Dorsoventral patterning in plants.

Abstract

Although mutations affecting dorsoventral patterning in plants have been known for centuries (Linnaeus 1749), the underlying mechanisms controlling this process have remained mysterious. Recent papers by Sawa et al. (1999b) and others have begun to shed light on this subject through the isolation and characterization of genes responsible for dorsoventrality. In plants, lateral organs and shoots are initiated from the flanks of the shoot apical meristem (SAM), a multilayered dome of self-organizing and self-perpetuating cells. Primordia emerge from the SAM with a characteristic asymmetry and identity that depend on both extrinsic and intrinsic factors (Fig. 1). For leaf primordia, this means having distinct upper and lower programs of growth and cellular differentiation so that cells in the upper half develop the appropriate epidermis, palisade mesophyll, and xylem strand, and cells in the bottom half develop a phloem strand, a spongy mesophyll, and appropriate epidermis. Organ shape and cell-type differences between the adaxial (upper) and abaxial (lower) sides of the mature leaf ensure that the plant puts the bulk of its light-gathering capacity toward the light. The asymmetry in the shape of most leaves is apparent from inception as they emerge as a crescent on the flank of the SAM, and this dorsoventrality is further accentuated by differential growth of the adaxial and abaxial sides. Early asymmetry is also revealed by adaxialand abaxial-specific expression of genes in incipient and young organ primordia (Fig. 1; see also Sawa et al. 1996). Dorsoventral patterning also occurs in lateral primordia that give rise to asymmetric shoots, such as the snapdragon flower, in which the upper and lower positioned petals and stamens differ. The dorsoventrality of this and other asymmetric lateral shoots is a result of the differences in the initiation and growth of organs on the adaxial and abaxial sides of the lateral SAM, and also to the asymmetry of the lateral SAM itself.