|Lateral Root Initiation (LRI)|
In Arabidopsis thaliana, Lateral Root Initiation (LRI) is hallmarked by coordinated asymmetric divisions in adjacent xylem pole pericycle cells. As many developmental processes it is strongly regulated by the plant hormone auxin.
A major advantage over other processes is that LRI can be manipulated easily, which allowed us to design a lateral root-inducible system (Himanen et al., 2002). Genome-wide analyses of transcriptional changes upon this synchronized induction of lateral roots resulted in the elucidation of some crucial aspects of LRI (Himanen et al., 2004 – Cell cycle; Vanneste et al., 2005 – SLR/IAA14; De Smet et al., 2008 – ACR4; De Rybel et al., 2010 – GATA23; De Rybel, Audenaert et al. 2012). A tool allowing to combine these datasets into a versatile compendium has been developed (Parizot et al., 2010) and allowed the selection of new candidate genes for the LRI. These datasets were also made available in the eFP browser inline tool : LRI eFP browser
One of these breakthroughs was the identification of the transcription factor GATA23 as an important regulator of lateral root founder cell specification. The formation of new lateral roots starts with the specification of lateral root founder cells in the basal meristem of the primary root (De Smet et al., 2007). This event, also called priming, is linked with the auxin response maximum oscillation (Figure 1) and is Aux/IAA28 and GATA23-dependent (Figure 2). Higher up in the root these founder cells can start to divide and give rise to new primordia when stimulated to do so by high levels of auxin, a process which is regulated by SLR/IAA14. A newly formed, dome-shaped primordium results from both formative and proliferative divisions. The receptor-like kinase ACR4 is involved in maintaining this fragile balance (Figure 3).
Lateral root priming occurs in the basal meristem. Schematic overview of the different root tissues (a) and the auxin signalling maximum present (b) as reported by the DR5::GUS marker line . The regularly spaced auxin accumulation sites in the protoxylem primes the adjacent pericycle cells to become pericycle founder cells. (Péret et al., 2009)
Schematic representation of the auxin signaling modules acting successively during lateral root founder cell specification in the basal meristem (module 1) and nuclear migration and lateral root initiation processes (modules 2 and 3). As a consequence of module 1, GATA23 is hypothesized to be involved in the determination of founder cell identity in XPP cells. (De Rybel et al., 2010)
Early events during lateral root initiation. Local auxin accumulation in primed xylem pole pericycle cells activates the auxin signalling cascade. Auxin causes the degradation of IAA14, thereby de-repressing ARF7 and 19 and activating downstream gene expression. The receptor-like kinase ACR4 promotes formative divisions in the primordium and represses cell divisions in surrounding pericycle cells . Abbreviation: LOB, LATERAL ORGAN BOUNDARIES. (Péret et al., 2009)
GATA23 Expression Analysis : Section through a stage I lateral root primordium showing pGATA23::GUS expression at one side of the xylem pole pericycle (XPP). Asterisks and arrowheads indicate phloem pole and XPP cells, respectively. (De Rybel et al., 2010)
LRI datasets compendium -
LRI datasets were compiled and integrated in the eFP browser online tool.
Here is a snapshot of the relative expression of GATA23 in these experiments (low expression values are masked in grey)
A Novel aux/IAA28 Signaling Cascade Activates GATA23-dependent Specification of Lateral Root Founder Cell Identity
Receptor-like kinase ACR4 restricts formative cell divisions in the Arabidopsis root.
Auxin-dependent regulation of lateral root positioning in the basal meristem of Arabidopsis
Cell Cycle Progression in the Pericycle Is Not Sufficient for SOLITARY ROOT/IAA14-Mediated Lateral Root Initiation in Arabidopsis thaliana
Transcript profiling of early lateral root initiation
Arabidopsis lateral root development: an emerging story.