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.