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loss of the meristematic zone was achieved by over-expressing CLV3 in the root.
So far the CLAVATA-like pathway controlling meristematic activity during root
development remains to be identified, though a mutation in a putative Zn
2
+
-
carboxypeptidase can suppress the over-expressor phenotype, suggesting that it is
involved in processing the ligand (Casamitjana-Martinez
et al.
, 2003). Furthermore,
a
WUSCHEL
-like gene has been recently shown to be expressed in the QC cells in
rice (Kamiya
et al.
, 2003). How exactly the QC keeps the surrounding initials un-
differentiated remains as yet unknown. On the basis of the analysis by Casamitjana-
Martinez
et al.
(2003), the CLAVATA pathway in the root meristem does not seem
to act directly to maintain the undifferentiated state of the initials (as in shoot apical
meristem). Clearly more analysis has to be carried out and yet other factors need to
be identified. Based on a genetic analysis (Sabatini
et al.
, 2003), another molecule
required for the integrity of the QC is SCARECROW, a transcription factor that has
a role also in radial patterning of the ground tissue (as described below).
8.4.2
Genetic control of initiation of secondary roots
Over the years auxin has been linked to the development of secondary roots by
several mutants that have auxin-related problems (for a recent review see Casimiro
et al.
, 2003). Indeed, recently several genetic and physiological studies have pro-
vided evidence that auxin is required to facilitate lateral root formation at several
specific developmental stages. By using auxin-responsive reporter genes and sophis-
ticated mass spectrometry techniques, the auxin concentrations in
Arabidopsis
root
have been mapped in detail. Furthermore, the changes in auxin concentration and
localization have been correlated with the function of various proteins (Casimiro
et al.
, 2003). For example, the
aux1
mutation alters auxin content and distribution in
the root, leading to reduced number of lateral root primordia. Thus, the auxin influx
carrier AUX1, which is expressed in lateral root primordia before the first pericli-
nal cell division, has been implicated in regulation of lateral root development by
facilitating auxin transport between auxin source and sink tissues (Marchant
et al.
,
2002). Auxin transport between adjacent cells seems to be the most important factor
initiating lateral root formation.
Recently, the auxin reporter DR5 was used to analyse the presence of auxin in
the developing lateral root in
Arabidopsis
(Benkova
et al.
, 2003). They showed that
already at stage 0 of lateral root initiation, before any cell divisions have taken place,
there is an auxin maximum in the pericycle cells that will divide. This maximum
was present in all cells after the anticlinal cell divisions that form the short initial
cells in stage I. During the next stage (stage II) the maximum was confined to the
central cells of the two layers of lateral root initials. During the following stages
a gradient of
DR5
expression was established with its maximum in the columella
initials of a newly formed lateral root primordium, forming a pattern identical to that
of the primary root (Sabatini
et al.
, 1999). This gradient was shown to be dependent
on polar auxin transport by differentially expressed, functionally redundant PIN
proteins, whose sub-cellular localization was rearranged during lateral root initiation
