Growth of plant cell walls: from expansin action to wall structure and mechanics

Speaker: Daniel Cosgrove, Penn State

Host: José A. Feijó

Abstract: Plant cell growth entails controlled yielding of the cell wall to turgor-generated wall stresses, but the molecular basis of wall yielding is unsettled. Expansin proteins stimulate yielding (creep) of plant primary cell walls. Their activity turns on and off rapidly under pH control. They lack wall lytic activity and they facilitate pH-dependent loosening of the wall without softening it (that is, they induce wall creep without changing wall stiffness). To understand how they promote wall creep, we need a molecular model of the growing wall, one that is quantitatively accurate, accounts for the transmission of tensile forces in the wall and identifies potential sites of polysaccharide shear. I will summarize current cell wall ‘models’ and our attempts to assess them with enzymatic treatments and mechanical assays, using epidermal walls of onion. We developed a computational model of the epidermal wall based on coarse-grained molecular dynamics and mechanical testing*. The model demonstrates how cellulose microfibrils may interact noncovalently to form a strong scaffold that can withstand high tensile forces generated by cell turgor, yet can undergo plastic deformation by tension-driven sliding of cellulose-cellulose junctions. This ‘sliding scaffold’ model differs in numerous respects from other current concepts of wall structure and provides insights into the molecular bases of cell wall elasticity, plasticity and yield threshold. Working expansin actions into this model remains a challenge for the future.

* Zhang, Y., Yu, J., Wang, X., Durachko, D.M., Zhang, S., and Cosgrove, D.J. (2021). Molecular insights into the complex mechanics of plant epidermal cell walls. Science 372, 706-711. DOI: 10.1126/science.abf2824.

Seminars start at 4:00 pm, and refreshments will be served at 3:45 pm. All seminars are held in the Conference Room (1116) of the Institute for Physical Science and Technology (IPST) Building unless otherwise noted.