Regulation of leaf shape

Article By:

Kang, Julie Department of Plant Biology, University of California, Davis, California.

Sinha, Neelima R. Department of Plant Biology, University of California, Davis, California.

Last reviewed:2011

DOI:https://doi.org/10.1036/1097-8542.YB110032

The arrangement and shape of leaves of flowering plants are some of the most diverse characteristics in plant biology. The striking morphological differences in these above-ground structures can range from simple leaves, with each leaf consisting of a one-blade unit, to compound leaves, in which a blade is subdivided into two or more separate parts called leaflets. Examples of such leaf-shape variation include simple leaves of Arabidopsis thaliana, peltate leaves (which have the petiole attached to the lower surface instead of the base) of Tropaeolum majus (Nasturtium), pinnately compound leaves of tomato (pinnate leaves have their parts arranged like a feather, branching from a central axis), and palmately compound leaves of Pachira aquatica (money tree; palmate leaves have lobes that radiate from a common point) [Fig. 1]. Within these different leaf shapes, the blade of the leaf may have smooth margins, toothlike serrations, or lobes, each of which may be elaborated to various degrees (Fig. 2a,b). Despite the great morphological diversity in leaves across seed plants, the common trait of all leaves is that they function as the main photosynthetic organ of the plant. Leaves also share certain defining traits that determine their early development independent of final leaf form. For instance, all leaves arise from the flanks of an organ called the shoot apical meristem. The shoot apical meristem is a dome-shaped structure composed of a population of stem cells, and it is these cells that regulate the patterns of cell division activity that lead to leaf initiation. Once leaves initiate from the flanks of the shoot apical meristem, their lateral position inherently polarizes the leaves along their dorsiventral axis. This dorsiventral polarity is also specified in both external and internal tissues. The common internal tissues of leaves differentiate during this stage of leaf development, during which structures such as the vascular system are formed. Despite these commonalities of early development in simple and compound leaves, evolutionary studies suggest that compound leaves may have arisen more than once in the flowering plant lineage, with frequent reversions back to the simple leaf form. With the advent of molecular biology, classical theories that suggest or refute homology between simple and compound leaves can be revisited.

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