Changes in leaf form: developmental and
environmental influences
The type of leaf formed by a plant at any one time is under
both developmental and environmental regulation. Some of
these changes may be very dramatic, such as the switch in
leaf form displayed by some aquatic plants as the stem enters
a gaseous environment. Other significant changes in leaf form
and differentiation may also occur in response to different
light regimes leading, for example, to the formation of shade
leaves. At the molecular level, the greatest advances in our
understanding have been gained from the developmentally
controlled change in leaf form during the lifetime of a plant,
so-called phase change.
In Arabidopsis, the juvenile and adult phases of vegetative
development are characterized by differences in leaf size and
shape and by the distribution of trichomes on the leaf blade.
As a result of screening for mutations in genes affecting this
phase change, Poethig and colleagues identified a number of
gene products involved in this process. Interestingly, one of
these is an ARGONAUTE-like protein implicated in the
processing of miRNAs (Hunter et al., 2003b). As described
above, miRNAs have now been implicated in the acquisition
of leaf adaxial/abaxial polarity and leaf shape and this provides
another example suggesting that they play a fundamental role
in many aspects of leaf development. Other genes implicated
in leaf phase have also been identified and these share the
characteristic of being implicated in the regulation of nuclear
transport (Bollman et al., 2003; Hunter et al., 2003a). The
precise significance of this is still to be elucidated, but the data
suggest that the control of movement of (as yet uncharacterized)
factors between the nucleus and the cytoplasm are
involved in phase change.