Chapter Summary

In plants many of the cells are differentiating continuously, and it is possible to see several stages of differentiation in a single plant by looking at different locations. The epidermis is an interesting organ in which to study differentiation in the root and the shoot. In the root, the epidermis will give rise to two cell types: hair cells, which are responsible for the uptake of nutrients, and nonhair cells, which provide structure to the root epidermis [fig. 11.1]. The hair cells are derived from hair cell precursors called trichoblasts, which have distinct cytological differences from nonhair cell precursors called atrichoblasts. In the shoot, the epidermis will also form hair cells on the leaves and stem, called trichomes, that are polyploid and are surrounded by accessory cells [figs. 11.2 and 11.6]. Epidermal cell fate in the root and shoot has been shown to require the glabra2 gene for proper differentiation [fig. 11.3].

Another differentiated epidermal structure present on leaves is the stomata, which regulates gas exchange. The stomatal complex is a patterned organ [fig. 11.4] and consists of two guard cells and two subsidiary cells, which are all derived from a single guard mother cell (GMC) [fig. 11.5]. The guard cells regulate the movement of gasses and release of water vapor while subsidiary cells help the guard cells to open and close.

The differentiation of vascular tissue includes the differentiation of xylem and phloem, each of which become highly specialized cell types important for transport. Woody plants have a specialized meristem, called the vascular cambium, that makes these two cell types. By studying clonal sectors, vascular differentiation has been shown to be dependant on position and not lineage. A useful system for studying differentiation of vascular plant cells is the culture of xylem mesophyll cells, photosynthetic cells that contain many chloroplasts from zinnia leaves [fig. 11.7].

It is clear that positional information is essential for differentiation of cell types in plants, and this suggests that plant cells must communicate with one another during development. How do plant cells communicate with each other through cell walls? Plants have a specialized structured, called the plasmodesmata, membrane-lined cytoplasmic channels that connect the cytoplasm of adjacent cells and contain a strand of endoplasmic reticulum [fig. 11.8]. Thus plants have two types of cellular continuity: symplastic, which allows electrical and molecular continuity, and aplastic, which occurs outside of the cytoplasm in the space between the cell walls. These connections are highly dynamic and can change under developmental or environmental influences [fig. 11.9]. These connections are responsible for the movement of macromolecules and proteins between cells, and these movements are facilitated by movement proteins, which increase the effective pore size of plasmodesmata to 40kD. Many proteins that are important for plant development have been shown to move between cells: the transcription factor knotted 1 (Kn1), leafy [fig. 11.10], and Shr [fig. 11.11].

Another way that plant cells communicate is by using kinase receptors. One example is the phenotype in corn that produces ears that are fasciated and results when the meristem becomes divided [fig. 11.12]. It is thought that genes producing fascinated phenotypes function in signaling pathways that limit growth in the meristem. The communication of plant cells during development is necessary for cells to coordinate their activities to provide final form and function for the plant [fig. 11.13].

Most plant developmental pathways involve the action of plant hormones [table 11.1], and each type of hormone plays a different role for the plant [fig 11.15]. The first group of hormones identified, the auxins, are responsible for cell elongation, xylem regeneration, apical dominance, adventitious root growth, gravitropy, fruit development, and phototropism (bending toward the light). These chemical messengers are translocated, moved from one part of the plant to another. In the case of auxin, this transport is polarized and active, and we refer to it as polar auxin transport [fig. 11.14]. The cytokinins are responsible for cell division, shoot formation, loss of apical dominance, and inhibition of senescence. The gibberellins are responsible for regulating plant height, absicisic acid is responsible for seed dormancy, and brassinosteroids are similar to animal-like steroid hormones. The simplest plant hormone is ethylene, which regulates fruit ripening, seed germination, cell elongation, leaf curling, root hair formation, and abscission.

In addition to hormones, light perception plays a major role in plant development [fig. 11.16]. The perception of light is mediated through photoreceptors, which include a family of pigment molecules called phytochromes that absorb light in the red and far-red spectrum. Phytochrome responses to light include seed germination, greening, stem elongation, flowering, and the induction of gene transcription. There are also two types of blue light receptors known as the cryptochromes and phototropins. All of the photoreceptors are essential for the circadian clock in plants, which is important for many developmental phenomena.

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