Initiation and growth of leaf blades is oriented by an adaxial/abaxial axis aligned with the original axis of polarity in the leaf primordium. To investigate mechanisms regulating this process, we cloned the Nicotiana tabacum ortholog of PHANTASTICA (NTPHAN) and generated a series of antisense transgenics in N. sylvestris. We show that NSPHAN is expressed throughout emerging blade primordia in the wild type and becomes localized to the middle mesophyll in the expanding lamina. Antisense NSPHAN leaves show ectopic expression of NTH20, a class I KNOX gene. Juvenile transgenic leaves have normal adaxial/abaxial polarity and generate leaf blades in the normal position, but the adaxial mesophyll shows disorganized patterns of cell division, delayed maturation of palisade, and ectopic reinitiation of blade primordia along the midrib. Reversal of the phenotype with exogenous gibberellic acid suggests that NSPHAN, acting via KNOX repression, maintains determinacy in the expanding lamina and sustains the patterns of cell proliferation critical to palisade development.

Leaf initiation in the peripheral zone of the shoot apical meristem involves a transition to determinate cell fate, but indeterminacy is maintained in the vascular cambium, a tissue critical to the continuous growth of vascular tissue in leaves and stems. We show that the orientation of cambial growth is regulated by microRNA (miRNA)-directed cleavage of mRNA from the Nicotiana sylvestris ortholog of PHAVOLUTA (NsPHAV). Loss of miRNA regulation in semidominant phv1 mutants misdirects lateral growth of leaf midveins and stem vasculature away from the shoot, disrupting vascular connections in stem nodes. The phv1 mutation also expands the central zone in vegetative and inflorescence meristems, implicating miRNA and NsPHAV in regulation of meristem structure. In flowers, phv1 causes reiteration of carpel initiation, a phenocopy for loss of CARPEL FACTORY/DICER LIKE1, indicating that miRNA is critical to the termination of indeterminacy in floral meristems. Results point to a common role for miRNA in spatial and temporal restriction of HD-ZIPIII mediated indeterminacy in apical and vascular meristems.

Filament elongation and the role of auxin in this process in Gaillardia grandiflora was investigated. Filament elongation in vivo occurred just prior to anthesis and was accompanied by cell elongation and fresh weight increase. Filaments isolated and exposed to auxin in vitro grew more rapidly than controls and their growth was comparable to that of filaments in vivo. Furthermore, the natural auxin content of disc flowers (determined by double‐standard isotope dilution analyses) increased just prior to anthesis and filament elongation. These results imply that auxin controls filament elongation. Applied ethylene slightly promoted filament elongation in vitro, and ethylene production of the flowers (determined by gas chromatography) slightly increased prior to filament growth. Fusicoccin and acidic buffers also stimulated elongation of isolated filaments. Thus, the role of auxin in controlling filament elongation in Gaillardia may involve stimulation of ethylene biosynthesis and acid growth.

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Filament elongation and the role of auxin in this process in Gaillardia grandiflora was investigated. Filament elongation in vivo occurred just prior to anthesis and was accompanied by cell elongation and fresh weight increase. Filaments isolated and exposed to auxin in vitro grew more rapidly than controls and their growth was comparable to that of filaments in vivo. Furthermore, the natural auxin content of disc flowers (determined by double‐standard isotope dilution analyses) increased just prior to anthesis and filament elongation. These results imply that auxin controls filament elongation. Applied ethylene slightly promoted filament elongation in vitro, and ethylene production of the flowers (determined by gas chromatography) slightly increased prior to filament growth. Fusicoccin and acidic buffers also stimulated elongation of isolated filaments. Thus, the role of auxin in controlling filament elongation in Gaillardia may involve stimulation of ethylene biosynthesis and acid growth.

Corolla elongation and the roles of plant hormones in this process in Gaillardia grandiflora Van Houtte ray flowers were examined. The sterile ray flowers elongated during a 2‐day period, and corolla growth was accompanied by fresh and dry weight increases and epidermal cell elongation (greatest near the base of the corolla) but not by cell division. Corollas excised from young ray flowers were measured during treatment in vitro with solutions of plant growth regulators. They elongated in response to gibberellins and fusicoccin but did not respond to auxins, cytokinins, abscisic acid, ethylene, or inhibitors of ethylene biosynthesis. Sequential and simultaneous hormone applications indicated no additive or synergistic effects between hormones, but auxin did reduce gibberellin‐promoted growth. Analyses of endogenous auxins showed no significant variation, and ethylene production decreased prior to elongation, while a 20‐fold increase in endogenous gibberellin activity was observed just prior to rapid corolla elongation. It appears that corolla growth in Gaillardia is accomplished by an increase in gibberellin activity alone, that multiple hormone interactions are not important in the control of corolla growth, and that part of the mode of action of gibberellin is acid‐induced growth.

Corolla elongation and the roles of plant hormones in this process in Gaillardia grandiflora Van Houtte ray flowers were examined. The sterile ray flowers elongated during a 2-day period, and corolla growth was accompanied by fresh and dry weight increases and epidermal cell elongation (greatest near the base of the corolla) but not by cell division. Corollas excised from young ray flowers were measured during treatment in vitro with solutions of plant growth regulators. They elongated in response to gibberellins and fusicoccin but did not respond to auxins, cytokinins, abscisic acid, ethylene, or inhibitors ofethylene biosynthesis. Sequential and simultaneous hormone applications indicated no additive or synergistic effects between hormones, but auxin did reduce gibberellin-promoted growth. Analyses of endogenous auxins showed no significant variation, and ethylene production decreased prior to elongation, while a 20-fold increase in endogenous gibberellin activity was observed just prior to rapid corolla elongation. It appears that corolla growth in Gaillardia is accomplished by an increase in gibberellin activity alone, that multiple hormone interactions are not important in the control of corolla growth, and that part of the mode of action of gibberellin is acid-induced growth.

No abstract