Flowering plants display a remarkable range of inflorescence architecture, and pedicel characteristics are one of the key contributors to this diversity. However, very little is known about the genes or the pathways that regulate pedicel development. The brevipedicellus (bp) mutant of Arabidopsis thaliana displays a unique phenotype with defects in pedicel development causing downward-pointing flowers and a compact inflorescence architecture. Cloning and molecular analysis of two independent mutant alleles revealed that BP encodes the homeodomain protein KNAT1, a member of the KNOX family. bp-1 is a null allele with deletion of the entire locus, whereas bp-2 has a point mutation that is predicted to result in a truncated protein. In both bp alleles, the pedicels and internodes were compact because of fewer cell divisions; in addition, defects in epidermal and cortical cell differentiation and elongation were found in the affected regions. The downward-pointing pedicels were produced by an asymmetric effect of the bp mutation on the abaxial vs. adaxial sides. Cell differentiation, elongation, and growth were affected more severely on the abaxial than adaxial side, causing the change in the pedicel growth angle. In addition, bp plants displayed defects in cell differentiation and radial growth of the style. Our results show that BP plays a key regulatory role in defining important aspects of the growth and cell differentiation of the inflorescence stem, pedicel, and style in Arabidopsis.

Arabidopsis thaliana (L.) Heynh. has been used as a model system to investigate the regulatory genes that control and coordinate the determination, differentiation and morphogenesis of the floral meristem and floral organs. We show here that benzylaminopurine (BAP), a cytokinin, influences flower development in Arabidopsis and induces partial phenocopies of known floral homeotic mutants. Application of BAP to wild-type inflorescences at three developmental stages results in: (i) increase in floral organ number; (ii) formation of abnormal floral organs and (iii) induction of secondary floral buds in the axils of sepals. These abnormalities resemble the phenotypes of mutants, clv1 (increase in organ number), ap1, ap2, ap3 (abnormal floral organs) and ap1 (secondary floral buds in the axils of first-whorl organs). In addition, BAP induces secondary floral buds in the axils of perianth members of ap2-6, ap3-1 and ag mutants, and accentuates the phenotype of the ap2-1 mutant to resemble the ap2-6 mutant. These observations suggest that exogenous BAP suppresses the normal functioning of the genes for floral meristem identity and thereby affects flower development and the later stages of floral organ differentiation.

DNA replication machinery is highly conserved in eukaryotes. DNA polymerase is essential for the synthesis of new DNA strands and for DNA repair. Despite the significant progress in the understanding of these processes in yeast and animal model systems, there is only scant information available for their counterparts in plants. Among different multisubunit-containing DNA polymerases, DNA polymerase α (POLA complex) is composed of four subunits. In this study, we report on the characterization of PolA2, which encodes the putative B subunit of DNA polymerase α in Arabidopsis thaliana (L.) Heynh. PolA2 is a single copy gene in Arabidopsis and shows highly conserved regions with putative homologs in other plant species. Insertional inactivation of PolA2 in Arabidopsis leads to embryo lethality, with developmental arrest at or before the four-cell stage during embryogenesis. The apical cell lineage is strongly affected in the mutant embryos and the endosperm initial cell fails to divide. PolA2 is expressed broadly in the early phases of embryo development during the period of active cell divisions, while during the later stages of development expression is reduced and more localized. Ectopic overexpression of PolA2 produced dominant negative phenotypes with gametic and embryo lethality suggesting that coordinated and parallel expression with other subunits is critical for its proper function in DNA replication and plant development.

Thidiazuron (TDZ), a urea derivative, was applied to the inflorescences of Arabidopsis thaliana at early stages of development to study its effect on flower development. Scanning electron microscope observations revealed that TDZ induced the formation of branched trichomes on sepals and carpels and stomata on petals that are not normally produced on these organs. Concurrently, TDZ also suppressed the development of pollen grains in the anthers and ovules in the ovaries. In addition, TDZ induced an increase in the size of all the floral organs except stamens. These observations suggest that thidiazuron alters the differentiation processes in floral organs by inducing the production of traits that are characteristic of cauline leaves, i.e., trichomes and stomata. Key words: Arabidopsis thaliana, flower development, stomata, thidiazuron, trichomes.