Rapid light-induced enhancement of sucrose catabolism in the apical hook of sunflower hypocotyls

Sunflower seedlings (Helianthus annuus L.) were grown for 3 days in darknes s. After irradiation with continuous white light (WL) the apical hook unfol ded. WL-induced hook opening was due to an enhancement of cell elongation o n the inner side of the curved organ. Two types of acid invertase (EC 3.2.1 .26) were detected in the apical hook: a soluble and a cell-wall-bound form . The activity of the soluble enzyme was enhanced by WL, whereas that of th e wall-associated form was unaffected. Detailed time-course studies reveale d that the activity of the soluble acid invertase increased within 2 h of W L-treatment. Concomitantly, the level of sucrose declined and that of gluco se and fructose increased. WL-induced sucrolysis was activated 2 h before a change in growth could be detected (hook opening, differential elongation of the epidermal cells). We conclude that the light-dependent activation of sucrose catabolism via soluble acid invertase represents one of the key ev ents that leads to the unfolding of the apical hook in developing sunflower seedlings.