15. The Regulation of Dorsiventral Symmetry in Plants
237
transformed with an antisense construct of the homeobox gene NTHl5 showed loss
of dorsiventrality in the midrib, suggesting that NTHl5 plays a role in establishing
adaxial cell fate of the midrib. Bifaciality is maintained in mutant leaves.
The argonaute1 mutant in Arabidopsis also has radially symmetric cauline leaves
and filamentous flower organs, which lack ab-adaxiality (Bohmert et al. 1998).
However, cotyledons and rosette leaves of agol still have ab-adaxiality even though
they are filamentous. This feature is very similar to lam in which ab-adaxiality of
leaf primordia is established, but leaf lamina formation is defective (McHale 1992,
1993). AGOl was cloned and shown to be a novel protein of unknown function.
AGOl-like genes have been found only in multicellular organisms suggesting the
importance of AGOl in developmental organization relating to multicellularity. In
tomato, several nonallelic wiry mutations have been described. Even though severity of the phenotype varies, all wiry mutants cause parts of the leaf to become radially symmetric. Flower organs in some wiry mutants are also radially symmetrical.
The fact that the same phenotype can be caused by different wiry loci suggests that
the specification of the adaxial cell fate may be caused by an interplay among several genes.
10.2 Flower Symmetry
In general, angiosperm flowers can be classified into four major types, 1) radially
symmetrical (regular, actinomorphic), 2) bisymmetric (bilateral) [such as the flower
of Dicentra spectabilis], 3) zygomorphic (dorsiventral, monosymmetrical), 4) asymmetrical flowers. A good example of genetic control of floral asymmetry was reported inAntirrhinum majus. Luo and coworkers (1996) reported the cloning of a
gene, CYCLOIDEA, controlling floral asymmetry. In cycioidea mutants (in a certain genetic background, dichotoma) a ventralization occurs in the dorsal region of
the flower transforming the zygomorphic flower into a radially symmetric structure. Two possible functions of CYCLOIDEA were suggested. CYCLOIDEA expression in the dorsal region of flower meristems at an early stage could increase
growth rate and the initiation of primordia in the dorsal region. At a later stage, the
continuous expression of CYCLOIDEA in the dorsal region of flower primordia
could affect petal and stamen morphology. However, to make a flower perfectly
radially symmetrical another locus, DICHOTOMA, is needed, suggesting several
loci control floral asymmetry in Antirrhinum. Support for this idea comes from the
fact that no mutants have been reported in symmetric flower species which cause
asymmetric flower formation. Mutants which change zygomorphic into actinomorphic flowers can be found easily. To test the two traditional hypotheses of the acquisition of flower symmetry (multiple vs. single origin of zygomorphy) an evolutionary study on CYCLOIDEA in different species was suggested by Coen and Nugent
(1994).
The number of planes of symmetry in a flower can vary depending on species.
Typical Arabidopsis flowers are bisymmetric (bilateral) with 4 sepals, 4 petals, 6
stamens, and 2 carpels. Running and Meyerowitz (1996) reported a mutant,
237
transformed with an antisense construct of the homeobox gene NTHl5 showed loss
of dorsiventrality in the midrib, suggesting that NTHl5 plays a role in establishing
adaxial cell fate of the midrib. Bifaciality is maintained in mutant leaves.
The argonaute1 mutant in Arabidopsis also has radially symmetric cauline leaves
and filamentous flower organs, which lack ab-adaxiality (Bohmert et al. 1998).
However, cotyledons and rosette leaves of agol still have ab-adaxiality even though
they are filamentous. This feature is very similar to lam in which ab-adaxiality of
leaf primordia is established, but leaf lamina formation is defective (McHale 1992,
1993). AGOl was cloned and shown to be a novel protein of unknown function.
AGOl-like genes have been found only in multicellular organisms suggesting the
importance of AGOl in developmental organization relating to multicellularity. In
tomato, several nonallelic wiry mutations have been described. Even though severity of the phenotype varies, all wiry mutants cause parts of the leaf to become radially symmetric. Flower organs in some wiry mutants are also radially symmetrical.
The fact that the same phenotype can be caused by different wiry loci suggests that
the specification of the adaxial cell fate may be caused by an interplay among several genes.
10.2 Flower Symmetry
In general, angiosperm flowers can be classified into four major types, 1) radially
symmetrical (regular, actinomorphic), 2) bisymmetric (bilateral) [such as the flower
of Dicentra spectabilis], 3) zygomorphic (dorsiventral, monosymmetrical), 4) asymmetrical flowers. A good example of genetic control of floral asymmetry was reported inAntirrhinum majus. Luo and coworkers (1996) reported the cloning of a
gene, CYCLOIDEA, controlling floral asymmetry. In cycioidea mutants (in a certain genetic background, dichotoma) a ventralization occurs in the dorsal region of
the flower transforming the zygomorphic flower into a radially symmetric structure. Two possible functions of CYCLOIDEA were suggested. CYCLOIDEA expression in the dorsal region of flower meristems at an early stage could increase
growth rate and the initiation of primordia in the dorsal region. At a later stage, the
continuous expression of CYCLOIDEA in the dorsal region of flower primordia
could affect petal and stamen morphology. However, to make a flower perfectly
radially symmetrical another locus, DICHOTOMA, is needed, suggesting several
loci control floral asymmetry in Antirrhinum. Support for this idea comes from the
fact that no mutants have been reported in symmetric flower species which cause
asymmetric flower formation. Mutants which change zygomorphic into actinomorphic flowers can be found easily. To test the two traditional hypotheses of the acquisition of flower symmetry (multiple vs. single origin of zygomorphy) an evolutionary study on CYCLOIDEA in different species was suggested by Coen and Nugent
(1994).
The number of planes of symmetry in a flower can vary depending on species.
Typical Arabidopsis flowers are bisymmetric (bilateral) with 4 sepals, 4 petals, 6
stamens, and 2 carpels. Running and Meyerowitz (1996) reported a mutant,