Jerry Cohen: Auxin Biochemistry and Molecular Biology Laboratory

Dr. Jerry Cohen's laboratory is a part of the Department of Horticultural Science situated in Alderman Hall on the St. Paul campus of the University of Minnesota, Twin Cities. Work in our laboratory is supported by grants from the National Science Foundation and the U.S. Department of Energy and from funds from the Gordon & Margaret Bailey Endowment for environmental horticulture.

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Arabidopsis thaliana is a small flowering plant that is widely used by plant science researchers as a model organism to study many aspects of plant biology. It is a member of the Brassica family, which includes species such as cabbage and radish. Arabidopsis is not of major agronomic significance, but it does have several important advantages for the researchers in many areas of plant biology - especially genetics and molecular biology

(Image: Heldt, 1996)

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Auxins are a class of hormones that promote the lengthwise growth of plants. They have a role in other cellular activities as well, such as cell division and differentiation, fruit development, root formation from cuttings, and in abscission. The key auxin, indole-3-acetic acid (IAA), has been implicated in the regulation of virtually every aspect of plant development, from embryogenesis through senescence. It is synthesized in meristems and young organs, transported through the plant by a dedicated transport system and ultimately produces a specific cellular response. Since the hormone functions in all higher plant species, a thorough understanding of auxin homeostasis and response will impact all aspects of agriculture. Qualities such as stature, shoot and root architecture, shoot strength, seed and fruit size, fruit ripening, and the senescence program are all regulated in part by auxin and may be targets for directed engineering.

SOME EXAMPLES OF IAA CONJUGATES

IAA conjugate structures

Most plants examined to date contain the majority (~90%) of their IAA in conjugated forms, with only minute amounts of free IAA. While relatively few IAA conjugates have been identified, IAA has been found to be conjugated to simple sugars, cyclitols, high molecular weight polysaccharides, or the carbohydrate component of glycoproteins via an ester or anhydride linkage. IAA can also be conjugated by amide linkage to single amino acids, peptides or proteins. Conjugates are thought to be involved in a variety of hormonally related processes such as: transport of IAA within the plant, the storage and subsequent reuse of IAA, protection of IAA from enzymatic destruction, as components of a homeostatic mechanism for the control of IAA levels and as an entery route into the subsequent catabolism of IAA. We have recently cloned from bean the first gene for a protein covalently modified by IAA and have identified related proteins in several other species, including Arabidopsis

For over 50 years, plant IAA biosynthesis research has focused on degradation of tryptophan by sequential oxidative deamination and decarboxylation. However, several recent model systems have been used by investigators to show that IAA biosynthesis occurs via two separate pathways--tryptophan-dependent (TD) and tryptophan-independent (TI). Recent results from our lab with bean axes suggest that wounding alters in a transient way the route of IAA biosynthesis, causing a change from synthesis via the TI pathway to synthesis via the TD pathway. Also, both pathways can be used at different times of development, with the TD pathway predominating during early embryogenesis and seed germination, and the TI utilized during late embryogenesis and vegetative growth (Cohen and Slovin, 1999; Normanly, 1997).

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Page last modified on September 18, 2002.

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