Determination of femtomol quantities of gibberellic acid by radioimmunoassay.

A highly sensitive and specific radioimmunoassay which allows the detection of as little as 5 fmol (2 pg) of gibberellic acid (GA3) in crude plant extracts is described. Antisera of high affinity and titer were obtained by immunizing rabbits with a conjugate of carboxyl-coupled GA3 and bovine serum albumin. [(125)I]Gibberellic acid-[N-(p-hydroxybenzyl) putrescine]amide of high specific activity, used as the immunotracer, is readily displaced by gibberellic acid methyl ester but not by free gibberellic acid. Thus, methylation of extracts prior to analysis is required. The assay is very specific; besides GA3, only the closely related GA7 is highly immunoreactive. Various gibberellins, related compounds, as well as other classes of plant hormones do not interfere with the assay. Levels of immunoreactive gibberellins (GA3, GA7) in actively growing tissues, among them cell suspension cultures of 33 different species, were determined.