EFFECT OF XYLOGLUCAN MONOSACCHARIDE ON CELL ELONGATION INDUCED BY 2,4-DICHLOROPHENOXYACETIC ACID AND INDOLE-3-ACETIC-ACID

Xyloglucan nonasaccharide (XG9) is recognized as an inhibitor of 2,4-D-induced long-term growth of segments of pea stems. In the presence of 10(-5) M 2,4-D, inhibition by 10(-9) M XG9 of elongation of third internode segments of pea seedlings was detected within 2 h after the start of incubation, in some experiments. Analysis by double-reciprocal (Lineweaver-Burk) plots of elongation in the presence of various concentrations of 2,4-D, with or without XG9, gave parallel lines, indicating that XG9 inhibited 2,4-D-induced elongation in an uncompetitive manner. XG9 did not influence the 2,4-D-induced cell wall loosening. Thus, XG9 does not fulfill the proposed definition of an "antiauxin". XG9 at 10(-11) to 10(-6) M did not influence IAA-induced elongation of segments from pea third internodes, azuki bean epicotyls, cucumber hypocotyls, or oat coleoptiles. Inhibition of IAA-induced elongation by XG9 was not observed even when the segments from pea or azuki bean were abraded. Furthermore, fucosyl-lactose at 10(-11) to 10(-4) M did not affect the IAA-induced elongation of segments of pea internodes or of azuki bean epicotyls. XG9 may be incapable of inhibiting the IAA-induced cell elongation (especially in oat) or, alternatively, the endogenous levels of XG9 may be so high that exogenously applied XG9 has no inhibitory effect on IAA-induced elongation.