N-(INDOL-3-YLACETYL)AMINO ACIDS AS SOURCES OF AUXIN IN PLANT-TISSUE CULTURE

N-(Indol-3-ylacetyl) derivatives (IAA conjugates) of aliphatic amino acids with a two- to six-carbon backbone including alpha-L-amino acids, omega-amino acids, and the alpha,omega-diamino acids ornithine and lysine were prepared, chemically characterized, and tested as sources of auxin in plant tissue culture. Stimulation of unorganized growth in Solanum nigrum L. callus and callus induction and developmental effects in tomato (Lycopersicon esculentum Mill. cv. Marglobe) hypocotyl explants were studied systematically. Relative auxin activities were estimated by comparing physiologically equivalent concentrations, in the optimal and suboptimal range, of the individual IAA conjugates. While the growth-promoting properties of some of the conjugates were species-dependent, those containing straight-chain two- to four-carbon alpha-L-amino acid moieties were generally up to 100 times more active than those of their five- to six-carbon homologues. Branching of the amino acid backbone at C-beta (norvaline vs. valine and norleucine vs. isoleucine) and C-gamma (norleucine vs. leucine) had a minor effect, but substitution of H-alpha by a methyl group (alpha-amino-L-butyric vs. alpha-aminoisobutyric acids) almost completely blocked growth-promoting activity. IAA conjugates of omega-amino acids were, in most cases, nearly as active as those of their alpha-amino-L-isomers. Among the conjugates of alpha,omega-diamino acids N(delta)-(IAA) ornithine was less active than N(epsilon)-(IAA)lysine. The activity of N(alpha)-(IAA)lysine was less than for the epsilon-(IAA) isomer, and that of N(alpha),N(epsilon)-(IAA)2-lysine was different in tomato and Solanum nigrum. The L-alanine and epsilon-lysine conjugates were also found to be useful for induction and development of Oenothera leaf callus and in tomato cell-suspension culture, two systems which require highly active sources of auxin.