Frederick D. Hempel

Bi-directional inflorescence development in Arabidopsis thaliana : acropetal initiation of flowers and basipetal initiation of paraclades

In this study we investigated Arabidopsis thaliana (L.) Heynh. inflorescence development by characterizing morphological changes at the shoot apex during the transition to flowering. Sixteen-hour photoperiods were used to synchronously induce flowering in vegetative plants grown for 30 d in non-inductive 8-h photoperiods. During the first inductive cycle, the shoot apical meristem ceased producing leaf primordia and began to produce flower primordia. The differentiation of paraclades (axillary flowering shoots), however, did not occur until after the initiation of multiple flower primordia from the shoot apical meristem. Paraclades were produced by the basipetal activation of buds from the axils of leaf primordia which had been initiated prior to photoperiodic induction. Concurrent with the activation of paraclades was the partial suppression of paraclade-associated leaf primordia, which became bract leaves. The suppression of bract-leaf primordia and the abrupt initiation of flower primordia during the first inductive photoperiod is indicative of a single phase change during the transition to flowering in photoperiodically induced Arabidopsis. Morphogenetic changes characteristic of the transition to flowering in plants grown continuously in 16-h photoperiods were qualitatively equivalent to the changes observed in plants which were photoperiodically induced after 30 d. These results suggest that Arabidopsis has only two phases of development, a vegetative phase and a reproductive phase; and that the production of flower primordia, the differentiation of paraclades from the axils of pre-existing leaf primordia and the elongation of internodes all occur during the reproductive phase.

Plant intercellular communication via plasmodesmata.

The development and function of multicellular organisms rely on cell-to-cell communication. Detailed studies of animal cells show that this communication can occur by secretion of chemical signals, such as hormones and neurotransmitters, and by contact-dependent signals transmitted through extracellular matrixand membrane-anchored molecules. Plants use similar modes of communication, although they are not as well characterized as those in animal systems. The transport of small signal molecules, such as hormones, regulates the proper growth of plant cells (see Creelman and Mullet, 1997; Kende and Zeevaart, 1997, in this issue), and cell-to-cell contact via an extracellular matrix-located glycoprotein and a receptor kinase plays a role in the self-incompatibility reaction between pollen and stigma (Stein et al., 1991). Plant cells, however, have an additional and unique mode of cell-to-cell communication derived from two of their characteristic features: the deposition of cell wall material, and the incomplete separation of the cytoplasm during cytokinesis. Plasmodesmata (PD) are structurally complex channels that span the cell wall and connect the cytoplasm of one plant cell with that of its neighbors, consequently facilitating communication between cells. In higher plant embtyos, initially all cells are interconnected by PD (Figure 1A; Schulz and Jensen, 1968; Mansfield and Briarty, 1991; see below) and integrated into a single symplast, the domain of common cytoplasm that is bounded by the plasma membranes of connected cells (Munch, 1930). As the plant differentiates, individual cells or groups of cells become isolated, possibly by loss of functional PD (Carr, 1976; Bergmans et al., 1993; Duckett et al., 1994). This symplastic isolation allows subsets of cells to function as distinct compartments within the plant. Indeed, mature flowering plants could best be described as mosaics of symplastic domains. Because PD provide the symplastic connections between cells, communication and transport within and between these symplastic domains are intimately linked to the frequency, distribution, and function of PD. Plasmodesmal frequencies among cells have been used to characterize symplastic transport pathways, but this approach assumes that PD are both structurally and functionally uniform, which, as described below, does not appear to be the case (van Bel and Oparka, 1995). Severa1 techniques have been used

Leaf-to-shoot apex movement of symplastic tracer is restricted coincident with flowering in Arabidopsis

Classical experiments in plant physiology showed that leaves are the source of signals that control the development of flowers from shoot meristems. Additional physiological and genetic experiments have indicated some of the molecules (e.g., gibberellins, cytokinins, and sucrose) that promote flowering in mustards including Arabidopsis. These small hydrophilic molecules are likely to move to the shoot apex symplastically via the phloem and/or via cell-to-cell movement through plasmodesmata. To analyze potential changes in the symplastic trafficking of small molecules during the induction of flowering in Arabidopsis, we measured changes in the flow of symplastic tracers from the leaf to the shoot apex. We previously found that the onset of flowering is coincident with an evident decrease in the leaf-to-shoot trafficking of symplastic tracer molecules; this decrease in trafficking is transitory and resumes when floral development is established. Here we provide detailed analyses of symplastic connectivity during floral induction by monitoring tracer movement under different photoperiodic induction conditions and in a number of genetic backgrounds with altered flowering times. In all cases, the correlation between flowering and the reduction of symplastic tracer movement holds true. The lack of tracer movement during the induction of flowering may represent a change in plasmodesmal selectivity at this time or that a period of reduced symplastic communication is associated with floral induction.

Floral induction and determination: where is flowering controlled?

Flowering is controlled by a variety of interrelated mechanisms. In many plants, the environment controls the production of a floral stimulus, which moves from the leaves to the shoot apex. Apices can become committed to the continuous production of flowers after the receipt of sufficient amounts of floral stimulus. However, in some plants, the commitment to continued flower production is evidently caused by a plants commitment to perpetually produce floral stimulus in the leaves. Ultimately, the induction of flowering leads to the specification of flowers at the shoot apex. In Arabidopsis, floral specification and inflorescence patterning are regulated largely by the interactions between the genes TERMINAL FLOWER, LEAFY and APETALA1/CAULIFLOWER.

Photoinduction of Flower Identity in Vegetatively Biased Primordia

Far-red light and long photoperiods promote flowering in Arabidopsis. We report here that when 30-day-old vegetative plants were induced with a continuous light treatment enriched in far-red light, flowers developed directly from previously initiated primordia. Specifically, plants induced with our continuous incandescent-enriched (CI) treatment produced an average of two primary-axis nodes with a leaf/flower phenotype, indicating that approximately two leaf/paraclade primordia per plant produced an individual flower from tissue that typically would differentiate into a paraclade (secondary inflorescence). Assays for APETALA1::β-glucuronidase activity during the CI photoinduction treatment indicated that the floral meristem identity gene APETALA1 was transcriptionally activated in primordia with a leaf/paraclade bias and in primordia committed to leaf/paraclade development. APETALA1::β-glucuronidase activity levels were initially highest in young primordia but were not correlated strictly with primordium fate. These results indicate that primordium fate can be modified after primordium initiation and that developing primordia respond quantitatively to floral induction signals.

Transcriptional regulation of a seed-specific carrot gene, DC8

Many late embryogenesis abundant (Lea) protein genes in plants are regulated by abscisic acid (ABA). The RNA level of a carrot gene, DC8, increases in response to ABA in developing seeds. However, DC8 cannot be induced by ABA in adult tissues. We used chimeric genes made of various DC8 promoter fragments fused to β-glucuronidase (GUS) to analyze the transcriptional regulation of DC8. DC8:GUS expression was measured in electroporated carrot protoplasts and in stably transformed carrots. The region of the DC8 promoter from −170 to −51 contained ABA-responsive sequences that required a 5′ upstream region for high levels of expression in embryogenic callus protoplasts. 505 bp of the DC8 promoter conferred GUS expression in stably transformed somatic and zygotic embryos. DC8:GUS was expressed only in tissues formed in the seed. This includes cells in the embryo, the endosperm and the germinating seedlings. Gel retardation and competition experiments were performed to analyze the embryo nuclear protein-DNA binding activities in vitro. No binding activity was detected on the putative ABA-responsive region; however the 5′ upstream regions located between -505 and -301 interacted with embryo nuclear factors. An additional site of DNA-protein interaction was located between positions -32 and +178. The nuclear proteins that bind these sequences were found in the embryo nuclei only, not in the nuclei from leaves or roots.