THE UPTAKE AND METABOLISM OF BRASSINOSTEROID BY TOMATO (LYCOPERSICON-ESCULENTUM) PLANTS

When brassinosteroids (BR) are applied to the roots of hydroponically grown tomato plants there is an accumulation of 1-aminocyclopropane-1-carboxylic acid (ACC) in the leaves. In this work, BR-induced ethylene production in tomato plants was studied further using (HBR)-H-3. Twenty day old tomato plants were incubated in 25 mL Hoagland's solution plus 1.1-mu-M BR and 3.7 x 10(6) cpm of (HBR)-H-3. During incubation the tomato plants accumulated ACC and extractable radioactivity. The 24-h plant extracts contained 2695 nmol kg-1 (fresh mass basis) of ACC compared with 73 nmol kg-1 in the control. The 24-h treated plant extracts also contained over 2.12 x 10(6) cpm kg-1 of tritium. Migration of the radioactivity on silica gel TLC plates showed there was no apparent metabolism of BR during the first four hours; however, by 8 h there were two radioactive spots on the plate. The 12-h extract contained 3 radioactive spots, the original BR plus two metabolic products. When plants were incubated with BR for 24 h and transferred to BR-free Hoagland's solution, the ACC content of the tissues decreased and there was an increase in the amount of tritium migrating as BR metabolites. The plant apparently metabolizes BR to inactive forms, and the overproduction of ethylene ceases. The identities of these metabolic products are unknown at this time.