by a finely adjusted balance between cell division, expansion and differentiation. The antagonism between auxin and cytokinins contribute to the maintenance of the proper root meristem structure.
In recent years, evidence accumulated on ethylene’s role in controlling root elongation at two different levels: regulating cell proliferation in the root apical meristem and cell elongation in the elongation zone. Strigolactones are also suggested to regulate primary root length. Indeed, exogenous application of GR24 led to elongation of the primary root and increased meristem cell number. In rice, the role of strigolactones on root elongation might be the control of cell division. In Arabidopsis, low ABA concentrations promote root growth by encouraging the quiescent centre and suppressing stem cell differentiation; in rice, under nonstressed conditions, ABA stimulates root hair elongation in a polar auxin transport-dependent manner. The fact that ABA supports root elongation was also demonstrated in transgenic rice lines in which one of the ABA-catabolic enzymes was knocked out.