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S. Gleissberg et al.
the leaf can be uncoupled from the site of its margin. It is, however, possible that
the divergence of the margin from the leaf apex occurs as a postgenital event as in
the formation of cross zones below a precursor tip (sepals of Passiflora caerulea,
Kunze 1985). Critical developmental studies in conjunction with studies on expression of the responsible genes in preprimordia and early post-initiation primordia
will help to decide the issues of proximal/distal versus marginal growth directions.
8 Adaxialized Leaves - the Exception?
In almost all species exhibiting unifaciality it is the adaxial leaf surface that is
suppressed (Napp-Zinn 1973). Loss-of-function mutations associated with radiality
and loss of transverse polarity like phan inAntirrhinum (Waites and Hudson 1995)
and Ibll in Zea (Timmermans et al. 1998) also show abaxialized leaves. This may
indicate that adaxial identity relies on a more labile developmental pathway that is
perturbed by knock-outs of a single gene or a few genes. Known mutants with
adaxialized leaves like phbl in Arabidopsis (McConnell and Barton 1998) and
Rldl and Cel in maize (Timmermans et al. 1998) are dominant and could thus
represent gain-of-function mutants of a factor conferring adaxial identity. The recessive agol mutant of Arabidopsis (Bohmert et al. 1998) could, however, represent a loss-of-function mutant in which abaxial identity is lost. agol leaves are
surrounded by trichomes typically developed on the adaxial surface of rosette leaves.
Adaxialized (inverse unifacial) leaf organs are known from the bracts of
Peperomia (Napp-Zinn 1973). A particularly interesting case is the leaves of two
varieties of Codiaeum variegatum, that exhibit both normal and inverse unifaciality
in a singlc leaf (Baum 1952). This curious phenotype is in accordance with the idea
that symmetry is controlled by the variation of the expression domain of a single
adaxializing factor.
9 Experimental Investigations of Ab-Adaxiality in
Leaves
As a leaf initiates on the shoot apical meristem it quickly arches over the SAM
(shoot apical meristem) due to more growth on the abaxial face. These cells are
usually more vacuolated and larger than cells on the adaxial face of the organ.
These features give the leaf a dorsi ventral symmetry almost from the point of inception at the apex. Is the acquisition of dorsiventrality in a leaf primordium conditioned by proximity to the apical meristem? Wardlaw (1949) suggested that an
inhibitory influence from the shoot apical meristem may prevent growth on the
adaxial side of the leaf primordium. Early investigators tried to answer this question by using experimental approaches like microsurgery. Sussex (1955) used fine
incisions to isolate the incipient leaf primordium (11 or Po) from the shoot meristem.
S. Gleissberg et al.
the leaf can be uncoupled from the site of its margin. It is, however, possible that
the divergence of the margin from the leaf apex occurs as a postgenital event as in
the formation of cross zones below a precursor tip (sepals of Passiflora caerulea,
Kunze 1985). Critical developmental studies in conjunction with studies on expression of the responsible genes in preprimordia and early post-initiation primordia
will help to decide the issues of proximal/distal versus marginal growth directions.
8 Adaxialized Leaves - the Exception?
In almost all species exhibiting unifaciality it is the adaxial leaf surface that is
suppressed (Napp-Zinn 1973). Loss-of-function mutations associated with radiality
and loss of transverse polarity like phan inAntirrhinum (Waites and Hudson 1995)
and Ibll in Zea (Timmermans et al. 1998) also show abaxialized leaves. This may
indicate that adaxial identity relies on a more labile developmental pathway that is
perturbed by knock-outs of a single gene or a few genes. Known mutants with
adaxialized leaves like phbl in Arabidopsis (McConnell and Barton 1998) and
Rldl and Cel in maize (Timmermans et al. 1998) are dominant and could thus
represent gain-of-function mutants of a factor conferring adaxial identity. The recessive agol mutant of Arabidopsis (Bohmert et al. 1998) could, however, represent a loss-of-function mutant in which abaxial identity is lost. agol leaves are
surrounded by trichomes typically developed on the adaxial surface of rosette leaves.
Adaxialized (inverse unifacial) leaf organs are known from the bracts of
Peperomia (Napp-Zinn 1973). A particularly interesting case is the leaves of two
varieties of Codiaeum variegatum, that exhibit both normal and inverse unifaciality
in a singlc leaf (Baum 1952). This curious phenotype is in accordance with the idea
that symmetry is controlled by the variation of the expression domain of a single
adaxializing factor.
9 Experimental Investigations of Ab-Adaxiality in
Leaves
As a leaf initiates on the shoot apical meristem it quickly arches over the SAM
(shoot apical meristem) due to more growth on the abaxial face. These cells are
usually more vacuolated and larger than cells on the adaxial face of the organ.
These features give the leaf a dorsi ventral symmetry almost from the point of inception at the apex. Is the acquisition of dorsiventrality in a leaf primordium conditioned by proximity to the apical meristem? Wardlaw (1949) suggested that an
inhibitory influence from the shoot apical meristem may prevent growth on the
adaxial side of the leaf primordium. Early investigators tried to answer this question by using experimental approaches like microsurgery. Sussex (1955) used fine
incisions to isolate the incipient leaf primordium (11 or Po) from the shoot meristem.