N-(indol-3-ylacetyl)amino acids as sources of auxin in plant tissue culture

Abstract

N-(Indol-3-ylacetyl) derivatives (IAA conjugates) of aliphatic amino acids with a two- to six-carbon backbone including α-l-amino acids, (ω-amino acids, and the α,ω-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 α-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-β (norvaline vs. valine and norleucine vs. isoleucine) and C-γ (norleucine vs. leucine) had a minor effect, but substitution of H-α by a methyl group (α-amino-l-butyric vs. α-aminoisobutyric acids) almost completely blocked growth-promoting activity. IAA conjugates of ω-amino acids were, in most cases, nearly as active as those of their α-amino-l-isomers. Among the conjugates of α,ω-diamino acids N _δ-(IAA) ornithine was less active than N _ε-(IAA)lysine. The activity of N _α-(IAA)lysine was less than for the ε-(IAA) isomer, and that of N _α,N _ε-(IAA)₂-lysine was different in tomato and Solanum nigrum. The l-alanine and ε-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.