Nanoscale movements of cellulose microfibrils in primary cell walls

Abstract

The growing plant cell wall is commonly considered a fiber-reinforced structure whose strength, extensibility and anisotropy depend on the orientation of crystalline cellulose microfibrils, their bonding to the polysaccharide matrix, and matrix viscoelasticity1–4. Structural reinforcement of the wall by stiff cellulose microfibrils is central to contemporary models of plant growth, mechanics, and meristem dynamics4–12. Although passive microfibril reorientation during wall extension has been inferred from theory and from bulk measurements13–15, nm-scale movements of individual microfibrils have not been directly observed. Here we combined nm-scale imaging of wet cell walls by atomic force microscopy (AFM) with a stretching device and endoglucanase treatment that induces wall stress relaxation and creep, mimicking wall behaviors during cell growth. Microfibril movements during forced mechanical extensions differ from those during creep of the enzymatically-loosened wall. In addition to passive angular reorientation, we observed a diverse repertoire of microfibril movements that reveal the spatial scale of molecular connections between microfibrils. Our results show that wall loosening alters microfibril connectivity, enabling microfibril dynamics not seen during mechanical stretch. These insights into microfibril movements and connectivities need to be incorporated into refined models of plant cell wall structure, growth and morphogenesis.

Gradients in Wall Mechanics and Polysaccharides along Growing Inflorescence Stems

“…This idea springs from the high stiffness of cellulose microfibrils but fails to consider how cellulose microfibrils are connected to one another to form a mechanically strong material. A recent study that combined extensometry with atomic force microscopy and different forms of cell wall strain showed that both cellulose microfibrils and matrix polymers, predominantly pectins, bear the tensile load of elastically stretched onion walls (Zhang et al, 2017). This confirms the mechanical significance of the matrix at the molecular scale and adds impetus to recent interest in the state of pectins in growing cell walls and the need for better models of cell wall mechanics (Peaucelle et al, 2012;Xiao et al, 2014;Levesque-Tremblay et al, 2015).…”

Diffuse Growth of Plant Cell Walls

“…These inferences about microfibril movements gain support from a recent study in which the nanoscale movements of cellulose microfibrils were directly monitored by AFM (Zhang et al, 2017). The cell wall was extended in a well-defined series of extensions that included elastic and plastic strains imposed by axial force as well as time-dependent creep induced by treatment with an endoglucanase with wallloosening activity.…”

“…These purely mechanical responses of walls differ in an essential way from the sustained wall expansion that occurs during cell growth (Cosgrove, 2016b;Zhang et al, 2017), which depends on continuous loosening by expansins or other wall-loosening agents (Cosgrove, 2016a). Wall loosening results in wall stress relaxation that drives cell growth.…”

“…The predominance of red shows that microfibrils have been pulled taut by the axial force, indicating they are load bearing, as are matrix components, though to a lesser extent. Images adapted from Zhang et al (2017).…”

Nanometrology of Biomass for Bioenergy: The Role of Atomic Force Microscopy and Spectroscopy in Plant Cell Characterization

“…AM-AFM has been successful in characterizing biomass (Kirby et al, 1996;Salvadori et al, 2014;Farahi et al, 2017) and in particular the architecture of plant cell walls at the molecular level which in turn is essential in improving lignocellulosic feedstock properties (Zhang et al, 2013b(Zhang et al, , 2017Keplinger et al, 2014;Torode et al, 2018). AM-AFM mapping of cellulose topography revealed the right-handed twisted nature of microfibrils and the periodic distribution of glucose and fiber unit along the microfibrils (Hanley et al, 1997).…”