Genetics of Plant Development
The recent work of my laboratory concentrates in two areas, the origin of developmental patterns in flowers, and the control of cell division in meristems. In our work we use the convenient laboratory plant Arabidopsis thaliana, which allows the parallel use of classical and of molecular genetics. The complete sequence of the Arabidopsis genome has also been determined, making genomic approaches to our scientific problems readily available.
Arabidopsis flowers originate as small groups of undifferentiated and apparently identical cells. These floral primordia, in the course of their two-week development, differentiate into complex structures with four organ types, with each organ type found in stereotyped numbers and positions. The four types are sepal, petal, stamen and carpel (the ovary subunit), and they are specified by the organ identity genes, which act in specific subregions of the floral primordium to combinatorially specify which type of organ will appear where. We have over the past few years identified and cloned a number of these genes. By changing the patterns of activity of these genes, both by mutation and by use of cloned genes and transgenic plant technology, we have established a working model of how floral organ patterns arise. The use of the predictive capabilities of the model, and the available genes and technologies, has allowed us to design and then grow new types of flowers with whatever patterns of organs we wish.
A related question in flower development is how the appropriate number of floral organs is achieved. Arabidopsis flowers typically have four sepals, four petals, six stamens and two carpels. We have collected mutants with more organs than usual, and this has shown us that there are two ways to get extra organs. One is for the floral primordium, or floral meristem, to have too many cells. The other is for too many organs to appear in a meristem with the usual number of cells, that is, to change the rules by which the primordium is measured out for the formation of organ primordia. We are studying both types of mutation.
The cell number mutations have led us to study the process of maintenance of cell number in shoot apical meristems. These meristems are the populations of stem cells at the growing tip of each shoot, they serve as the ultimate source of all of the above-ground cells of the plant. One mysterious property of the meristems is their constancy &endash; despite continued departure of cells from the meristem to become parts of stems, leaves and flowers, the meristem itself is maintained in constant size for much of the life of the plant. We have shown that some of the coordinate control of the cell number in different meristem regions is the result of communication between different populations of meristem cells, and that this communication involves extracellular ligands and a plant-specific type of transmembrane receptor serine-threonine kinase. Several hundred such kinases are found encoded in the Arabidopsis genome; by studying them we expect to discover numerous pathways of cell-cell communication in plants.