Annulus Plasmodesma Desmotubule
▲ FIGURE 6-34 Structure of a plasmodesma.
(a) Schematic model of a plasmodesma showing the desmotubule, an extension of the endoplasmic reticulum, and the annulus, a plasma membrane-lined channel filled with cytosol that interconnects the cytosols of adjacent cells. Not shown Is a gating complex that fills the channel and controls the transport of materials through the plasmodesma. (b) Electron micrograph of thin section of plant cell and cell wall containing multiple plasmodesmata. [E. H. Newcomb and W. P Wergin/Biological Photo Service.]
Only a Few Adhesive Molecules Have Been Identified in Plants
Systematic analysis of the Arabidopsis genome and biochemical analysis of other plant species provide no evidence for the existence of plant homologs of most animal CAMs, adhesion receptors, and ECM components. This finding is not surprising, given the dramatically different nature of cell-cell and cell-matrix/wall interactions in animals and plants.
Among the adhesive-type proteins apparently unique to plants are five wall-associated kinases (WAKs) and WAK-like proteins expressed in the plasma membrane of Arabidopsis cells. The extracellular regions in all these proteins contain multiple epidermal growth factor (EGF) repeats, which may
▲ EXPERIMENTAL FIGURE 6-35 An in vitro assay used to identify molecules required for adherence of pollen tubes to the stylar matrix. In this assay, extracellular stylar matrix collected from lily styles (SE) or an artificial matrix is dried onto nitrocellulose membranes (NC). Pollen tubes containing sperm are then added and their binding to the dried matrix is assessed. In this scanning electron micrograph, the tips of pollen tubes (arrows) can be seen binding to dried stylar matrix. This type of assay has shown that pollen adherence depends on stigma/stylar cysteine-rich adhesin (SCA) and a pectin that binds to SCA. [From G. Y Jauh et al., 1997, Sex Plant Reprod. 10:173.]
directly participate in binding to other molecules. Some WAKs have been shown to bind to glycine-rich proteins in the cell wall, thereby mediating membrane-wall contacts. These Arabidopsis proteins have a single transmembrane domain and an intracellular cytosolic tyrosine kinase domain, which may participate in signaling pathways somewhat like the receptor tyrosine kinases discussed in Chapter 14.
The results of in vitro binding assays combined with in vivo studies and analyses of plant mutants have identified several macromolecules in the ECM that are important for adhesion. For example, normal adhesion of pollen, which contains sperm cells, to the stigma or style in the female reproductive organ of the Easter lily requires a cysteine-rich protein called stigma/stylar cysteine-rich adhesin (SCA) and a specialized pectin that can bind to SCA (Figure 6-35).
Disruption of the gene encoding glucuronyltransferase 1, a key enzyme in pectin biosynthesis, has provided a striking illustration of the importance of pectins in intercellular adhesion in plant meristems. Normally, specialized pectin molecules help hold the cells in meristems tightly together. When grown in culture as a cluster of relatively undifferentiated cells, called a callus, normal meristematic cells adhere tightly and can differentiate into chlorophyll-producing cells, giving the callus a green color. Eventually the callus will generate shoots. In contrast, mutant cells with an inactivated glucuronyltrans-ferase 1 gene are large, associate loosely with each other, and do not differentiate normally, forming a yellow callus. The introduction of a normal glucuronyltransferase 1 gene into the mutant cells by methods discussed in Chapter 9 restores their ability to adhere and differentiate normally.
The paucity of plant adhesive molecules identified to date, in contrast with the many well-defined animal adhesive molecules, may be due to the technical difficulties in working with the ECM/cell wall of plants. Adhesive interactions are often likely to play different roles in plant and animal biology, at least in part because of their differences in development and physiology.
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