Quantitative structure-activity relationships for N-(indol-3-ylacetyl)amino acids used as sources of auxin in plant tissue culture

Abstract

The methods of quantitative structure-activity relationship (QSAR) analysis were applied to two sets of N-(indol-3-ylacetyl)amino acids tested in vitro as promoters of unorganized callus growth in cultures derived from Solanum nigrum L. and from soybean (Glycine max L.). Two types of statistical models were developed describing molecular structure: (a) by molecular connectivity indices or (b) by experimentally determined biochemical constants. The parameter structure of those models indicated that the mode of action of the indole-3-acetic acid (IAA) conjugates is similar in both plant systems. Growth-promoting activities decreased with molecular size and with increasing involvement of the amino acid C-α and C-β in branched or cyclic structures. This was more clearly visible when molecular connectivity indices were employed as descriptor variables. The biochemical constants with most accurately describe the auxin-like effect of N-(IAA)amino acids are lipophilicity and resistance to degradation by peroxidase. This suggests that biological activity mostly depends on uptake and metabolic stability. The latter includes hydrolysis to free IAA, but the models do not support the popular hypothesis that this is the only ratedetermining step.