Lessons from the vegetative shoot apex - Intercellular Communication in Plants - page 169

Figure 6.5 Factors regulating the patterning of lateral organs. This figure shows genetic interactions
that are likely to pattern the emerging organ primordium. The Class III HD-ZIP genes - PHABULOSA ,
PHAVOLUTA and REVOLUTA - are expressed in the adaxial domain of a developing organ and in the
meristem. The adaxial expression pattern in organs is established in part by the miRNA-mediated
degradation of HD-ZIP RNA in the abaxial domain. HD-ZIPs, which also have a sterol-binding domain
(START domain), promote adaxial cell identity perhaps following activation by a meristem-derived
sterol (indicated with an arrow). Loss of HD-ZIP activity results in an abaxialisation of organs, a
phenotype similar to that seen when the abaxially expressed KANADI ( KAN ) genes are expressed
throughout a developing organ. Thus one function of HD-ZIP s may be to limit KAN expression to the
abaxial domain. KAN s promote abaxial identity as in their absence organs become adaxialised. As
HD-ZIP expression is detected in the abaxial domain of kan mutant organs, it is likely that KAN s
restrict HD-ZIP stothe adaxial domain. They may do this directly or perhaps by promoting miRNA
expression. The juxtaposition of cells with adaxial and abaxial identity triggers lateral growth, a
process that presumably requires signalling between cells. Redrawn from Emery et al. (2003).

adaxial cell types, ultimately leading to the formation of completely abaxialised
needle-like organs (Waites & Hudson, 1995). Sensitivity of the phan mutant pheno-
type towards cold suggested that PHAN , which is the orthologue of AS1 , promotes
adaxial cell identity redundantly with cold-sensitive factors (Waites & Hudson,
1995; Waites et al. , 1998). At the most severe, cold-grown phan mutants fail to
produce any organs and the meristem becomes inactive. A link between adaxial cell
identity and meristem activity is also apparent in Arabidopsis ,aslines ectopically
expressing KAN sinthe adaxial domain of the cotyledons fail to develop a shoot
apical meristem (Eshed et al. , 2001).
The characterisation of mutants with defects in adaxial-abaxial patterning has re-
vealed extensive signalling between the developing organ and meristem. The nature
of these signals is presently not well understood, but clearly the ability of the meris-
tem to impart adaxial identity on organs argues for the existence of a morphogen.

6.9 Conclusion

This chapter has described how diverse signalling pathways, which operate over dis-
tances of several cells to many hundreds of cells, are integrated into a functioning
shoot apical meristem. Analysis of the Arabidopsis genome has shown that there