Tips from the Plant Prop Shop: The Foundation for Clonal Propagation: Plant Shoot Meristems

To master the basics of successful clonal plant propagation, it’s necessary to understand the process of vegetative reproduction in plants. This information will be especially beneficial when discussing specific plant propagation techniques in future newsletter articles. Plant growth originates in localized regions called meristems (derived from the Greek word Meritos meaning Divisible)In animals, growth is determinate and ceases when a specific size and shape is achieved. In contrast, except for most leaf and floral development, plants exhibit indeterminate growth where shoot and root meristems continue to produce new organs under favorable conditions and available resources.  

Primary meristems are characterized by areas of active cell division, enlargement, and differentiation into specialized cells, tissues, and organs such as leaves (Fig. 1). The shoot apical meristem produces three primary meristems: 1) the procambium from which the vascular tissues (xylem and phloem) originate; 2) ground meristem from which the inner pith and cortex tissues are produced; and 3) the protoderm from which the epidermal layer is differentiated (Fig. 1). In perennial plants, secondary meristems (lateral), the vascular cambium and cork cambium, give rise to secondary vascular tissue and the periderm of the bark, respectively. This results in thickening of stems and roots.  This developmental sequence is outlined in Fig. 2.

Salient Features of a Shoot-tip

Note that there are significant differences in shoot-tip organization between taxonomic plant groups.  A typical shoot-tip (Fig. 3) is comprised of an apical meristematic dome consisting of zones of actively dividing cells called apical initials which produce genetically identical cells to the plant via the two-step process of mitosis (genetic duplication) and cell division during which one cell gives rise to two genetically identical cells.  It’s interesting to note that no vascular connection exists between the apical meristem and the rest of the plant. The highly structured nature of the meristem is believed to provide genetic stability in the cells produced, however, mutations do occur. Leaf primordia develop below the apical meristematic dome with the internal leaf tissue layers and vascularization forming through marginal meristem activity. Within the axil of each leaf a lateral bud consisting of an apical meristem enclosed in a bud scale (a modified leaf). The outgrowth of these lateral buds is inhibited by the shoot-tip through the phenomenon called apical dominance.  Inhibition of lateral bud elongation down the stem is controlled by production of the plant hormone, auxin (indole-3-acetic acid) in developing leaf primordia in the shoot-tip.  This and its subsequent stem transport via the vascular system throughout the plant. (Fig. 3). Another class of plant hormones, cytokinins, produced in the roots and transported up the stem, antagonize the inhibitory effects of auxin. Unfortunately, this interaction explains why trimming your garden hedges results in vigorous lateral shoot outgrowth several weeks later!

Mistakes in the Meristem: Plant Chimeras

Various theories have been put forth to explain shoot apex cell arrangement organization.  The Tunica-Corpus theory states that most angiosperm shoot meristems consist of a multi-layered tunica [L1/L2] and a corpus [L3] (Fig. 4).  Cells within the tunica are characterized as dividing anticlinally (cell division occurring perpendicular to the surface).  Anticlinal divisions contribute to the surface (sheet) growth of the meristem but not its thickness. The L1 layer gives rise to the epidermis of all leaves and stem while the L2 produces most of the subepidermal tissues of leaves and flowers. The corpus, the inner tissue zone, is several layers thick.  The cells first divide periclinally (divisions parallel to surface) forming the core of the shoot apex with subsequent irregular cell division in all planes resulting in increased shoot apex growth and volume.  The corpus give rise to the cortex/ground tissues.

We might assume that all plant cells that comprise a plant are genetically identical, but this is often not the case. Genetic changes occur frequently in plants. Plants or plant parts that are a comprised of two or more genetically different cell types are called chimeras.  Chimeras may arise following grafting of two distinct plants.  Chimeras may also develop following random mutations in dividing cells, especially in meristematic tissues. The type of chimera and its stability largely depends on where the mutation occurs in the plant.  A single cell may mutate spontaneously due to genetic errors occurring during replication or following exposure to ultraviolet light or to other mutagenic agents. Chimeras will be observed if they result in visible changes in morphology, pigmentation, and growth habit.

Three types of chimeras exist: 1) mericlinal; 2) sectorial; and 3) periclinal chimeras (Fig. 5).  Mericlinal chimeras originate first in a section of single layer while sectorial chimeras consist of mutated tissue which develop into each layer of the meristem (Fig. 5). Both mericlinal and sectorial tend to be very unstable.  Periclinal chimeras are the most stable type as the mutation occurs in an apical initial cell such as in the L1 which, following periclinal divisions of the mutant cell, results in all cells in that layer being mutated. (Fig. 5). Lateral shoots which form down the stem will maintain this chimera thus allowing clonal propagation. Many commercially valuable food and ornamental crops are chimeras (Fig. 6).

                                                            Michael Kane, Professor Emeritus

                                                  Environmental Horticulture Department, University of Florida