Department of Plant Biology
Carnegie Institution for Science
260 Panama Street
Stanford, CA 94305
Phone: (650) 325-1521 x224
Fax: (650) 325-6857
Although leaf shape and arrangement may differ greatly between species, all plants share a similar body plan consisting of repeating units of stem and leaf. These units of stem and leaf have their origin in the shoot apical meristems located at the growing tips of shoots. The goal of our lab is to understand how these basic developmental units of the plant are made by the shoot apical meristem.
Figure 1. Parts list for the Arabidopsis plant. The plant body consists of leaf and stem segments. Some stem segments (also called internodes) are compressed (such as those in the rosette and flower) while others are elongated (such as those in the inflorescence stem). Leaves can vary in shape and size and may be highly modified as in the case of the sepals, petals and carpels. The main shoot apical meristem is made during embryogenesis, between the cotyledons (or seed leaves). Plant growth is reiterative with new shoot apical meristems, or axillary buds, made on the body of the plant. Axillary meristems form in the leaf axils - the point where the leaf base joins the stem.
Shoot apical meristems have two important functions. First, they are the home of a small population of self-renewing stem cells. The stem cells are located in the central region of the meristem. When these cells divide, daughter cells are pushed out into the peripheral zone of the meristem where they ultimately “exit” the meristem and take part in the development of differentiated tissues.
Figure 2. Scanning electron micrograph of an Arabidopsis shoot apical meristem. The stem cells are located in the center of the meristem. Cells in the peripheral zone of the meristem participate in leaf formation. In Arabidopsis, leaves form in a spiral with 137 degrees between successive leaves.
Second, shoot apical meristems are a site of pattern formation. For example, spatial cues in or around the meristem determine where the next new leaf will form. In addition, fundamental coordinates of the leaf are likely specified while the leaf founder cells reside in the meristem. For instance, it is likely that the future adaxial (top) and abaxial (bottom) domains of the leaf are specified based on their distance from the center of the meristem.
Figure 3. Schematic of a transverse section showing organization of the Arabidopsis shoot apical meristem and young leaves. The cross hatched area in the center of the meristem is the home of the self-renewing stem cells. As these cells divide, their descendants are displaced into the peripheral zone where they take part in leaf formation. Leaf primordia are labelled P1, P2 etc. The SHOOTMERISTEMLESS gene is expressed in both the central and peripheral zones and is downregulated in early leaf development (in the P0 leaf) before the leaf is morphologically evident. The positions of even younger leaves (P-1 to at least P-3) are detected by high levels of the PINHEAD transcript in leaf traces below the meristem. The P0 stage primordium appears to lack polarity. Both adaxial transcripts (such as phabulosa and pinhead) and abaxial transcript (such as filamentous flower) are expressed throughout the primordium. Polarity along the adaxial/abaxial dimension of the leaf is established by the P2 stage in response to an unknown signal, perhaps from the center of the meristem. By this stage, the phabulosa and pinhead transcripts are localized to the adaxial side of the primordium while the filamentous flower trnscript is localized to the abaxial side of the primordium.
One gene that plays a critical role in meristem formation is the Arabidopsis SHOOTMERISTEMLESS gene. This gene is required for meristem formation; in its absence, no meristem forms in the notch between the cotyledons during embryogenesis. The SHOOTMERISTEMLESS gene is also sufficient for meristem formation; when the gene is ectopically expressed in the leaf, many ectopic meristems form. Thus, it is important that both activation and repression of the SHOOTMERISTEMLESS gene product be carefully controlled.
Figure 4. A. Wild-type seedling with a shoot apical meristem formed between the cotyledons. B. Shootmeristemless mutant. No shoot apical meristem has formed. C. Seedling ectopically expressing the shootmeristemless protein. The adaxial sides (tops) of the cotyledons are covered with ectopic meristems. (Photographs by J. Long)
Several genes that are candidates for regulators of SHOOTMERISTEMLESS expression have been identified in the lab. Some of these appear to act to establish the pattern of positional identities in the embryo. For instance, the TOPLESS gene appears to be involved in establishing apical/basal polarity in the embryo. Normally, the root forms at the basal pole of the embryo and the shoot (including both cotyledons and shoot apical meristem) forms at the apical pole of the embryo. In topless mutant embryos, both ends of the embryo develop into roots. Transcripts associated with shoot apical meristem formation, including that for the SHOOTMERISTEMLESS gene, never appear. An in depth study of the defect in topless embryos should help clarify how opposite ends of the plant embryo differentiate into shoot and root poles.
Figure 5. A wild-type embryo (left) showing domains of the embryo and what they give rise to in the seeding (right). The cells that express the SHOOTMERISTEMLESS gene are stained in red (Photograph by J. Long).
Figure 6. A topless mutant embryo showing development of a root at both poles of the embryo (Photograph by J. Long).
Another gene required for proper regulation of SHOOTMERISTEMLESS expression is the PINHEAD locus. pinhead mutants form a meristem during embryogenesis but the meristem terminates in a centrally placed leaf. Given that the PINHEAD gene product bears homology to gene products implicated in translational control, it is possible that the PINHEAD gene plays a role in promoting the translation of genes such as the SHOOTMERISTEMLESS gene that are required for meristem maintenance. However, the PINHEAD gene likely plays a wider role in plant development since double mutants carrying mutations in the PINHEAD gene and in the related ARGONAUTE gene arrest during embryogenesis.
Figure 7. Scanning electron micrograph showing a pinhead seedling in which the shoot apical meristem has terminated inappropriately in a radially shaped leaf. Axillary buds often develop in the cotyledons of pinhead mutant seedlings. (Photograph by K. Lynn).
Patterning within the leaf also appears to influence the process of meristem formation. Axillary meristems (buds) develop in the axil of the leaf (the junction where the leaf meets the stem) on the adaxial (top) side of the leaf base. When adaxial fates are duplicated, as they are in the PHABULOSA mutant, ectopic axillary meristems develop on the duplicated adaxial leaf bases. Thus, positional cues associated with the polar development of the leaf are also used to direct axillary meristem formation. The PHABULOSA gene product contains a predicted DNA binding domain and also a predicted binding domain for a steroid or a ligand. We hypothesize that this protein acts as a receptor for a steroid or steroid like signal that establishes polarity in the leaf. The lab is currently working on establishing the nature of this signalling process as well as the mechanism underlying the link between leaf polarity and meristem formation.
Figure 8. A plant carrying the dominant phabulosa mutation. In this mutant the leaves develop as rod or tube-shaped organs with adaxial (top) traits all around their circumference. Whereas wild-type plants make axillary meristems (buds) only from the adaxial leaf bases, these mutants make extra buds from the duplicated adaxial leaf bases.