Longitudinal root growth is mainly controlled by the final cell length in the root elongation zone

Abstract The longitudinal growth of the primary root is determined by consecutive events that take place along the main axis of the root. These events divide the root in several zones based on criteria including cellular activities, such as cell proliferation, and morphological features, such as cell size. In general, the root apex is divided into four zones. The meristematic zone is characterised by active cell division, resulting in cells that subsequently move to the transition zone for slow elongation. Following this, cells enter the elongation zone, where the elongation rate increases significantly. Finally, cells reach the mature zone, where elongation ceases. In the case of maize roots, the delimitation of these zones is achieved through a mathematical procedure that calculates the breakpoint in cell length, thereby identifying the limit between two contiguous zones. It is well established that the exogenous application of auxin and cytokinin inhibits root elongation. The present work investigates these effects by analysing changes in cell length in the various zones. The results showed that external hormones slightly modify the meristematic zone. However, the reduction of the maximum cell length, along with a shortage in the extension of the elongation zone, can explain the root inhibition, as a strong correlation was measured.