4.3. Hormones in root angle determination
Root growth angle determines whether the root will grow deep into the
soil or opposite at the surface in the upper layers of the soil. The
final root growth angle reflects three environmental responses:
gravitropism, phototropism and hydrotropism. Roots show positive
gravitropism. The ability to sense gravity is correlated with the
presence of statoliths in the columella cells (forming the root cap).
Statoliths are amyloplasts that sediment at the bottom of the columella
cells. When gravity orientation changes by rotating plants, the
statoliths start falling to the new bottom side of the cells. It is
assumed that this movement distorts the endoplasmic reticulum, releasing
Ca2+ into the cytoplasm and consequently modifying the
intracellular pH. Changes in pH might affect polar auxin transport while
inducing the re-localization of PIN proteins. This results in an unequal
distribution of auxin from columella cells to the lateral root cap and
auxin transport from the root cap to the epidermal cells of the
elongation zone. This leads to differential cell elongation and root
curvature in the elongation zone. PIN proteins and the auxin influx
carrier AUXIN RESISTANT 1 (AUX1) have been reported to function in root
gravitropism. It is fascinating to note that the root angle varies
between the different classes of roots (primary, lateral, seminal). It
has been proposed that this behaviour reduce self-competition and
maximise the soil volume to be explored.
Root growth angle does not solely rely on response to gravitropism but
also nutrient availability, temperature or hydropatterning. For this
developmental process, literature exclusively mentions auxin’s role
while other hormones are not reported. This is understandable as all
mutants affected in root angle growth are related to the auxin
signalling pathway. Almost 20 years ago, Aloni and coworkers suggested
for the first time that cytokinins control the early response to
gravitropism. Indeed, a gravistimulation induced an asymmetric
accumulation of cytokinin in the statoliths within less than 30 min.
This undoubtedly caused the downward curvature near the root apex. In
Arabidopsis, cytokinins could interact with auxin, ethylene and glucose
signalling to trigger directional root growth. Whether such interactions
are conserved in cereal crops is unknown. Interestingly, mutant
loss-of-function in CRL4/OsGNOM1 , CRL1 or CRL5genes affected crown root development and gravitropic response. To
understand the genetic control of root system architecture, including
root growth angle, several QTLs and genes have been determined to play a
central role in root growth angle in cereals.