Studies on the kinetics of auxin-induced growth

Abstract

The auxin-induced growth reaction of the Avena coleoptile has been treated by methods of classical enzyme kinetics. The kinetic treatment makes it possible to characterize the growth promoting activity of an auxin by two parameters, K[subscript s] and V[subscript max]. These express respectively the affinity of the auxin for the auxin-receptive site within the plant and the ability of the complex thus established to promote growth. Treatment of Avena section growth by the methods of enzyme kinetics has made it possible to determine rigorously whether or not inhibitors of auxin-induced growth are true antiauxins and act by competing with auxin for the auxin-receptive site within the plant. Certain auxin-inactive compounds have been shown to possess antiauxin activity. Among such substances are 2,4-dichloroanisole, 4,-chloro- and 2,4-dichlophenoxyisobutyric acids, and 2,6-dichloro-and 2,4,6-trichlorophenoxyacetic acids. Each of these substances can be considered as derived from an active auxin (2,4-dichlorophenoxyacetic acid) by elimination of one of the structural features essential to auxin activity and thereby capable of combining at one point of the auxin-receptive site but incapable of consumating the two-point attachment requisite for auxin activity. It is shown that chemically different auxins compete with one another for the same receptive sites within the plant. Auxins of low v[subscript max] are capable of inhibiting or augmenting the activity of auxins of greater V[subscript max]. Whether inhibition or promotion result depend on the concentrations of the two substances and the differential in V[subscript max] exhibits apparent antiauxin activity. This activity is shown to be different from competitive inhibition of true antiauxins. The relationship between Avena section growth rate and auxin concentration has been demonstrated to be predictable on the basis of a requirement for two-point attachment of the auxin molecule to some receptive entity within the plant. The growth inhibition resulting from high auxin concentrations is not alleviated by antiauxins but rather the auxin inhibition is augmented by the presence of sufficiently high antiauxin concentrations. A necessary corollary to the single point attachment antiauxin concert and the bimolecular complex formation concept for inhibitory auxin concentrations is therefore confirmed. A preliminary investigation concerning herbicidal activity of mixtures of an antiauxin and an auxin on bean plants is presented. The data obtained do not unequivocally establish that inactive bimolecular auxin receptor complex formation at high auxin concentrations is a factor contributing to herbicidal action of 2,4-D, but the possibility that this is in fact so is considered. A cultural technique for obtaining isolated flax root clones is described and data for some experiments with an isolated flax root clone are presented. The inhibitory action of certain antimetabolites on the growth of young tomato plants is described.