A model of leaf formation
The information in this review provides an indication of the
complexity of the process of leaf formation. Many different
Tansley review
New Phytologist (2005) 166: 9–20 www.newphytologist.org © New Phytologist (2005)
18 Review
steps are involved, yet these steps must be coordinated in time
and space. An overview of the entire process is provided in
Fig. 6 in the form of a tentative flowchart of events.
The first phase in leaf formation involves patterning of
a band of cells around the periphery of the SAM. Strong
evidence indicates that the growth regulator auxin is intimately
involved in this patterning process. Although the target genes/
processes of this flux of auxin are not verified, a change in
cell wall extensibility to allow for the switch in growth vector
required for leaf primordium outgrowth seems necessary. A
number of lines of evidence suggests that the cell wall protein
expansin is likely to be involved as the mediator of this process.
Before leaf initiation occurs, KNOX gene expression is
down-regulated. This may also be a direct consequence of
auxin flux at the site of presumptive leaf formation. One consequence
of decreased KNOX gene expression is to promote
GA biosynthesis which may fix and/or amplify the altered
growth vector characteristic of the new leaf primordium. At
the same time, PHANTASTICA-like gene expression is upregulated
in the primordium. Gain of PHANTASTICA activity
and loss of KNOX activity determines the fate of the cells
incorporated into the leaf. At the same time, signalling processes
emanating from the SAM lead to the fixation of adaxial
tissue identity (via the expression of PHB/PHV genes in
this tissue) and, conversely, loss of PHB/PHV activity in the
abaxial tissue leads to gain of abaxial identity (via expression
of KANADI and YABBY genes). The juxtaposition of these
adjacent tissue identities allows lateral growth to occur to form
the lamina. As the leaf grows from an initial bulge on the
SAM, intercellular patterning signals are created within the
leaf which lead internally to the specification of different
tissues. In the case of the vascular tissue, auxin is intimately
involved. Similarly, in the leaf epidermis, intercellular signalling
pathways delineate the cell-specific expression patterns of
transcription factors which define the fate of those cells.
This model provides a general outline of the process of leaf
formation. Some aspects have been characterized in some
detail, but many parts involve speculation, in particular how
the different aspects are coordinated. It is clear that appropriate
timing and spatial coordination of transcription factor
activity is key to the formation of a functioning leaf and the
characterization of these factors continues apace (e.g. Eshed
et al., 2004). At the same time, it is clear that intricate and
possibly novel forms of communication occur within and
between the SAM and the young leaf. Elucidation and manipulation
of these signalling pathways will be essential for a full
understanding of the control of leaf development.