Abstract
The growing cell wall in plants has conflicting requirements to be strong enough to withstand the high tensile forces generated by cell turgor pressure while selectively yielding to those forces to induce wall stress relaxation, leading to water uptake and polymer movements underlying cell wall expansion. In this article, I review emerging concepts of plant primary cell wall structure, the nature of wall extensibility and the action of expansins, family-9 and -12 endoglucanases, family-16 xyloglucan endotransglycosylase/hydrolase (XTH), and pectin methylesterases, and offer a critical assessment of their wall-loosening activity
Highlights
The growing cell wall of plants is both strong and extensible
Synthesis and incorporation of new structural components into the growing walls are required in the long term to prevent loss of mechanical integrity, but wall synthesis in most plant cells is not linked mechanistically to expansion, as it is in bacteria
Unlike α-expansins, pollen β-expansins greatly reduced the tensile strength of grass cell walls, at least in part by weakening the middle lamella between cells, whereas they had negligible effect on cell walls from eudicot species[69]. These results suggest that eudicot walls lack the specific target of pollen β-expansins, or that the target has a minor mechanical role in eudicot walls
Summary
The growing cell wall of plants is both strong and extensible. Its mechanical strength lets it resist the tensile stresses in the plane of the wall (~10 MPa or more) generated by the internal hydrostatic pressure (turgor) typical of plant cells (~0.5–1 MPa). An XTH with strong transglucosylase activity from tomato was tested for its ability to induce wall creep or to weaken the wall as measured in stress/strain assays, with negative results[25] These results are consistent with results obtained with xyloglucan-specific GH12 endoglucanases[26], described above, showing that cutting of xyloglucan is not sufficient for cell wall loosening. Prospectus Recent studies are converging on the concept that the primary cell wall contains limited cellulose-cellulose junctions that are sites of initial wall loosening and stress relaxation, and that are the selective targets of expansins and potentially other wall-loosening agents How these sites are formed is unknown; are they the result of a well-controlled cellular process or of a purely physical, stochastic interaction? I confirm that the funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript
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