Chapter 8
central cylinder and the other to the cap. The inherent organizing capacity of the quiescent center for root development was demonstrated by in vitro culture of isolated quiescent centers of corn that were able to form whole roots directly without intermediate callus formation (Feldman and Torrey, 1975). Beyond the RAM, cells elongate (elongation zone), then differentiate. 5.1.2. Origin of the primary root meristem
Very detailed studies of cell fates have been published for the embryonic origin of the Arabidopsis RAM (Dolan et al., 1993, 1994; Scheres et al., 1994). The RAM comes from two clonally distinct cell populations: the upper tier of stem cells is derived from the apical daughter cell of the zygote whereas the quiescent center and the lower tier of stem cells originate, via the hypophysis, from the basal daughter cell of the zygote. Cell lineage analysis was performed with transgenic plants expressing the 35S:Ac:GUS construct, the excision of the Ac transposable element activating the GUS expression in sectors of the RAM that serve as cell lineage markers. This approach confirmed cell fates predicted by the anatomical analysis. Common origin for external layer and lateral root cap cells was demonstrated as well as for cortical and endodermal cells. Despite the apparent rigid organization, the root tip was also demonstrated as capable of flexibility as revealed by laser cell ablation experiments (Van den Berg et al., 1995). 5.2. GENES INVOLVED IN PRIMARY ROOT IDENTITY AND PATTERNING
5.2.1. Control of RAM, quiescent center and initials identity
Root development initiates with root specification. The HBT (HOBBIT), BDL (BODENLOS) and AXR6 (AUXIN RESISTANT 6) genes are required during embryogenesis and appear to have a pivotal role in root formation (Weigel and Meyerowitz, 1994; Willemsen et al., 1998). Mutations in these three genes give seedlings that lack a primary root but are capable of forming roots post-embryonically. These genes appear to mediate auxin-dependant processes. Arabidopsis mutants homozygous for a mutation in the ROOT MERISTEMLESS (RML1) gene are unable to establish an active root meristem and are glutathiol depleted; this mutation abolishes cell division in the root but not in the shoot and the gene that codes for a gamma-glutamylcysteine synthetase involved in glutathiol synthesis, was found
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to affect root development only in the post-embryonic period (Cheng et al., 1995; Vernoux et al., 2000b). Unfortunately, very few genes isolated up to now have been observed to be specifically involved in the determination of the primary root meristem as was the STM gene for the SAM. One gene, RCH1, which encodes a putative receptor kinase as the CLV1 gene in the SAM, have been shown to be RAM specific (Casamitjana-Martinez et al., 2003). The expression of other CLAVATA-like (CLE) genes, CLE19 and CLE40 was also detected in the RAM (CasamitjanaMartinez et al., 2003; Hobe et al., 2003). A WUSrelated homeobox gene, WOX5, has been found to be expressed in the quiescent center of Arabidopsis root (Haecker et al., 2004) as does its rice homolog QHB (Kamiya et al., 2003). These findings suggest that a WUS-CLV pathway might exist in the RAM (Fig. 8.12) as in the SAM but the role of the genes must be investigated further (Birnbaum and Benfey, 2004). Monopteros mutants lack root and hypocotyl in addition to their vascularization defects in the postembryonic phase; during embryogenesis, the first defect is observed at the octant stage with aberrant axialization of the lower tier of the embryo and aberrant hypophysis (Berleth and JĂźrgens, 1993). Interestingly, the MP protein is very similar to the AUXIN RESPONSE FACTOR 1 (ARF1), a transcription factor that binds auxin-responsive promoter elements, and is thought to mediate responses to auxin (Hardtke and Berleth, 1998). The use of ARF-binding elements (DR5) fused to reporter gene (GUS) to visualize free auxin, has revealed a maximum in the distal region of RAM and asymmetric auxin distribution was found to establish an organized pattern and polarity in the root meristem (Sabatini et al., 1999). Control of root growth and patterning would be exerted by the PIN auxin efflux protein (Blilou et al., 2005). 5.2.2. Control of radial patterning
Concerning the radial size of the RAM, a range of diverse root morphologies has been described in transgenic root clones expressing the rol genes from the root-inducing plasmid of Agrobacterium rhizogenes and/or bacterial genes involved in auxin biosynthesis such as tms1/tms2 from A. tumefaciens or their A. rhizogenes counterparts. Thick and ramified roots characterized the clones expressing genes for auxin synthesis and provided with high endogenous auxin levels, while roots expressing only the rol genes were thin and sparsely ramified (Prinsen et al., 1992; Chriqui et al., 1996; Guivarc’h et al., 1999).