Role of growth regulators in the bean hypocotyl hook opening response

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Abstract
The opening of the hypocotyl hook in bean seedlings is due to a rapid elongation of cells on the inner side of the hook elbow. Red light promotes hook opening by inducing this cell elongation. Opening is inhibited by low concentrations of indoleacetic acid (IAA) and 2,4-dichlorophenoxyacetic acid (2,4-D), and higher concentrations of these auxins cause a closure of the hook. In darkness, opening is induced slightly by p-chlorophenoxyisobutyric acid (PCIB), whereas in red light this auxin antagonist promotes opening only when IAA is added simultaneously to inhibit opening. The amount of diffusible auxin released by the hook tissue is not affected by red illumination that is sufficient to induce maximal hook opening. Gibberellic acid (GA) promotes the hook opening. The magnitude of its effect is, however, rather small, especially in darkness. (2-Chloroethyl)-trimethylammonium chloride (CCC) and 2′-isopropyl-4′-(trimethylammonium-chloride)-5′-methylphenyl piperidine-1-carboxylate (Amo-1618) inhibit hook opening in red light, and this inhibition is completely overcome by addition of GA. Cytokinins and abscisic acid at rather high concentrations inhibit hook opening in light but produce no significant effect in darkness. Hook opening is promoted by Ca++ and K+, and notably by Co++ and Ni++. It is concluded that 1. endogenous gibberellin assists in hook opening, but light does not act by changing the gibberellin level; 2. light does not act by decreasing the endogenous auxin level; and 3. cytokinins or abscisic acid do not seem to have a special role in the response.