Several pieces of evidence have been presented herein to support the view that the plant epidermal cell layer may play an important role in regulating plant growth and morphology. However, it is very likely that mechanisms also exist to tailor the growth of the epidermal cell layer to that of underlying cells. The observations made regarding the potential roles of WOX proteins in organ initiation, and particularly the fact that their over expression can lead to ectopic growth of the epidermis, are important indicators to the existence of such mechanisms.
The molecular basis for the co-ordination of plant epidermal growth with that of underlying tissues may, as previously proposed, involve the movement of developmentally important transcriptional or cell-cycle regulators between cell-layers. A good example of this involves the well-documented movement of the transcription factor KNOTTED1 (KN1) from the L2 (where it is expressed) to the L1 cell layer of the maize shoot apical meristem. Studies of the regulation of the movement of KN1 in maize, and that of related proteins in Arabidopsis, suggest that it is mediated by sequences within the proteins and may be necessary for endogenous protein function (Lucas et al. 1995; Kim et al. 2003).
In addition to the movement of proteins through plasmodesmata, it is also possible that the growth of cell layers may be co-ordinated by apoplastic signalling. This possibility is supported by the observation that three Leucine Rich Repeat RLKs, ERECTA (ER), ERECTA-LIKE1 (ERL1) and ERL2 act syn-ergistically to control organ growth in Arabidopsis (Shpak et al. 2003, 2004). Triple mutants between the three genes are severely dwarfed and show patterning defects in floral organs. Shpak and colleagues propose that ER, ERL1 and ERL2 are responsible in part for linking cell proliferation to the growth and morphogenesis of organs. This is an interesting hypothesis especially in light of recent work from the same group showing that in the epidermis, the synergistic activity of these three proteins is apparently modified by the superposition of epidermis-specific apoplastic signalling, involving the TOO MANY MOUTHS receptor-like protein, involved in deciding the developmental fate of cells in the stomatal lineage (Shpak et al. 2005). This is an interesting example of how a proliferative vs. differentiation decision can in volve local modifications of globally expressed signalling pathways and may provide a paradigm for another mechanism by which proliferation in different tissues/cell layers could be co-ordinated.
In conclusion, the plant epidermis, in addition to its obvious and critically important physiological roles, also appears to play an important role in controlling the growth and morphogenesis of shoot organs. Because loss of epidermal identity or integrity is usually fatal, investigating this role presents serious technical challenges. However, as our knowledge of the types of molecules potentially involved both in epidermal specification/organisation, and in the regulation of plant growth increases, so does the number of tools available to investigate how these developmental processes interact.
Acknowledgements I would like to thank the Royal Society of London and the EPSRC (RCUK fellowships) for financial support, Dr. Justin Goodrich for access to his data, and Prof. Andrew Hudson for helpful comments.
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