The elaboration of leaf form
In a typical angiosperm, a newly formed primordium undergoes
lateral growth (i.e. becomes flatter) and growth along the
proximal–distal axis (i.e. becomes longer). It has become
apparent that the phase of lateral growth is dependent on
the generation of a gradient within the primordium which
differentiates adaxial and abaxial tissue. In essence, after
primordium initiation, the future adaxial tissue becomes
defined by a set of specific transcription factor activities, as
does the abaxial tissue, and it is the juxtaposition of these two
Fig. 4 KNOX (Kn) and PHANTASTICA (Phan) gene expression
patterns presage the switching from indeterminate to determinate
cell fate. (a) The auxin patterning process leads to a local area of auxin
accumulation. (b) The local site of auxin accumulation corresponds to
the area in which KNOX gene expression is lost. This leads to derepression
of PHANTASTICA-like gene expression which is required for repression
of KNOX gene expression in the young leaf primordium. Loss of KNOX
gene expression and gain of PHANTASTICA-like gene expression
leads to determinancy of cell fate.