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Vascular land plants share a common body plan comprising an underground portion, the root system, and an aboveground shoot portion that consists of the stem, leaves, and flowers. Extensive research in model organisms has uncovered numerous genes that play a role in shoot morphogenesis. However, the level of functional conservation of these genes during evolution required to generate the plant body is not known. In addition to variation in gene expression, the regulatory sequences, or noncoding regions of a gene, can also act as significant factors driving morphological traits. A comparative approach that uses analysis of natural variation (e.g., within families and among species) presents several challenges. Progress has been made in utilizing these approaches, but there are limitations to these studies, such as the inability to do forward genetics due to both the lack of mutants and the long generation time of many species. We also emphasize the urgent need to develop genetic and transformation tools in ...

Genetic and molecular analyses of shoot development in model organisms, maize, and Arabidopsis have shown that the KNOTTED‐like homoebox Class I (KNOX I) of genes plays an important role in maintaining indeterminacy at the shoot apical meristem (SAM) and may also be involved in the production of determinate leaf primordia. In order to investigate whether this class of genes is also involved in shoot development in plants with divergent morphology, we analyzed the developmental morphology and gene expression patterns in Welwitschia mirabilis, a gymnosperm with an interesting phylogenetic placement and a peculiar growth habit. It produces only two photosynthetic leaves over its entire life span, and these leaves continue to grow from a basal meristem throughout the life of the plant. Since KNOX I genes have been hypothesized as required for maintenance of indeterminate shoot apices, and their absence as necessary for determinate leaf production, we were able to test robustness of the specificity of KNOX I g...

We examined leaf morphology, anatomy, and margin development in wild‐type and mutant leaves in order to define the genetic pathways controlling the different stages of leaf development in tomato. Tomato mutants were placed into four categories based on the degree of leaf complexity and margin development. Mutant phenotypes ranged from little or no blade expansion (Type I mutant wiry [w]), reduced leaf complexity (Type II mutants Lanceolate [La] and entire [e]), and reduced margin complexity (Type III mutants solanifolia [sf], trifoliate2 [tf2], and potato leaf [c]) to excessive leaf complexity (Type IV mutants Mouse ears [Me], Curl [Cu], clausa [clau], and complicata [com]). Based on our analyses, we conclude that the central domain of the terminal leaflet is established first, followed by blade expansion and further partitioning of the blade into a proximal domain that will promote the initiation of lobes and lateral leaflets. The mutant w causes a defect in blade expansion, La inhibits the establishment...

The plant epidermis is only rarely involved in fusion events. These events often occur when the epidermis has been wounded or removed. The adherentl (adl) mutation in maize shows fusions between cells and organs in the presence of an intact epidermis. Fusions are not limited to a particular stage of organ development. The first fusion events occur very early in development and adherent leaves can be seen in ungerminated embryos. In the vegetative parts of the plant the coleoptile can fuse to the first leaf and juvenile leaves can fuse to each other. At the onset of reproduction, tassel branches can adhere to each other, the flag leaf can fuse to the tassel, and the glumes can fuse to the anthers. While fused regions appear to have an intact cuticle, cell surfaces are perturbed in the mutation. Time-lapse replicas reveal that trichome fusion on organ surfaces is postgenital. The extracellular matrix can exert control over organs and tissues in plants; the Adl gene may represent a product that determines th...