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of auxin to sites of organ formation (Reinhardt
et al.
, 2003b). According to this
model, different auxin distributions within the apex largely determine the plant's
phyllotaxis (Fig. 6.4C). The distribution of auxin may be influenced by a number of
factors including the relative auxin sink strengths within the apex, as well as the size
and growth rates of the central and peripheral zones of the shoot apical meristem.
6.7.3 Organ outgrowth involves physical forces
Given the likely role of auxin in phyllotaxis, it seems unlikely that physical forces
have a direct role in determining the position of organ formation. However, there
is evidence that physical forces are involved in the process of organ outgrowth.
Work in which the flexibility of meristem cell walls was altered using exogenously
applied expansins, proteins that weaken the bonds between cell wall polysaccharides
(Cosgrove, 2000), showed that an outgrowth with leaf-like qualities could be induced
to form on tomato apicies (Fleming
et al.
, 1997). Similar studies in tobacco, where
expansin activity was induced in all three layers of the meristem, rather than just the
epidermis, resulted in an outgrowth that formed a near perfect leaf (Pien
et al.
, 2001).
In both studies, formation of an organ at the site of expansin application affected
phyllotaxis, in many cases reversing the spiral arrangement of organs. These studies
imply that changes to cell wall flexibility, and by inference the physical forces acting
on these cells, are sufficient to induce the formation of organs. However, expansin
application generated outgrowths only at sites that would normally form organs
during the course of development. This suggests that cell wall loosening resulted
in the early outgrowth of an existing initial rather than the
de novo
formation of an
organ. Interestingly, localised changes to the pattern of cell division and proliferation
within the tobacco apex did not lead to organ formation (Wyrzykowska
et al.
, 2002;
Wyrzykowska & Fleming, 2003), suggesting that cell wall loosening is the first step
in organ outgrowth.
The process of organ formation and outgrowth may therefore involve a combi-
nation of signalling and physical forces. According to this model, accumulation of
auxin determines the site of organ formation, triggering the localised expression
of expansin and cell wall loosening. It should also be noted that auxin itself can
induce cell expansion. The pressure from underlying layers subsequently triggers
cell division and organ outgrowth. Consistent with this model is the observation
that certain isoforms of expansin are expressed specifically at sites of organ forma-
tion prior to any sign of organ development (Fleming
et al.
, 1997; Reinhardt
et al.
,
1998). Localised expansin expression also occurs at sites where auxin is applied
exogenously to the meristem (Reinhardt & Kuhlemeier, 2002).
6.8
Signalling between organ primordia and the meristem
As organs emerge from the shoot apical meristem, they typically flatten laterally
before developing unique cell types along the adaxial (facing the meristem) - abaxial
