A Companion to Plant Physiology, Fifth Edition by Lincoln Taiz and Eduardo Zeiger
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Topic 15.6

Wall Degradation and Plant Defense

The plant cell wall is not simply an inert, static exoskeleton. In addition to acting as a mechanical restraint, the wall serves as an extracellular matrix that interacts with cell surface proteins, providing positional and developmental information. It contains numerous enzymes and smaller molecules that are biologically active and that can modify the physical properties of the wall, sometimes within seconds. In some cases, wall-derived molecules can also act as signals to inform the cell of environmental conditions, such as the presence of pathogens. This is an important aspect of the defense response of plants (see Chapter 13).

Walls may also be substantially modified long after growth has ceased. For instance, the cell wall may be selectively degraded, such as occurs in ripening fruit or in the endosperm or cotyledons of germinating seeds. In cells that make up the abscission zones of leaves and fruits (see Chapter 22), the middle lamella is digested, with the result that the cells become unglued and separate. Cells may also separate selectively during the formation of intercellular air spaces, during the emergence of the root from germinating seeds, and during other developmental processes. Plant cells may also modify their walls during pathogen attack as a form of defense.

Walls may also be substantially modified long after growth has ceased. For instance, the cell wall may be selectively degraded, such as occurs in ripening fruit or in the endosperm or cotyledons of germinating seeds. In cells that make up the abscission zones of leaves and fruits (see Chapter 22), the middle lamella is digested, with the result that the cells become unglued and separate. Cells may also separate selectively during the formation of intercellular air spaces, during the emergence of the root from germinating seeds, and during other developmental processes. Plant cells may also modify their walls during pathogen attack as a form of defense.

Enzymes Mediate Wall Hydrolysis and Degradation

Hemicelluloses and pectins are modified and broken down by a variety of enzymes that are found naturally in the cell wall. This process has been studied in greatest detail in ripening fruit, in which softening is thought to be the result of disassembly of the wall (Rose and Bennett 1999), with multiple enzymes working cooperatively (Cantu et al. 2008). Glucanases and related enzymes hydrolyze the backbone of hemicelluloses. Xylosidases and related enzymes remove the side branches from xyloglucan (particularly xyloglucan fragments, or oligomers). Transglycosylases cut and join hemicelluloses together. Expansins make the wall more accessible to enzymatic digestion. These biochemical processes may alter the physical properties of the wall, for example, by changing the viscosity of the matrix or by altering the tendency of the hemicelluloses to stick to cellulose.

Messenger RNAs for expansin are expressed in ripening fruit, suggesting that they play a role in wall disassembly (Rose et al. 1997). Similarly, softening fruits express high levels of pectin methyl esterase, which hydrolyzes the methyl esters from pectins. This hydrolysis makes the pectin more susceptible to subsequent hydrolysis by pectinases and related enzymes. The actions of these and related enzymes are important factors in fruit softening and susceptibility to necrotrophic pathogens causing fruit rot.

Oxidative Bursts Accompany Pathogen Attack

When plant cells are wounded or treated with certain low molecular weight elicitors (see Chapter 13), they activate a defense response that results in the production of high concentrations of hydrogen peroxide, superoxide radicals, and other active oxygen species in the cell wall. This “oxidative burst” is part of a defense response against invading pathogens (Brisson et al. 1994; Otte et al. 2001) (see Chapter 13).

Active oxygen species may directly attack the pathogenic organisms, and they may indirectly deter subsequent invasion by the pathogenic organisms by causing a rapid cross-linking of phenolic components of the cell wall. In tobacco stems, for example, proline-rich structural proteins of the wall become rapidly insolubilized upon wounding or elicitor treatment, and this cross-linking is associated with an oxidative burst and with a mechanical stiffening of the cell walls.

Degradation of cell walls can result in the production of biologically active fragments 10 to 15 residues long, called oligosaccharins, that may be involved in natural developmental responses and in defense responses. Some of the reported physiological and developmental effects of oligosaccharins include stimulation of phytoalexin synthesis, oxidative bursts, ethylene synthesis, membrane depolarization, changes in cytoplasmic calcium, induced synthesis of pathogen-related proteins such as chitinase and glucanase, other systemic and local “wound” signals, and alterations in the growth and morphogenesis of isolated tissue samples (John et al. 1997).

The best-studied examples are oligosaccharide elicitors produced during pathogen invasion (see Chapter 13). For example, the oomycete Phytophthora secretes an endopolygalacturonase (a type of pectinase) during its attack on plant tissues. As this enzyme degrades the pectin component of the plant cell wall, it produces pectin fragments—oligogalacturonans—that elicit multiple defense responses by the plant cell (Web Figure 15.6.A). The oligogalacturonans that are 10 to 13 residues long are most active in eliciting these responses.

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Web Figure 15.6.A   Scheme for the production of oligosaccharins during fungal or oomycete invasion of plant cells. Enzymes secreted by the plant, such as chitinase and glucanase, attack the fungal or oomycete wall, releasing oligosaccharins that elicit the production of defense compounds (phytoalexins) in the plant. Similarly, fungal or oomycete pectinase releases biologically active oligosaccharins from the plant cell wall. Fungal, but not oomycete, walls contain chitin. (After Brett and Waldron 1996). (Click image to enlarge.)

Plant cell walls also contain a (1,3)-β-D-glucanase that attacks the (1,3)-β-D-glucan that is major component of oomycete but not most plant cell walls. When this enzyme attacks the oomycete wall, it releases glucan oligomers with potent elicitor activity. The wall components serve in this case as part of a sensitive system for the detection of pathogen invasion.

Plants and microbes also possess inhibitory proteins that block the activity of the each other’s degradative enzymes (York et al. 2004). For example, plants have inhibitory proteins that inhibit or otherwise modify the activity of microbial (but not plant) polygalacturonases, glucanases, and xylanases, presumably to thwart microbial attacks. A similar trick is played out by some plant pathogens, which secrete proteins that inhibit plant defense enzymes.

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