Auxins

Auxin's discovery traces back to Charles Darwin's 1880 observation that Canary grass coleoptiles bend unilaterally toward light; the substance responsible was not named until F. W. Went isolated it from oat (Avena) coleoptiles in 1926 and coined the term 'auxin' (Greek for 'to grow'). Kogl and Haugen Smith later isolated a related substance from human urine in 1931 (Auxin A), and a similar one from corn grain oil in 1934 (Auxin B), before Kogl's group chemically identified the plant's own version as Indole Acetic Acid (IAA) - still the principal natural auxin. Auxin is generally produced at the growing tips of stems and roots and migrates from there to its site of action (Figure 15.11). Natural auxins found in plants include IAA, Indole Propionic Acid, Indole Butyric Acid and Phenyl Acetic Acid, while synthetic auxin-mimics such as 2,4-D, 2,4,5-T and NAA are manufactured artificially; anti-auxin compounds like TIBA do the opposite, blocking auxin's effect when applied. Auxin exists in a freely diffusible ('free') form like IAA and a non-diffusible ('bound') form like IAA-Aspartic acid, and its amino-acid precursor is tryptophan, whose conversion needs zinc. Structurally, natural auxin closely resembles IAA, and it is transported polarly - basipetally through the phloem (shoot to root) and acropetally through the xylem (root to shoot). Its activity is classically measured using the Avena curvature (Went) bioassay (Figure 15.12), where a coleoptile bends toward the side carrying an auxin-soaked agar block. Physiologically, auxin promotes cell elongation in the stem and coleoptile, suppresses lateral bud growth from the apical bud (apical dominance), prevents abscission, drives cambial cell division important for secondary growth and tissue culture, stimulates respiration and vascular d …