Organ Formation at the Vegetative Shoot Meristem.

Abstract

OVERVIEWIn higher plants, organ formation is critical for generating the vegetative portion of the plant. Above-ground organ formation occurs at a collection of stem cells termed the shoot meristem (SM), which is established during embryogenesis. How the SM functions as a site of continuous organ formation is a central question for plant developmental biology. This is not only because all above-ground organs are initiated by the SM but also because their position and identity are established there.The SM retains the capability to form organs through two fundamental processes (Figure 1). The first function of the SM is to maintain a pool of undifferentiated cells. Without undifferentiated cells to draw upon, new organ initiation would not be possible. The second process is to direct appropriately positioned undifferentiated cells toward organ formation and eventual differentiation. Although the structure of meristems is variable across the plant kingdom, all complex multicellular plants regulate the balance between an undifferentiated and differentiated fate at their respective meristems. Thus, unraveling the mechanisms by which model plants such as Arabidopsis regulate meristem development is expected to have implications for a wide variety of plant species.The SM has been the focus of many studies over the past several decades. These studies have investigated the diversity, morphology, histology, cell division patterns, and cell lineages of the SM. Because this work has been summarized in detail elsewhere (Steeves and Sussex, 1989; Lyndon, 1990Lyndon, , 1994, this review focuses on recent advances in understanding the genetic control of organ formation at the SM. MERISTEM STRUCTUREThe SM can be divided conceptually into several regions (Figure 1 C). The distinctions between the inner two regions, the central zone (Cz) and the peripheral zone (PZ), are

LATERAL ORGAN FUSION1andLATERAL ORGAN FUSION2function in lateral organ separation and axillary meristem formation inArabidopsis

“…In contrast to animals, which form organs primarily during embryogenesis, plants produce organs throughout their life cycle. The SAM contains a population of self-renewing stem cells at its center, whereas cells at the periphery are recruited into organ primordia or into stem tissue during stem elongation Clark, 1997;Fletcher, 2002;Gallois et al, 2002;Long et al, 1996). A balance between maintenance of the SAM and recruitment of cells into organ primordia is crucial for precise patterning.…”

The KNAT2 Homeodomain Protein Interacts with Ethylene and Cytokinin Signaling

“…In Angiosperms like Arabidopsis, the cells within the shoot apical meristem (SAM) are distributed in three layers: L1, L2, and L3 (Clark, 1997; Barton, 1998). The cells derived from the L1 will preferentially form the epidermis, whereas L2 and L3 layers will give rise to the inner parts of the organs.…”

Anomalous somatic embryos in Acca sellowiana (O. Berg) Burret (Myrtaceae)

“…Arabidopsis thaliana (L.) Heynh. mutant plants for normal organization of shoot apical meristems as the shootmeristemless (stm), wuschell (wus) and gurke (gk) present profound phenotypical changes (Torrez Ruiz et al 1996, Clark 1997. Two other functionally redundant genes CUP-SHAPED COTYLEDON 1 (CUC1) and 2 (CUC2), are also necessary for the separation of the cotyledon primordia and for the formation of a functional shoot apical meristem in arabidopsis (Aida et al 1999).…”