Sucrose metabolism during Agrobacterium tumefaciens — induced tumor growth in sunflower hypocotyls

Tumor growth induced by the wild-type strain B6 of Agrobacterium tumefaciens in hypocotyls of sunflower seedlings (Helianthus annuus L.) was investigated with respect to the catabolism of imported sucrose. Tumor development was associated with a large increase in dry mass of the infected stem region and a cessation of hypocotyl growth. In non-infected control seedlings the hypocotyls continued to elongate. Sucrolysis in the growing tumor tissue was analyzed. The enzyme soluble (vacuolar) acid invertase (EC 3.2.1.26) was found to represent the dominant sucrose breakdown activity, whereas the specific catalytic activity of sucrose synthase (EC. 2.4.1.13) was very low. During tumor growth the level of acid invertase displayed a large increase. In the control, a decrease in the activity of this enzyme was measured. The time-course changes in the concentrations of sucrose, hexoses (glucose and fructose) and the osmolality of the tissue sap were determined during tumor development. The data indicate that the growing tumor cells import and cleave large amounts of sucrose, i.e., they represent a strong sink. As a result, in the infected sunflower seedlings, sucrose may no longer be available in sufficient amounts for the maintenance of stem elongation.