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.