Chapter Summary

Before one can understand plant development, one has to review the basic organization of plants and some of their fundamental differences from animals. First and foremost, from a developmental perspective, plants continue to produce new organs throughout their lifetime. This is due to the presence of the meristem, a group of multipotent cells that can generate organs, has regenerative properties, and is self-organizing. Second, plants have cell walls, which affect morphogenesis (because the cells have fixed positions and cannot move) and cell-cell communication (because signaling molecules need to pass through the cell wall barrier). These key differences mean that most of plant development is controlled by positional information and not by lineage-based information in the meristem.

There are two types of meristem in plants, the above ground portion in the shoot and the below ground portion of the root [fig. 9.1]. The shoot apical meristem produces leaves, buds, and stem. The root apical meristem produces the root and responds to gravitational cues. The shoot apical meristem is organized into zones and layers [fig. 9.2]. There are two zones, a central zone located in the center of the meristem, surrounded by a peripheral zone. Cells in the central zone are similar to stem cells in animals and will replenish and organize the meristem [fig. 9.4]. Cells in the peripheral zone will form organs and undergo morphogenesis. The three layers of the peripheral zone ar: the outer layers of the tunica (L1 and L2) and the inner layer of the corpus (L3). Each layer contributes to different tissues to the developing organs [fig. 9.3]. The tunica will form the epidermis (L1) and the leaves and flowers (L2). The corpus will form the stem (L3).

The meristem continually produces new leaves, which are visible as they initiate as slight bumps, or primordia, at the periphery of the meristem. Leaves are initiated at a certain frequency and in a certain pattern [fig. 9.5]. The timing between successive leaf initiations is called the plastochron and can be described in stages of leaf development from initiation to full differentiation. The plastochron stages are numbered P1, P2, P3, . . . , Pn, where n is the number of leaves differentiating and varies widely between different plant species. The leaf continues going through plastochron stages until it is fully differentiated. The pattern in which new leaves are initiated is phyllotaxy. There are two possible phyllotactic patterns: spiral phyllotaxy, where leaf primordia bud at certain angles from the previous primordia, and whorled phyllotaxy, where leaf primordia form in alternating patterns of one (distichous), two (decussate), or three (tricussate) leaves per whorl.

There are many models to explain how phyllotactic patterning occurs, most falling under two concepts: morphogenetic fields or biophysical constraints. The concept of morphogenetic fields and related models suggest that an initiated primordia makes a diffusible substance that inhibits the initiation of another primordia, so that the next primordia will form only after escaping this biochemical constraint. Alternatively, the concept of biophysical constraints suggests that the positions of existing primordial effect initiation of new primordia by limiting the available physical space. The available space model can include either biochemical or biophysical constraints and is supported by studies of mutants producing more organs than normal because they have larger meristems, and thus more room to escape biochemical or biophysical constraints [fig. 9.6]. However, these mutants do not change the phyllotactic pattern of leaves, only the number of leaves produced in that pattern. Surgical manipulations can be performed in corn that will change phyllotactic patterning from whorled to spiral [fig. 9.7]. This mimics the natural change in phyllotactic patterning that occurs in corn during the progression from leaf formation (whorled pattern) to flower formation (spiral pattern).

Another important factor for leaf initiation is the fact that plant cells have cell walls that affect morphogenesis of new primordia. One way that plant cells can rapidly change in shape is by expanding their volume, and this is controlled by the protein expansin. Experiments using beads coated with expansin show that the expansion of plant cells within the P0 plastochron domain affect primordium initiation [fig. 9.8].

When new leaves are initiated and develop, they differentiate with a dorso-ventral, or adaxial-abaxial polarity [fig. 9.10]. The adaxial side of the leaf faces the meristem and is dorsal; the abaxial side of the leaf faces away from the meristem and is ventral. This polarity is apparent in the organization of tissues and cell types in the leaf and is important for plant physiological processes like gas exchange (location of stomata), photosynthesis (palisade mesophyll and bundle sheath cells), and vasculature (xylem and phloem). The meristem is important for establishing dorso-ventral polarity, and many mutants have been described with defects in adaxial-abaxial polarity.

Not only are leaves initiated from meristems during plant development, but also new branches are initiated from axillary meristems located at leaf axil junctions along the main stem. The growth of axillary meristems is normally suppressed by the shoot apical meristem. However, if the shoot apical meristem is removed, by pruning for example, then the axillary meristems will grow into new flowering shoots and branches [fig. 9.9]. This phenomenon by which the growth of apical meristems is suppressed by the shoot apical meristem is called apical dominance. Apical dominance also suppresses the growth of adventitious meristems, which form along the surface of stems and leaves. Hormonal signals are important for the mechanism of apical dominance and affect the growth of both axial and adventitious meristems.

Another important meristem is the root apical meristem [fig. 9.11], which is organized into four zones at the tip of the root: a root cap, a zone of cell division, a zone of elongation, and a zone of differentiation. The root cap is the zone that senses gravitational forces, and is gravitropic. Root apical meristems have several different tissue types radially organized around the inner vasculature: the pericycle, the endodermis, the cortex, and the epidermis. In addition there is a set of very slowly dividing cells, called the quiescent center, and a set of four initial cells, each of which produces a set of cell types in the root apical meristem. However, the initial cells do not produce these cell types in a lineage-specific manner. When initial cells are killed with a laser beam, they are replaced by neighboring differentiated cell types in the root apical meristem. Differentiation of cell types in the root apical meristem is dependent on position and involves tissue-specific expression of genes like scarecrow (scr) in the quiescent center and cortical endoderm cells [fig. 9.12] and short root (shr) in the stele.

The initiation of lateral organs from the root apical meristem is different from the initiation of organs from the shoot apical meristem. Lateral roots are not initiated in the epidermis of the meristem, as they are in leaves [fig. 9.13]. Rather, they initiate far from the root apical meristem internally and break through the epidermis. Also, lateral roots are not initiatied in a phyllotactic pattern like leaves. Instead, they respond to environmental cues: the main root grows toward gravity, that is, into the soil, whereas lateral roots grow toward nutrients.

Further Reading

If this is your first exposure to plant anatomy and physiology, it would be a good idea to review these chapters in your introductory biology textbook. In addition, there is a Web site entitled "Plant Anatomy Laboratory, Micrographs of Plant Cells and Tissues," with explanatory text. Written by James D. Mauseth from the Department of Integrative Biology at the University of Texas, it features many micrographs of different plant structures.

In particular, check out the chapters on meristems

and the epidermis


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