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Further investigation of a possible role for abscisic acid in controling the growth effects of indole-3-acetic acid in expanding Arabidopsis leaves?

Indole-3-acetic acid (IAA), a plant morphogenic hormone, controls multiple aspects of plant growth including vascular development and root and leaf initiation. Earlier we have examined how IAA impacts leaf expansion in the model plant, Arabidopsis. Increasing the IAA content of young expanding intact attached, intact detached, and wounded attached leaves resulted in inhibition of growth, while iAa treatment of excised leaf strips or wounded detached leaves stimulated rather than inhibited growth. (1) These results suggested that leaf tissue must be both wounded and detached from the plant for IAA to induce increased growth.

The current study addresses the requirement for detachment from the plant for IAA-induced growth. Implied is that an entity, continuously supplied by the rest of the plant, somehow interacts with IAA or auxin signal transduction in leaf blades of the intact plant to inhibit growth and preventing IAA-induced growth. A chemical signal seems most likely. Given the root to shoot movement of xylem transport, the chemical signal would seem most likely to originate from the root. Several candidate root derived chemical signals are known but the isoprenoid compound abscisic acid (ABA; 5-(1-Hydroxy-2,6,6-trimethyl-4-oxo-2-cyclohexen-1-yl)-3methyl-2,4-pentanedienoic acid) seems a particularly strong candidate as it is a known plant growth inhibitor. At this venue, we have already reported that we have found ABA treatment to be a potent inhibitor of leaf growth in Arabidopsis and that ABA and IAA treated leaf strips grew less than IAA treated strips while ABA and IAA treated strips grew more than strips treated with ABA alone.

As for previous experiments for this ongoing project, seedlings of Arabidopsis were grown in moist potting soil in a growth chamber at 19[degrees]C, with continuous illumination. After 10-14 days, plants were selected with both the first two true leaves 2.8-3.2 mm in diameter and rapidly expanding. One of these first two leaves from each plant served as the experimental leaf and the other leaf served as a paired control. For most experiments, scaled digital images of the individual leaves were prepared for subsequent determination of initial leaf area; for others initial images of excised leaf strips (0.7 mm wide cut transversely across the midpoint of the leaves) were prepared. Either leaf strips or intact attached or intact detached leaves were treated 24 hours with full strength Murashige and Skoog media (with 10 mM KCl and 0.1 mM Mes/Btp (pH 6.0)) or the same +/- 50 [micro]M IAA and/or 10 [micro]M ABA. After 24 hours increased area of leaves and strips was determined from digital images. The effect of IAA treatment on endogenous leaf ABA content in intact attached and in excised strips was determined using a new high-sensitivity high-throughput assay.

Here we report first that ABA was found to inhibit leaf strip expansion of control and of IAA treated tissue. Mutant plants (CS5736: Arabidopsis Biological Resource Center), deficient in ABA synthesis, produced leaves with more robust growth detached from the plant which were not growth-inhibited by IAA. Attached mutant leaves also were not growth inhibited by IAA. We are also testing the effect of abamineSG (a new potent ABA synthesis inhibitor) on the growth of IAA treated detached leaves. An initial pilot study (n=24) found a statistically insignificant trend towards IAA-induced leaf growth in the presence of 30 [micro]M abamineSG. These results are consistent with an hypothesis that IAA treatment of Arabidopsis leaves induces ABA synthesis in roots and intact leaf tissue leading to slower growth. Conflicting with the results described above, however, so-far limited assays of endogenous leaf tissue ABA content found ABA levels not to differ between IAA-treated and untreated intact detached leaves or strips.

This project is supported by grants from the National Center for Research Resources (P20RR0I647I) and the National Institute of General Medical Sciences (P20 GM103442) from the National Institutes of Health.

(1) Keller CP, Grundstad ML, Evanoff M, Keith JD, Lentz DS, Wagner SL, Culler AH, Cohen JD (2011) Plant Signaling & Behavior 6: 1997-2007

(2) Mark Wagner SL, Roise AM, Keller CP (2011) Proc. North Dakota Acad. Sci. 65: 20

Jakob R. Zerr [1] *, Samuel L. Wagner [1], Amanda M. Roise [1], Jo Heuschele [2], Jerry D. Cohen [2] and Christopher P. Keller [1]

[1] Department of Biology, Minot State University, Minot, ND 58707; [2] Department of Horticultural Science, University of Minnesota, St. Paul, MN 55108
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Author:Zerr, Jakob R.; Wagner, Samuel L.; Roise, Amanda M.; Heuschele, Jo; Cohen, Jerry D.; Keller, Christo
Publication:Proceedings of the North Dakota Academy of Science
Date:Apr 1, 2013
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