Diffuse Growth of Plant Cell Walls

The primary wall of a growing cell is a versatile, subtle, and dynamic structure, with unique properties and functions in the life of the plant (Burton et al., 2010). When a cell grows, its wall stretches irreversibly as the cell enlarges in volume. Cells can start and stop this process quickly, in less than a minute in some cases, revealing that the molecular processes underlying irreversible wall expansion are dynamically controlled. Such dynamic behavior may be mediated, at least in part, by changes in wall pH (Hager, 2003; Barbez et al., 2017), which modulates the wall-loosening action of expansins (Cosgrove, 2015) and potentially other wall-modifying agents. Wall pH in turn is dynamically modulated by plasma membrane H+-ATPase activity (Haruta et al., 2014, 2015) and other processes in the wall. Because growing cell walls are thin and in close physical contact with plasma membranes, wall pH can be rapidly modulated (Bibikova et al., 1998; Monshausen et al., 2007; Barbez et al., 2017). As a result of the pH-dependent activity of expansins, the growing cell wall behaves like a “smart material”—one whose properties (extensibility in this case) reversibly and rapidly change with environment (e.g. pH). Slower changes in wall structure that influence the wall’s ability to expand also occur as part of the natural course of cell development, e.g. as cells are displaced through the elongation zone of a stem (Phyo et al., 2017), or in response to external perturbations, e.g. Sahaf and Sharon (2016). These slower changes may include changes in mechanics, such as wall stiffening, and in the density or accessibility of sites where expansins or other proteins can loosen the wall. The wall itself is synthesized in a team effort: mobile cellulose synthesis complexes (Paredez et al., 2006; Li et al., 2016b) produce long, thin, strong, stiff cellulose microfibrils at the cell surface, while matrix polysaccharides and glycoproteins are deposited to the cell surface via the secretory apparatus (Zhu et al., 2015; Kim and Brandizzi, 2016). The cytoskeleton guides the wall synthesis machinery to supply wall components to appropriate locations on the cell surface (Szymanski and Staiger, 2017), where the components assemble to form an organized, mechanically strong structure that can withstand the in-plane tensile forces generated by the outward push of cell turgor pressure yet is able to expand in a controlled manner. The structural requirements for orderly expansion of the cell wall are not well defined at this time. Moreover, except with the possible exception of tip-growing cells (Dumais et al., 2006; Rojas et al., 2011), synthesis, secretion, and wall assembly are only distantly coupled to the wall extension process itself. For instance, cellulose synthesis in carbon-limited Arabidopsis (Arabidopsis thaliana) hypocotyls was temporally distinct from cell expansion (Ivakov et al., 2017); likewise, wall deposition did not keep up with cell expansion in dark-grown hypocotyls, resulting in substantial wall thinning (Refrégier et al., 2004). On the other hand, gradients in wall thickness in the growing trichome of Arabidopsis precisely matched predictions for mechanical stability of the wall, implying good coordination between local wall deposition and expansion (Yanagisawa et al., 2015). We still have much to learn about how plant cells build a stable yet extensible wall. When the normal molecular assembly of the cell wall is disturbed, for instance, by mutations that affect synthesis of cellulose (Fagard et al., 2000), xyloglucan (Cavalier et al., 2008), or pectic polysaccharides (Mouille et al., 2007), cell expansion may be disrupted in unpredictable ways. Genetic results suggest that errors in wall assembly trigger surface receptor-like kinases such as FERONIA that may act as sensors of cell wall integrity (CWI; Humphrey et al., 2007; Höfte, 2015) and mechanosensation (Hamant and Haswell, 2017). The ensuing responses, which include production of reactive oxygen species and inactivation of the plasma membrane H+-ATPase, likely give rise to some of the complex phenotypes that originate from rather simple modifications of wall polysaccharides (Voxeur and Höfte, 2016). For example, growth defects stemming from mutation of a cellulose synthase gene in Arabidopsis were partially suppressed by mutation of THESEUS1 (Hématy et al., 2007), another member of the same receptor kinase family as FERONIA (Cheung and Wu, 2011; Li et al., 2016a). Evidently, CWI responses compound and confound the direct effects of cell wall defects. Defects in pectin metabolism appear particularly prone to trigger CWI responses that activate the brassinosteroid pathway, leading to diverse growth phenotypes (Wolf et al., 2012, 2014). On the other hand, FERONIA and its extracellular peptide ligand (“rapid alkalinization factor”) are also required for normal root growth and auxin responses (Haruta et al., 2014; Shih et al., 2014; Velasquez et al., 2016; Barbez et al., 2017). Cell expansion thus appears to be intimately linked to these wall sensor pathways in ways we are only beginning to fathom. This Update focuses on the growing cell wall, in particular, the structural, mechanical, and physicochemical processes underlying irreversible wall enlargement during diffuse cell growth. Diffuse growth refers to surface expansion occurring on entire facets of cell walls, for instance, the side walls of elongating cells in the body of a growing root or stem. Diffuse growth may occur with or without a directional bias, which depends partly on wall structure and partly on patterns of mechanical stress in the wall (Baskin and Jensen, 2013). Its intensity may vary along a cell wall surface and on different cell wall facets. For instance, side walls of a hypocotyl cell may elongate rapidly, whereas its end walls may not enlarge much at all (Peaucelle et al., 2015). In the jigsaw-puzzle-like pavement cells of the Arabidopsis leaf epidermis, a complex pattern of local wall surface expansion occurs in the periclinal (outer epidermal) wall as well as in the anticlinal (side) walls (Szymanski, 2014; Armour et al., 2015). These complex expansion patterns have been linked to cytoskeletal dynamics within the cell and to spatial patterns of tensile stress (Szymanski and Cosgrove, 2009; Zhang et al., 2013; Sampathkumar et al., 2014a). Diffuse growth is the dominant pattern for most cells in the plant body and is traditionally contrasted with tip growth, for instance, in pollen tubes and root hairs, where surface expansion is localized to limited regions of the hemispherical tip (Campàs et al., 2012; Sanati Nezhad and Geitmann, 2013; Velasquez et al., 2016). Despite differences in spatial patterning, tip growth may involve some of the same cell wall processes as occur in diffuse growth, but in a more intense, spatially localized manner. This point is supported by the stunted elongation of root hairs in plants with genetic lesions in expansin genes specifically expressed in root hairs (Yu et al., 2011). Although expansins have been characterized largely in the context of diffuse growth, they also are necessary for root hair growth. Leaf trichomes in Arabidopsis are fiber-like cells with a conical shape that arises from highly anisotropic diffuse growth; its tip-biased gradient in surface expansion has been related to spatial gradients in cell diameter, wall stress, and wall thickness (Yanagisawa et al., 2015). In this Update, I begin with a review of the biophysical basis of cell wall growth, followed by our changing concepts of the role and interactions of cell wall components that compose the cell wall, and end with recent insights from atomic force microscopy (AFM), showing the details of microfibril organization and motions during wall enlargement. What happens when a cell grows? Cell volume increases as a result of water uptake, and wall surface area enlarges irreversibly by local separation of cell wall components (Cosgrove, 2016b). The phrase “turgor-driven growth” (or its variants) is often used to describe this process, but this phrasing can be misleading if it is taken to imply that growth is simply a mechanical stretching of an inherently pliant cell wall, like stretching a piece of putty. To understand this point better, it is useful to distinguish different patterns of cell wall stretching in response to an applied tensile force (Fig. 1). Elastic deformation is by definition reversible and instantaneous upon applied force. Small wall deformations are typically elastic, whereas after large, rapid, transient deformations, many cell walls do not fully return to their initial size. The irreversible part is a plastic deformation, which occurs when the wall is stretched beyond a yield point. When force is applied to a wall in a sustained manner, the stretch (“strain” in engineering terms) is partly elastic and partly plastic or partly viscoelastic and partly viscoplastic, depending on the time scales involved (Boudaoud, 2010; Moulia, 2013). These latter to sustained cell wall a force applied to a wall typically results in a that a in a depending on the and its and Such deformation is a result of the of plant cell walls, but the structural basis for these mechanical properties is largely and of different patterns of cell walls stretched with a force elastic Cell walls often a wall motions are not of elastic and plastic the yield point is partly elastic and partly and motions time to Wall results in sustained extension These mechanical responses of walls in an from the sustained wall expansion that occurs during cell growth (Cosgrove, Zhang et al., 2017), which depends on by expansins or other wall-loosening (Cosgrove, 2016a). Wall results in wall stress that cell growth. The of wall stress for plant growth was by et and by and results (Cosgrove, 2017). As a result of the complex interactions of cell wall are ways in which cell walls may mechanically more by mechanical but they do not result in an in wall and growth, and they do not as wall processes and For instance, with et al., may walls, but they do not cell growth or cell wall extension and et al., On the other hand, stress and enlargement of cell walls, but they wall activity and they do not the wall, as by tensile in et These are that to on of cell walls and To the point in another hypocotyl walls from the of Arabidopsis are more in tensile with walls, yet hypocotyls more than the et al., 2016). Moreover, their mechanical the walls more in less stress and are less to with walls and Cosgrove, the side of the cell wall expansion in but wall was et al., other have been where mechanical do not on the growth properties of the cell wall (Cosgrove, 2016b). Despite this to wall is taken to be with cell wall growth this is a of is to of growth et al., 2010; et al., have been in recent to of and cells with mechanical (e.g. et al., 2012; Nezhad et al., 2013; Sanati Nezhad et al., 2013; et al., et al., 2015; et al., 2017). To be are where with cell growth, as Such may a change in wall structure that to growth by wall stress or may be resulting from structural changes of wall Elastic changes may also be a of the growth. For instance, it has been that auxin results in wall in many and 2017). the of growth is and the change in wall which may changes in wall structure by auxin or growth itself. in the between and may be the definition of such as wall and We do not have an to distinguish the many facets of wall As wall refers to a or of a part of the wall, tensile stress in the wall and cell which is the to wall The in turgor water by which stretches the cell wall, Such be a 2013; Moulia, 2013; Cosgrove, 2016b). this a change in within the cell wall, which not appear to be necessary for cell wall cell wall without of or other of wall (Cosgrove, 2016a). 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In Arabidopsis hypocotyls of the et al., cellulose microfibrils were more and than in the an that xyloglucan may of microfibrils within a is also possible that these changes in cellulose organization are a of CWI responses to wall defects. As the of a direct cellulose within primary cell walls has have also as of wall enlargement. The has been most for pollen tubes and Geitmann, Rojas et al., 2011; Sanati Nezhad et al., where wall structure, and for the 2016). In Arabidopsis cell walls, results from interactions et al., 2012, 2015). This is such interactions are not in in et al., recent of cell wall properties along the growth gradient of the Arabidopsis stem (Phyo et al., 2017). in the and of the stem are more more more and more with in the growing and of the stem. these structural changes in pectin properties influence cell wall mechanics, and growth In a different of pectin was in a that characterized a with linked pectin and et al., 2013). 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These to of and in which wall stress, mechanics, and the cytoskeleton et al., 2012; et al., 2013; Sampathkumar et al., concepts in the of to understand the of dynamics et al., can also be used to cell wall at to cellulose microfibrils and revealing cellulose organization in et al., 2014). that the to microfibrils in the complex and of cell microscopy limited about microfibril organization in cell walls, with the exception of the et al., which has been by has for microfibril and 2014; et al., but to the wall be which wall may or as the water is In can be of walls in a The surface can be with and mechanically to an to surface showing the of cellulose microfibrils and matrix on the surface of the periclinal wall of force showing microfibril and organization of from Zhang et of in microfibrils are to of of microfibril details are by stiff on a which and a which to The area in regions of stiff matrix with matrix regions are they are on a and a deformation of matrix between microfibrils are in this from Zhang et (2016). of wall in a the of to of the same wall upon of stretching the wall in the by the The of that microfibrils have been by the they are as are matrix to a from Zhang et We used to cellulose microfibril organization of the wall of scales leaf of the from scales have been used in to mechanics, growth, and cellulose et al., and et al., 2009; et al., In were with We an to in which the wall from the of the This the surface for by and a for mechanical of walls rather than a of cells with complex the walls in many plant Arabidopsis hypocotyls et al., 2011), the wall in is or more depending on which is and has a In microfibrils in the surface are to form a with microfibrils and of regions where or more microfibrils are and in close contact (Fig. and Zhang et al., 2016). are in a in and microfibril by to between a wall of highly anisotropic in a of The result of this structure is that the wall has much structural than the that up the wall. with and deformation we can microfibrils in the context of and regions of the matrix (Fig. and Moreover, by stretching the wall, we can microfibril motions and which components more to an that they some of the tensile force (Fig. and This has for the patterns of microfibril separation during cell growth. is that cell walls expand in the at to the of cellulose et al., structural beyond the of this review et al., 2010; and Jensen, 2013). 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As and have been applied to cell growth, it has that our of the growing cell wall to have rather different than in of the growing cell wall of the for the role of in wall structure and are yet and the responses by CWI sensors the of cell wall the of the of cellulose organization in the growing wall for the molecular of diffuse growth. that of pectin and cellulose are for of these