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Development and Growth Regulation

Regulation of Indole-3-Acetic Acid Biosynthetic Pathways in Carrot Cell Cultures ¹

Department of Botany, University of Maryland, College Park, Maryland 20742, Horticultural Crops Quality Laboratory, Agricultural Research Service, United States Department of Agriculture, Beltsville Agricultural Research Center, Beltsville, Maryland 20705

2,4-Dichlorophenoxyacetic acid (2,4-D) promotes the accumulation of tryptophan-derived indole-3-acetic acid (IAA) in carrot cell cultures during callus proliferation by a biosynthetic pathway that is apparently not active during somatic embryo formation. The effects of 2,4-D were examined by measuring the isotopic enrichment of IAA due to the incorporation of stable isotope-labeled precursors (deuterium oxide, [¹⁵N]indole, and ²H₅-L-tryptophan). Enrichment of IAA from deuterium oxide is similar in both cultured hypocotyls and cell suspension cultures in the presence and absence of 2,4-D, despite the large differences in absolute IAA concentrations. The enrichment of IAA due to the incorporation of [¹⁵N]indole is also similar in callus proliferating in the presence of 2,4-D and in embryos developing in the absence of 2,4-D. The incorporation of ²H₅-L-tryptophan into IAA, however, is at least 7-fold higher in carrot callus cultures proliferating in the presence of 2,4-D than in embryos developing in the absence of 2,4-D. Other experiments demonstrated that this differential incorporation of ²H₅-L-tryptophan into IAA does not result from differential tryptophan uptake or its subsequent compartmentation. Thus, it appears that differential pathways for IAA synthesis operate in callus cultures and in developing embryos, which may suggest that a relationship exists between the route of IAA biosynthesis and development.

¹ These studies were supported by U.S. Department of Agriculture competitive research grant 89-37261-4791 (to T.J.C.), National Research Initiative Plant Growth and Development grant 91-03079 (to J.D.C.), National Science Foundation grants ECE-85-52670 (to T.J.C.) and DCB-8917378 (to J.D.C and T.J.C.), and the U.S.-Israel Binational Research and Development fund US-1362-87. This work was carried out, in part, under cooperative agreement No. 58-32U4-8-34 of the U.S. Department of Agriculture-Agricultural Research Service and the University of Maryland. Scientific article No. A6292, contribution No. 8461 of the Maryland Agricultural Experiment Station.

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