Maintaining L1 Identity in Meristems and Organ Primordia

It seems likely that the molecular mechanisms involved in specification of protoderm cell fate during early embryogenesis may be closely related to those involved in maintaining that fate later in embryogenesis and during post germination growth. The maintenance of epidermal identity via an active signalling pathway is crucial to the maintenance of an organised epidermal monolayer, which in turn is of fundamental importance to proper plant function, especially in leaves. It is probably almost as physiologically important to prevent cells which leave the L1 layer via periclinal division during organ development from maintaining their epidermal fate, as it is to ensure that the cells on the outside of the plant maintain theirs.

Two possible mechanisms can again be envisaged for the maintenance of epidermal cell fate. The first is a constant perception of "outside" (the environment, or a lack of neighbours) which permits only externally situated cells to develop with L1 fate. The second is constant "lateral" signalling between epidermal cells, so that if cells leave the monolayer they lose their epidermal fate. Compelling evidence that a variation on the latter rather than the former mechanism may be the case comes from the observation that after removal of the epidermal cell layer, cells at the wound site never re-assume an epidermal fate, although they can redifferentiate as parenchyma tissues. Thus, it would seem that the positional information required for epidermal differentiation is irretrievably lost. This type of observation led Bruck and Walker to describe "epidermal differentiation as a one-time event" during their studies of Citrus jambhiri (Bruck 1985b). What form this lateral signalling takes is still an open question. One intriguing possibility is that signals deposited in the external cell wall or cuticle (which is present in a rudimentary form even in meristems) are responsible for part of this signalling, echoing possible mechanisms of cell-fate specification in the developing embryo.

The phenotypes of weak alleles of CR4 and DEK1 in maize, and the acr4 and ADEK1 RNAi phenotypes in Arabidopsis all indicate that both classes of protein are likely to be involved in interpretation of the positional signals required for maintaining epidermal identity and organisation. Ongoing studies of these proteins should thus allow the isolation of other potential signalling components in the pathways involved both in L1 maintenance and protoder-mal specification during embryogenesis.

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