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 (fid)mutation '(called leafy in Arabidopsis)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 and squamosa giving phenotypes similar to that of floricaula (flo)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.
The flo 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.
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.
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