Similarly, the use of Antirrhinim
has the following advantages : (i) well characterized transposable elements, (ii) large flowers, (iii) easy to emasculate and cross and (iv) ease of vegetative propagation.
(a) Genetic control of first steps.
A large number of mutants for flower development have been isolated, which could be broadly classified into two groups, (i) homeotic mutations
that cause the transformation of one flower whorl into a whorl of a different type (e.g. apetala,
where petals are absent and converted in bracts, stamens or carpels) and (ii) meristic mutations
that alter the number of a structure within a floral whorl (e.g. clavata,
where number of carpels increased from two to three or four). A number of mutations including both these types, known in Arabidopsis
are listed in Table 38.1. Analogous mutations have been observed in other plant species, indicating that the fundamental processes controlling floral morphogenesis are highly conserved. In wild type plants, after a period of vegetative growth, the apex undergoes a transition to become an inflorescence meristem to produce indeterminate inflorescences, which do not produce a terminal flower. The inflorescence bears bracts, in the axils of which flowers develop. Two classes of mutants showing deviation from wild type have been detected in Antirrhinum
: (i) floricaula
)mutation '(called leafy
)leads to failure in transition from inflorescence meristem to floral meristem, so that proliferating inflorescence shoots develop in place of flowers. Two other mutations squemata
giving phenotypes similar to that of floricaula
)were reported, (ii) centroradialis
mutation has an effect opposite to that of flo
and leads to the development of terminal flower converting indeterminate inflorescences into determinate type. Mutations producing proliferating inflorescences have also been obtained in alalfa, tomato, maize, etc.
gene has been isolated and was shown to produce a transcript of 1.6kb, encoding a protein FL0 of 396 amino acids showing some similarity with transcription factors (see Expression of Gene : Protein Synthesis 2. Transcription in Prokaryotes and Eukaryotes
). In situ
hybridization shows that flo
is expressed from a very early stage in wild type inflorescence in a very specific temporal and spatial sequence. It expresses in bract primordia followed by expression in primordia for sep.als, petals and carpels but not in the primordia for stamens. Therefore, flo+
may be involved in transition from vegetative to floral axis and in the initiation of floral primordia. Absence of its expression, however, is required for normal stamen development. The flo+
may also interact with other genes, like those meant for whorl identity.
(b) Mutations that alter the symmetry of the flower (class II mutants). Mutations that change zygomorphic flowers into actinomorphic or vice versa have also been identified. In Antirrhinum, cycloidia and centroradialis are two mutants, both conferring radial symmetry (actinomorphic flowers) on otherwise zygomorphic flowers. As reported above, cantroradialis gives radial symmetry due to the terminal position of the flowers, which are normally developed only in the axils of bracts.
(c) Genetic control of whorl identity (class III mutants).A major class of whorl identity genes (which will affect the identity of organs in a whorl) contains those that affect the identities of organs in two adjacent whorls (Table 38.1). For convenience, the following three overlapping regions, each consisting of two adjacent whorls, were recognized: A (whorls 1 and 2), B (whorls 2 and 3) and C (whorls 3 and 4) (see Fig. 38.6). Consequently, as shown in Table 1, three classes of mutants (called homeotic mutants) are known, (i) those affecting whorls 1 and 2 (type 1); (ii) those affecting whorls 2 and 3 (type 3) and (iii) those affecting whorls 3 and 4 (type 2). The genes acting in these three regions (A, B and C) are required for three functions a, b and c.
Experiments on isolation and cloning of homeotic genes revealed that many homeotic genes encode transcription factors. For instance def gene encodes DEFA protein which resembles transcription factors like serum response factor (SRF) in mammals. Similarly, the product of minichromosome maintenance gene (MCM 1) in yeast, has been shown to be a transcription factor. Similar is the situation with the agamous (ag)gene in Arabidopsis producing a product AG. These genes seem to have a conserved domain in common with each other and include the gene products MCM 1, AG, DEF A and SRF. Therefore, this domain has been given the name MADS-box.
|Fig. 38.6. Three regions of floral meristem, i.e., A, B and C, such that genes (a, b and c) acting in these regions have regulatory functions; note that out of four whorls (1 to 4), whorl 1 is controlled by a alone; whorl 2 is controlled by a and b; whorl 3 is controlled by b and c and whorl 4 is controlled by c alone.
The study of the phenotypes in several mutants as above followed by a study of cloned genes led several workers to formulate models for floral development. But the information is inadequate yet to suggest complete models based on unequivocal evidence. Knowledge about regulation of the expression of these genes is being generated at a rate, which should permit formulation of better models for the understanding of the pathways for flower development.