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application of auxin (Reinhardt
et al.
, 2000). Similarly, local application of auxin
onto wild-type tomato and
Arabidopsis
inflorescences can induce organ outgrowth,
with the size of the primordium being proportional to the amount of applied auxin
(Reinhardt
et al.
, 2000, 2003b). However, application of auxin to the central zone
does not lead to organ outgrowth, showing that auxin is only active in the peripheral
zone of the apex. These observations suggest that a patterned distribution of auxin
might largely account for phyllotaxis.
Although the loss of polar auxin transport and auxin signalling in the
pin1
and
pid
mutants does not affect the expression of genes involved in meristem maintenance,
genes regulating organ identity are no longer expressed in a phyllotactic pattern
(Christensen
et al.
, 2000; Vernoux
et al.
, 2000; Reinhardt
et al.
, 2003b). These genes
are instead expressed in concentric rings around the naked stem, suggesting that one
function of auxin is to limit their expression to sites of organ formation. Based
on these and other observations, a model of phyllotaxis involving two signalling
processes has been proposed (Reinhardt
et al.
, 2000; Vernoux
et al.
, 2000; Reinhardt
&Kuhlemeier, 2002). One, that does not involve auxin, promotes the formation of
evenly spaced rings of cells that are competent to assume lateral organ identity in the
periphery of the meristem. And a second auxin-dependent mechanism that partitions
each ring into lateral organ and non-lateral organ initials. According to this model,
partitioning is achieved by the initiating organ primordia generating a localised
zone of auxin depletion which prevents neighbouring cells from forming organs, as
originally proposed by Sachs (1991). A higher auxin concentration at more distant
positions leads to primordium formation (and perhaps promotes increased auxin
accumulation).
This model assumes that the distribution of auxin is patterned within the meris-
tem, and that this pattern determines future sites of organ initiation. Two elegant
studies have recently used the distributions of PIN1 protein to infer the likely move-
ment and distribution of auxin within the meristem (Benkova
et al
., 2003; Reinhardt
et al.
2003b). PIN1 protein is present in the plant membranes and accumulates in the
side of the cell that is actively involved in auxin efflux (Galweiler
et al.
, 1998). In the
meristem, PIN1 accumulates in the apical side of L1 and L2 cells, suggesting that
auxin moves towards the summit of the apex through the epidermal and subepider-
mal tissue. However, around the sites of future organ formation, PIN1 accumulation
is redirected to membranes facing the incipient primordium, showing that a change
in auxin movement is one of the earliest markers of organ formation. Following
primordium outgrowth, PIN1 distribution indicates that auxin flows from the sur-
rounding tissue into the centre of the primordium; then later as the organ enlarges,
auxin moves through the epidermis towards the apex of the organ before being redi-
rected inwards towards the centre. Auxin movement in maturing organs as inferred
by PIN1 localisation is consistent with the process of canalisation, the selective
channelling of auxin that leads to the formation of the vasculature (Sachs, 1991).
The use of PIN1 as a marker for auxin movement clearly shows that auxin is
patterned within the meristem, and that auxin accumulation is closely associated with
organ formation (see Fig. 6.4). However if auxin is the signal underlying phyllotaxis,
