ИСТИНА

Wild-type plants of Arabidopsis develop racemose inflorescences that lack a terminal flower, but some mutants consistently develop closed inflorescences. In the present study, we compare development and vasculature in wild type and mutant (tfl 1-2 and tfl 2-1) plants of Arabidopsis. All examined individuals of Arabidopsis mutants possessed a terminal flower, but its morphology was highly unstable. Some terminal flowers possessed sepals, petals, stamens and carpels and/or organs of ‘hybrid’ morphology, whereas others lacked petals or petals and sepals. The first-formed sepals of the terminal flower continued the spiral of lateral flower arrangement, irrespective of presence or absence of a flower-subtending bract in lateral flowers. In the most reduced form, the terminal flower consisted of two phyllomes intermediate between carpels and sepals. Cases in which one or two of the uppermost lateral flowers are closely associated with the terminal flower can result in pseudanthium formation, but organs belonging to the terminal flower still can be distinguished. In wild-type, flower-subtending bracts are absent, and a single bundle supplies each lateral flower. Mutant tfl 1-2, sporadically produce flowers subtended by a bract. In bracteates flowers, three bundles depart from inflorescence axis to supply the flower-subtending bract and its axillary flower. These bundles form a compact group in which the outer (abaxial) bundle enters the flower-subtending bract and paired inner (adaxial) bundles enter the pedicel. If no visible flower-subtending bract is present in tfl 1-2 mutants, flower supply still consists of two bundles. This could be explained by the occurrence of a considerable auxin flow from the site of a cryptic flower-subtending bract (in contrast to the wild type). The terminal flower of tfl 1-2 mutants is supplied by four to seven vascular bundles that enter the pedicel directly from inflorescence axis. Mutants tfl 2-1 usually produce very sparse non-bracteate inflorescences with weakly developed vasculature. Both terminal flower and lateral flowers are supplied by a single bundle each. This contradicts a rule that in plants with stable occurrence of terminal flowers they are supplied by more bundles than the lateral flowers. We compare data on Arabidopsis with those on basal monocots that exhibit variation in inflorescence tip structure in natural populations.