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 darkness. After irradiation with continuous white light (WL) the apical hook unfolded. WL-induced hook opening was due to an enhancement of cell elongation on 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 the wall-associated form was unaffected. Detailed time-course studies revealed that the activity of the soluble acid invertase increased within 2 h of WL-treatment. Concomitantly, the level of sucrose declined and that of glucose 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 events that leads to the unfolding of the apical hook in developing sunflower seedlings.