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Effect of petiole treatment with the auxin transport inhibitor N-1-naphthylphthalamic acid on leaf blade auxin concenration in the common bean (Phaseolus vulgaris).

Plant growth and development is controlled by several classes of morphogenic hormones. Chief among these are the auxins (from the Greek for "to grow"). The principle naturally occurring auxin is indole acetic acid (IAA) which occurs ubiquitously in all plants. It is synthesized in leaves, especially young leaves. IAA is transported downward in the plant by a complex cell to cell mechanism that is sensitive to inhibition by N-1-naphthylphthalamic acid (NPA). Below the leaves, IAA has a controlling role in diverse aspects of development including stem elongation, tropisms, and generation of lateral roots. The role of IAA in the control of leaf growth was long thought to be limited. I have found, however, a developmentally sensitive electrical response by excised leaf strips from tobacco to the hormone and, more recently, have found that exogenous auxin applied directly to leaves results in the development of smaller leaves. This finding suggests IAA plays an inhibitory role in the development of the leaves in which it is synthesized. I also found that placing a band of lanolin containing NPA around the petiole of leaves also produced significantly smaller leaves.

This last experiment also suggests increased leaf auxin inhibits leaf growth if I assume that the NPA on the petiole, by inhibiting the movement of IAA out of the developing leaf results in an elevation of the level of the endogenous hormone in the treated leaf. That assumption is open to question, however. Others have recently reported that growing Arabidopsis seedlings on agar containing NPA resulted in plants with smaller leaves which were shown to contain lower levels of IAA. The researchers involved speculated that inhibition of auxin transport feeds back through some unknown mechanism to inhibit IAA synthesis. It is also possible in their experiment, however, that the inhibited root development resultant from NPA treatment causes poor leaf development as a consequence of poor nutrition. Poor nutrient status of the plant could then somehow down regulate IAA synthesis.

In July of 2002, I visited the lab of Jerry Cohen in the Department of Horticulture at the University of Minnesota where I measured the IAA content of NPA treated leaves and in the opposite control leaves using an established protocol. The protocol involved amino anion minicolumn and HPLC-based purification of IAA and quantification using gas chromatography-selected ion monitoring mass spectrographic analysis. As in previous experiments Phaseolus vulgaris L. var. Contender were grown under greenhouse conditions for 10-13 days. Plants with both monofoliate leaves having midribs 30-40 mm were selected for experimentation. The petiole of one monofoliate leaf on each plant was treated with 1.0 % NPA. One, 3, and 6 days following petiole treatment, treated and untreated leaves were harvested, weighed, frozen to -80[degrees], and transported to Minnesota on dry ice. Individual leaves were then analyzed for IAA content.

I found that the IAA content was consistently higher in the NPA treated leaf on all three days, for example, averaging 53.2 ng IAA / g fr. wt. in the treated leaves compared to 29.3 ng IAA / g fr. wt. in the untreated leaves on day 1. The results support the assumption that auxin transport inhibition results in elevated auxin levels and supports the hypothesis that auxin inhibits leaf development.

Christopher P. Keller

Department of Biology, Minot State University, Minot, ND 58707
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Title Annotation:BRIN Symposium: MSU Interactive Video Network Studio: 8:00-10:00 am
Author:Keller, Christopher P.
Publication:Proceedings of the North Dakota Academy of Science
Geographic Code:1USA
Date:Mar 1, 2003
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