Daniel Cosgrove - Expansin action and the biomechanics of growing cell walls at the nanoscale

Expansins are wall-loosening proteins that characteristically induce irreversible extension of primary cell walls and are implicated in many developmental processes that involve changes in cell wall properties. We now divide expansins into three groups: (1) Alpha-expansins were discovered as mediators of 'acid growth" and are linked to stimulation of cell enlargement by auxin and other stimuli. Although they induce cell wall creep (slow, irreversible wall extension), they do not change wall stiffness and and lack evidence of enzymatic modification of wall polysaccharides. (2) Beta-expansins are likewise implicated in growth, but a subset has evolved a specialized function in grass pollen. We have studied the maize pollen protein, ZmEXPB1 in great detail. It promotes pollen penetration of the stigma and style by targeting arabinoxylan and greatly weakening the middle lamella. ( 3) Expansins are also found in many bacteria and fungi that colonize or infect plant tissues. We have carried out extensive structure-function studies of the expansin from Bacillus subtilis. The action of expansins on cell wall mechanics and rheology challenges long-standing models of cell wall structure and calls for more a more critical exploration of wall structure in combination with nano-scale mechanics.