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Actinoiiiycini D, an inllibitor of RNA synthesis (10, 12), will prevent auxin-induced cell elongation (4. 8. 12, 16). It has been assumed that it acts by inhibiting the action of auxin (8, 12, 16). If this is correct, it would indicate that RNA svnthesis is required for auxin action. Auxin-induced elongation is a complex process which requires a number of factors (5). In Avena coleoptile tissues the factor which normally controls elongation is the extensibility of the cell wall (2, 7, 21). Addition of auxin to a tissue causes a loosening of the cell wall which, in turn, results in rapid elongation. Any inhibitor that blocks the auxin-induced wall loosening will inhibit elongation. But a compound that inhibits elongation does not necessarily have to have any effect on wall loosening. Mannitol, for example, inhibits rapid elongation but does not affect wall loosening (2). Since other auxin-insensitive factors are needed for growth, inhibition of any of these will also lead to an inhibition of elongation. Thus a compound could inhibit elongation either by blocking the auxin-induced wall loosening or by affecting some other factor. In the past, it has not been l)ossil)le to separate these 2 possibilities. Recently Olsen et al. (18) have shown that it is l)ossible to directly measure the extensibility of the wall by using a modification of the technique of Heyn (9). At the end of the incubation period, the bulk of the protoplasm is removed from the cells with boiling methanol followed by Pronase. Then an Instron stress-strain analyzer is used to measure the load which develops across the walls as the walls are subjected to a constant rate of strain. The strain per unit stress (WEx) is determined from the resulting load-extension curve. It should be noted that WEx is a measure of the combined elastic and plastic extensibility of the walls. Using this technique, Olson et al. (18) have shown that auxin increases the extensibility of Avena coleoptile sections. This increase occurs even when elongation is osmotically inhibited (17). This technique can be used to study the effects of inhibitors on auxin-induced wall extensibility. This investigation was undertaken in order to deter-

Auxin-induced cell wall loosening in the presence of actinomycin D.