Pericycle Cell Proliferation and Lateral Root Initiation in Arabidopsis

Abstract

In contrast with other cells generated by the root apical meristem in Arabidopsis, pericycle cells adjacent to the protoxylem poles of the vascular cylinder continue to cycle without interruption during passage through the elongation and differentiation zones. However, only some of the dividing pericycle cells are committed to the asymmetric, formative divisions that give rise to lateral root primordia (LRPs). This was demonstrated by direct observation and mapping of mitotic figures, cell-length measurements, and the histochemical analysis of a cyclin-GUS fusion protein in pericycle cells. The estimated duration of a pericycle cell cycle in the root apical meristem was similar to the interval between cell displacement from the meristem and the initiation of LRP formation. Developmentally controlled LRP initiation occurs early, 3 to 8 mm from the root tip. Thus the first growth control point in lateral root formation is defined by the initiation of primordia in stochastic patterns by cells passing through the elongation and young differentiation zones, up to where lateral roots begin to emerge from the primary root. Therefore, the first growth control point is not restricted to a narrow developmental window. We propose that late LRP initiation is developmentally unrelated to the root apical meristem and is operated by a second growth control point that can be activated by environmental cues. The observation that pericycle cells divide and lateral root primordia form without intervening mitotic quiescence suggests that lateral organ formation in roots and shoots might not be as fundamentally different as previously thought.

Isolation, Characterization, and Pericycle-Specific Transcriptome Analyses of the Novel Maize Lateral and Seminal Root Initiation Mutant rum1

“…Remarkably, 19 of the 51 functionally annotated proteins that were preferentially expressed in wild-type pericycle cells were regulatory genes involved in signal transduction, transcription, and cell cycle. Preferential expression of these gene classes in the wild-type pericycle might indicate that those genes are involved in the regulatory network that finally leads to the dedifferentiation of the pericycle cells (Dubrovsky et al, 2000) and consequentially to cell division and lateral root primordia formation, a process that is blocked in the rum1 mutant. Several developmental processes in plants, e.g.…”

“…The initiation of lateral roots, i.e. the first division of founder cells (Lloret and Casero, 2002), is induced by endogenous signals of the maize plant, such as auxin, that stimulate pericycle cell division (Dubrovsky et al, 2000). In contrast to Arabidopsis (Arabidopsis thaliana), the sites of lateral root initiation cannot be predicted in maize roots (Bell and McCully, 1970).…”

Root system architecture: insights fromArabidopsisand cereal crops

“…It has traditionally been thought that the pericycle cells require a de-differentiation stage with exit from an arrested G 2 phase before renewal of cell division can occur [60,61], but observations suggest that the period between pericycle cell exit from the root apical meristem and the formation of founder is relatively short [62], thus allowing cells to remain division-competent without the requirement for dedifferentiation [63,64]. However, it does seem to be the case that a subset of pericycle cells remain in G 2 phase for an extended period of time after exit from the meristem, and it is these cells which are most susceptible to lateral root initiation [64].…”

SHORT-ROOT Regulates Primary, Lateral, and Adventitious Root Development in Arabidopsis

“…LRP initiation involves the asymmetric division of xylem pole pericycle founder cells (Dubrovsky et al, 2001). Based on data showing that the pericycle remains in G1 phase of the cell cycle (Beeckman et al, 2001) and that the xylem pole pericycle continues to cycle after leaving the RAM (Dubrovsky et al, 2000), it has been postulated that the pericycle acts as an extended monolayered meristem (Casimiro et al, 2003). While pericycle division is first visible in the differentiation zone well above the dividing RAM, xylem pole pericycle founder cells first need to be primed in the basal meristem (close to the elongation zone) in order to trigger LRP initiation (De Smet et al, 2007).…”