Characterization of the IPA-N3 auxin activity. (a,b) Comparison of the phenotypes exerted by various auxin-related compounds. (a) Seedlings grown for 48 h in the liquid culture supplemented with various compounds (5 μM concentration). Application of IAA, IPA and IPA-N3, but not L-tryptophan induced identical phenotypes. Note the epinastic cotyledons, enhanced hypocotyl but inhibited root elongation. These phenotypes were less pronounced in the case of the precursor of IBA and synthetic auxin NAA. The non-transportable synthetic auxin 2,4-D caused severe retardation of growth, and the auxin transport inhibitor NPA showed dissimilar phenotypes – e.g. cotyledons did not open. (b) Quantification of the root and hypocotyl length of above seedlings. (c) Sensitivity test shows similar activity range for the four related auxinic compounds tested. The mean values of at least 12 measurements ± SE. The difference in root length between mock and 10 nM IPA-N3 is statistically significant. *p < 0.01 (Student’s t-test). (d) Morphology of root tips of seedlings from (b). Note the root tip swelling in the seedlings grown in the three auxinic compounds indicated. (e–g) Two assays indicating that IPA-N3 exhibits morphogenic-like activity. (e,f) IPA-N3 is able to induce ectopic lateral root initiation. (e) Depiction of lateral root initiation in mock, IPA-N3 and IAA treated plants (5 µM, 48 h) (f) Quantification of the effect. Number of lateral root initiates from stage IV. The mean values from at least 9 roots ± SE. *p < 0.01 (Student’s t-test). (g) IPA-N3 triggers organ formation on the pin1 shoot meristem. The position of the application of the lanolin paste is indicated by a closed arrowhead and the outgrowth of an organ by an open arrowhead. The experiment was repeated twice on at least 10 plants for each treatment. The number of the positive cases of outhgrowth from all samples in total are indicated. Scale bars = 5 mm (a), 50 μm (d,e), 1 mm (g).