15. The Regulation of Dorsiventral Symmetry in Plants
233
6 Adaxial, Abaxial, and Marginal Identities
In bifacial leaves, ad-abaxial leaf polarity is cytohistologically recognizable soon
after initiation, when earlier vacuolation of the larger abaxial portion leads to
acrovergent curvature of the primordium (Hagemann 1970; Hagemann and
Gleissberg 1996). Cross sections of early-stage primordia typically show the densely
staining meristematic marginal domains that are connected by a band of procambium. Cells adjacent to this band in both the abaxial and adaxial domains are already more vacuolated at that time. It is possible that adaxial and abaxial domain
identities specify not only the sites of the meristematic margins but also the densely
staining band of procambial cells by which the developing margins are connected
(Mcdard 1988). This procambial sheet develops into the arc-shaped row of vascular
bundles typical for petioles of bifacialleaves.
A marginal blastozone and the sheet of procambial tissue can also be seen in
leaf sectors that become unifacial at maturity. The circular arrangement is achieved
by the additional differentiation of adaxially located bundles from the adaxial meristem. It remains to be shown how mutant leaves with abaxialized sectors at maturity behave developmentally in this respect. Early stages of phan mutant leaves as
seen in the scanning electron micrograph (SEM) (Waites and Hudson 1995) show
external flattening. More detailed examination is necessary to determine (1) if early
leaf development of cylindrical, abaxialized mutant leaves exhibit densely cytoplasmic marginal and procambial domains; (2) if an adaxial meristem is active; (3)
if the adaxial components of the vascular circle differentiate later than their abaxial
counterparts; and (4) if mature mutant leaves retain bifaciality at their bases.
7 Marginal Identity and Proximo-Distal Growth
The specification of adaxial versus abaxial domains may occur as early as in the
preprimordium (leaf anlage) (Timmermans et al. 1998; Waites and Hudson 1995).
This may coincide with and could result in the formation of a crescent-shaped,
small-celled meristematic zone at the shoot apex flank that precedes the outgrowth
of the leaf primordium (Hagemann 1970). Hagemann and Gleissberg (1996) considered the leaf apex to be an integrated part of the marginal blastozone by which
the ridge-shaped growth direction of the leaf is specified. If the determination of a
marginal blastozone requires the presence of a contact zone of adaxial and abaxial
identity, one would suggest at least an initial brief imposition of adaxial identity in
the preprimordium. In contrast, Timmermans et al. (1998) concluded that the formation of a proximal/distal growth axis can still occur in the absence of adaxial and
marginal domains in the lbll mutant. The same is suggested by the phan mutant
studied by Waites and Hudson (1995) and Waites et al. (1998), and the adaxialized
leaves of the phabulosa mutant of A rabi do psis (McConnell and Barton 1998). Similarly, the "sympodial leaf model" of unifaciality suggested that the growth axis of
233
6 Adaxial, Abaxial, and Marginal Identities
In bifacial leaves, ad-abaxial leaf polarity is cytohistologically recognizable soon
after initiation, when earlier vacuolation of the larger abaxial portion leads to
acrovergent curvature of the primordium (Hagemann 1970; Hagemann and
Gleissberg 1996). Cross sections of early-stage primordia typically show the densely
staining meristematic marginal domains that are connected by a band of procambium. Cells adjacent to this band in both the abaxial and adaxial domains are already more vacuolated at that time. It is possible that adaxial and abaxial domain
identities specify not only the sites of the meristematic margins but also the densely
staining band of procambial cells by which the developing margins are connected
(Mcdard 1988). This procambial sheet develops into the arc-shaped row of vascular
bundles typical for petioles of bifacialleaves.
A marginal blastozone and the sheet of procambial tissue can also be seen in
leaf sectors that become unifacial at maturity. The circular arrangement is achieved
by the additional differentiation of adaxially located bundles from the adaxial meristem. It remains to be shown how mutant leaves with abaxialized sectors at maturity behave developmentally in this respect. Early stages of phan mutant leaves as
seen in the scanning electron micrograph (SEM) (Waites and Hudson 1995) show
external flattening. More detailed examination is necessary to determine (1) if early
leaf development of cylindrical, abaxialized mutant leaves exhibit densely cytoplasmic marginal and procambial domains; (2) if an adaxial meristem is active; (3)
if the adaxial components of the vascular circle differentiate later than their abaxial
counterparts; and (4) if mature mutant leaves retain bifaciality at their bases.
7 Marginal Identity and Proximo-Distal Growth
The specification of adaxial versus abaxial domains may occur as early as in the
preprimordium (leaf anlage) (Timmermans et al. 1998; Waites and Hudson 1995).
This may coincide with and could result in the formation of a crescent-shaped,
small-celled meristematic zone at the shoot apex flank that precedes the outgrowth
of the leaf primordium (Hagemann 1970). Hagemann and Gleissberg (1996) considered the leaf apex to be an integrated part of the marginal blastozone by which
the ridge-shaped growth direction of the leaf is specified. If the determination of a
marginal blastozone requires the presence of a contact zone of adaxial and abaxial
identity, one would suggest at least an initial brief imposition of adaxial identity in
the preprimordium. In contrast, Timmermans et al. (1998) concluded that the formation of a proximal/distal growth axis can still occur in the absence of adaxial and
marginal domains in the lbll mutant. The same is suggested by the phan mutant
studied by Waites and Hudson (1995) and Waites et al. (1998), and the adaxialized
leaves of the phabulosa mutant of A rabi do psis (McConnell and Barton 1998). Similarly, the "sympodial leaf model" of unifaciality suggested that the growth axis of