Floral organ primordia
Flavin adenine dinucleotide
flavin adenine dinucleotide (FAD) a coenzyme composed of riboflavin phosphate and adenylic acid. FAD forms the prosthetic group of enzymes such as d-amino acid oxidase and xanthine oxidase. flavin mononucleotide riboflavin phosphate, a coenzyme for a number of enzymes including l- amino acid oxidase and cytochrome c reductase.
flavoprotein a protein requiring FMN or FAD to function. flint corn See corn. flora the plant life in a given region or period of time. floral organ identity mutations homeotic muta- tions (q.v.) in which one floral organ has been re- placed by another. For example, in Arabidopsis thali- ana (q.v.) the apetala 2 mutations have sepals converted into carpels and petals into stamens. In apetala 3 mutants, petals are converted into sepals and stamens into carpels. In the agamous mutant, stamens are converted to petals and carpels to sepals. The ingenious “ABC model” illustrated on page 162 explains the bizarre phenotypes of the mutant flow- ers. In the normal flower (matrix I) a class A gene (APETALA 2) produces an A morphogen, a class B gene (APETALA 3) produces a B morphogen, and a Class C gene (AGAMOUS) produces a C morpho- gen. Whorl 1 (W1) meristems produce sepals when A morphogen is present.
Whorl 2 (W2) meristems produce petals when A and B morphogens are pres- ent. W3 meristems produce stamens when B and C are present, and W4 meristems produce carpels when only C is present. The apetala 2 mutant lacks morphogen A (matrix II), the apetala 3 mutant lacks morphogen B (matrix III), and the agamous mutant lacks morphogen C (matrix IV). Furthermore, C genes must inhibit A genes in W3 and W4 (matrix IV), and A genes must inhibit C genes in W1 and W2 (matrix II). Homeotic mutations allow the genes, normally inactivated, to express themselves. The po- sitions in the matrices where this happens are starred. The proteins encoded by homeotic floral identity genes contain a conserved sequence of 58 amino acids.
This presumably binds to DNA in much the same way as the homeobox of Hox genes (q.v.). See Appendix C, 1996, Krizek and Mayerowitz; An- tennapedia, Antirrhinum, cadastral genes, floral organ primordia, meristems. floral organ primordia meristematic cells that give rise to the organs of flowers. The primordia are ar- ranged into four concentric whorls as shown on page 162. The leaf-like sepals form whorl number 1 at the periphery. The showy petals form whorl 2, inside the first. The stamens, the male reproductive organs,
form whorl 3 (inside whorl 2), and the carpels, the female reproductive organs, form central whorl 4. See Appendix A, Plantae, Angiospermae; Arabidopsis thaliana, floral identity mutations, meristems. floret a small flower from an inflorescence, as in a grass panicle. flour corn See corn. flow cytometry a technology that utilizes an in- strument in which particles in suspension and stained with a fluorescent dye are passed in single file through a narrow laser beam. The fluorescent signals emitted when the laser excites the dye are electronically amplified and transmitted to a com- puter. This is programmed to instruct the flow cytometer to sort the particles having specified prop- erties into collecting vessels. Human mitotic chro- mosomes can be sorted to about 90% purity by this technique.
flower the specialized reproductive shoot of angio- sperms (q.v.). Perfect flowers bear both pistils and stamens. Imperfect flowers bear either pistils or sta- mens. A plant bearing solely functional staminate or pistillate flowers is said to be dioecious. A plant bear- ing (1) imperfect flowers of both sexual types (corn is an example), or (2) perfect flowers (the garden pea), or (3) staminate, pistillate, and perfect flowers (the red maple, for example) is said to be monoe- cious. See androgynous. FLP/FRT recombination a system of site-specific recombination (q.v.) found in the yeast and based on the yeast two-micron plasmid. This plasmid is an autonomously replicating, circular plasmid of 6,318 base pairs, which exists in many copies in most strains of S. cerevisiae (q.v.).
It encodes a site-specific recombinase (q.v.) called the FLP (pronounced ‘flip’) protein. FLP acts on the FLP recombination target (FRT) located within two 599-base pair in- verted repeats in the plasmid DNA and catalyzes re- combination between the inverted repeats. This re- combination event inverts, or ‘flips’, part of the plasmid with respect to the remainder, and this is essential for plasmid amplification. FLP can also in- duce recombination between direct FRT repeats or between FRTs on different DNA molecules. In the former case, the recombination event results in exci- sion of intervening DNA and an FRT. This fact has been exploited to experimentally induce site-specific recombination in Drosophila and other organisms.
In Drosophila this is done by placing FLP under the control of a heat shock protein promoter, and the gene under study between two flanking FRT direct repeats. These DNAs are then integrated into the Drosophila genome by germ line transformation (q.v.). Under heat-shock conditions, FLP catalyzes recombination between the FRTs, resulting in exci- sion of the gene of interest. The descendants of cells without such a gene show a null phenotype. This system can thus produce somatic or germ line mosa- ics for specific genes. fluctuation test a statistical analysis first used by Luria and Delbru¨ck to prove that selected variants (q.v.), such as bacteriophage-resistant bacteria, are spontaneous mutants that arose prior to the expo- sure to the selective agent. They reasoned that if