Hendrickson Culler, Angela , Cohen, Jerry D. .
Novel Pathway for IAA Biosynthesis in Maize Endosperm.
How plants make the hormone indole-3-acetic acid (IAA) has yet to be fully solved. We have approached this question using maize endosperm where we are able to isolate and partially purify tryptophan-dependent IAA biosynthesis in vitro. Using [2H5] and [13C1115N2] tryptophan along with [13C6]IAA as an internal standard, we were able to quantify the amount of tryptophan converted to IAA, which occurred at a rate of 140 pmol/min. We then synthesized a series of intermediates in IAA biosynthesis using stable-labeled precursors that represented all major proposed pathways, and tested their involvement in IAA biosynthesis by asking three questions: 1) is the intermediate converted to IAA, 2) can the intermediate interfere in tryptophan conversion to IAA, and 3) is the intermediate endogenous to endosperm? We were able to measure and distinguish both IAA produced from precursors and IAA from tryptophan on the GC-MS. None of the proposed precursors proved to be good candidates for intermediates in IAA biosynthesis in this system. Indole-3-acetonitrile (IAN) completely inhibited IAA biosynthesis, but we did not find any endogenous IAN in the endosperm. [3H1] and [14C1] tryptophan supplied to the endosperm enzyme system allowed us to isolate a small membrane protein that covalently bound tryptophan and released IAA. Tryptophan was linked via a thioester and when hydrolyzed, released racemic tryptophan. The tryptophan-protein complex was an effective substrate for IAA biosynthesis. The small size of the protein (between 8-16 kDa) is quite unique from other systems studied, and sequence data should answer more questions about the mechanism of this system for IAA biosynthesis.
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1 - University of Minnesota, Horticultural Sciences, 1970 Folwell Ave., Saint Paul, MN, 55108, USA
2 - University of Minnesota, Horticultural Sciences
Presentation Type: ASPB Minisymposium
Location: Continental B/Hilton
Date: Tuesday, July 10th, 2007
Time: 4:35 PM