The Physiology of Flowering Plants - KHAM PHA MOI

CELL EXPANSION: MECHANISM AND CONTROL 217a member of a family of regulatory proteins called 14-3-3 proteins.This complexes with the H + ATPase to stimulate activity, althoughthe effect on growth is not as long-lived as that observed with auxin.Excised plant organs elongate more rapidly, and exhibit increasedcell wall plasticity, when placed into buffers of low pH (Fig. 8.10).The acid growth is not a direct consequence of low pH on the cellwall polysaccharides but rather is mediated by a series of proteinscalled expansins. If a section of cucumber hypocotyl is excised,frozen, and then thawed, the cells within it are killed but the cellwall still retains active proteins. An abraded section (to remove theimpermeable cuticle) of the hypocotyl is clamped into an extensometer,in which extension in response to a small weight is measuredusing an electronic position transducer (Fig. 8.11). Changing the pHof the buffer surrounding the section from pH 6.8 to 4.5 results in anincrease in extension although, again, this is not as long-lived as thatcaused by auxin, presumably as auxin-induced alterations in geneexpression are not induced by low pH. If the enzymes in the hypocotylare denatured by a brief heat treatment this acid-induced growthis abolished. However, it can be restored if proteins extracted fromthe walls of non-heat-treated hypocotyls are added. This bioassaywas used to identify two proteins necessary for this response, whichare called expansins (McQueen-Mason et al. 1992).Plants contain many different expansins whose roles are unclear.However, they are likely to be involved in numerous aspects of plantLength (mm)ABCutFreeze, thawand abradepH 6.8 pH 4.5 pH 4.50.6 0.6Length (mm)Not heat-treated0.4 0.4weight0.2 0.2Heat-treatedHeat-treatedElectronic positiontransducerAdd wall proteins00 1 200 1 2Time (h)Time (h)CbufferFig: 8:11 The extensometer andits use for the bioassay of expansins.(A) Sections of cucumber hypocotylare excised and killed by freezing.They are then thawed and thesurface abraded to allow solutionsto enter the section. The section isplaced in a buffer and a small weightattached. Increases in the length ofthe section are measured using anelectronic position transducer. (B)Transferring sections from a bufferof near-neutral pH to one of acidicpH results in cell extension. Thisresponse is abolished in heattreatedsections as key proteinshave been inactivated bydenaturation. (C) acid growth canbe restored to heat-treatedhypocotyl sections by the additionof isolated cell wall proteins.Redrawn from Cosgrove (1998).