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The research work in our laboratory is directed to understanding development - what is the mechanism by which a single cell, a fertilized egg, turns into a complex multicellular organism? To find out, we study the processes of development in the laboratory plant Arabidopsis thaliana, which grows from a fertilized egg cell to a mature embryo, ready to germinate, and then to a mature plant, all in the course of a few weeks. During the course of these few weeks both root and shoot develop, and the shoot grows to produce a stem surrounded first by leaves, and then by flowers.

We concentrate on two processes in plant development - the development of flowers, and the growth of shoots.

Flower Development:
In the flower area we have had a long-standing interest in the genes that regulate organ identity in flowers. Flowers are compressed shoots, but in place of the leaves that surround a vegetative shoot, there are four types of floral organs: sepals, petals, stamens, and carpels (which fuse to form the ovary). In Arabidopsis flowers there are four sepals, four petals, six stamens, and two carpels. Furthermore, flowers are determinate - they stop growing when carpels form, while vegetative shoots grow without a definite stopping point. In the past we identified a set of genes, called the ABC genes, whose overlapping expression specifies the identity of the floral organs that form in different flower regions. Current work is directed to understanding how these genes come to be activated in appropriate domains, how the overall pattern of floral organs (including number as well as type) is established, and what other genes the ABC genes, which are all transcription factors, activate and repress. This work involves extensive mutageneses and studies of mutant phenotypes, molecular cloning and misexpression of genes in transgenic plants, and use of microarrays for transcriptome analysis.

The organ identity mutants apetala1 (ap1), apetala2 (ap2), apetala3 (ap3), pistillata (pi), and agamous (ag) show homeotic transformations of floral organs. On the top left is a wild type flower. From Riechmann and Meyerowitz, 1997.

Schematic representation of the ABC model of floral organ determination, showing how the three organ-identity functions (A, B, and C) combinatorially specify the identity of the different types of organs in the four whorls of a wild type flower. From Riechmann and Meyerowitz, 1997.

Shoot Growth:
In the area of shoot growth we study the patterns of cell division, and the mechanisms of cell-cell communication, in the shoot apical meristem; and also genes necessary for formation of the shoot apical meristem. Recent work has established that cells in at least two meristematic regions, the central zone and rib meristem, communicate via a secreted protein ligand (produced in the central zone) and a transmembrane receptor kinase found in the rib meristem cells. Mutations in either ligand or receptor genes cause excess meristem growth. Current work includes computer modeling of cellular behavior in shoot meristems, and detailed analysis of cell division patterns, gene expression domains, and hormonal gene activation in the meristem. This analysis depends heavily on reporter gene expression in transgenic plants, as visualized in living material by laser scanning confocal microscopy.

(A) View of the shoot apical meristem (SAM) and the adjacent floral meristems (FM) of wild-type Arabidopsis. False colors are applied to illustrate (B) the three zones of the SAM: the peripheral zone (PZ), the central zone (CZ) and the rib meristem, and (C) the three clonal layers: the epidermal L1, the subepidermal L2, and the corpus or L3. From Meyerowitz, 1997.

See examples of our meristem growth and gene expression data in the Meyerowitz Movie Gallery!

Our current work is funded by the National Institutes of Health, the National Science Foundation, the Human Frontier Science Program, and the U.S. Department of Energy.

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