ROLE OF EXPANSIN IN CELL ENLARGEMENT OF OAT COLEOPTILES - ANALYSIS OFDEVELOPMENTAL GRADIENTS AND PHOTOCONTROL

Expansins are wall proteins that mediate a type of acid-induced extens ion in isolated plant cell walls (S. McQueen-Mason, D.M. Durachko, D.J . Cosgrove [1992] Plant Cell 4:1425-1433). To assess the role of these proteins in the process of cell enlargement in living tissues, we com pared the spatial and temporal growth patterns of oat (Avena sativa L. ) coleoptiles with four wall properties related to expansin action. Th ese properties were (a) the ability of isolated walls and living segme nts to extend in acidic buffer, (b) the ability of heat-inactivated wa lls to extend upon application of expansins, (c) the amount of immunol ogically detectable expansin in wall protein extracts, and (d) the ext ractable expansin activity of walls. Growth rate was maximal in the ap ical half of dark-grown coleoptiles and negligible in the basal region . This growth pattern correlated with properties a and b; in contrast, the amount and activity of extractable expansin (properties c and d) were reduced only in the most basal region. Upon exposure to white lig ht, coleoptiles abruptly ceased elongation at 8 to 10 h after start of irradiation, and this cessation correlated with reductions in propert ies a to c. The growth cessation at 8 to 10 h also coincided with the loss of growth response to exogenous auxin and fusicoccin in excised c oleoptile segments. These results lend correlative support to the hypo thesis that expansin action is important for growth responses of livin g oat coleoptiles (e.g. responses to acidic buffers, auxin, fusicoccin , aging, and light). our results suggest that changes in the susceptib ility of the wall to expansin action, rather than changes in expansin activity, may be a key determinant of the growth patterns in oat coleo ptiles.