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DEVELOPMENT AND HORMONE ACTION

Long-Term Inhibition by Auxin of Leaf Blade Expansion in Bean and Arabidopsis¹

Department of Biology, Minot State University, 500 University Avenue West, Minot, North Dakota 58707 (C.P.K.); Department of Botany, Box 351330, University of Washington, Seattle, Washington 98195 (R.S.); and Department of Horticultural Science, Center for Microbial and Plant Genomics, University of Minnesota, Saint Paul, Minnesota 55108 (L.S.B., J.D.C.)

The role of auxin in controlling leaf expansion remains unclear. Experimental increases to normal auxin levels in expanding leaves have shown conflicting results, with both increases and decreases in leaf growth having been measured. Therefore, the effects of both auxin application and adjustment of endogenous leaf auxin levels on midrib elongation and final leaf size (fresh weight and area) were examined in attached primary monofoliate leaves of the common bean (Phaseolus vulgaris) and in early Arabidopsis rosette leaves. Aqueous auxin application inhibited long-term leaf blade elongation. Bean leaves, initially 40 to 50 mm in length, treated once with -naphthalene acetic acid (1.0 mM), were, after 6 d, approximately 80% the length and weight of controls. When applied at 1.0 and 0.1 mM, -naphthalene acetic acid significantly inhibited long-term leaf growth. The weak auxin, -naphthalene acetic acid, was effective at 1.0 mM; and a weak acid control, benzoic acid, was ineffective. Indole-3-acetic acid (1 µM, 10 µM, 0.1 mM, and 1 mM) required daily application to be effective at any concentration. Application of the auxin transport inhibitor, 1-N-naphthylphthalamic acid (1% [w/w] in lanolin), to petioles also inhibited long-term leaf growth. This treatment also was found to lead to a sustained elevation of leaf free indole-3-acetic acid content relative to untreated control leaves. Auxin-induced inhibition of leaf growth appeared not to be mediated by auxin-induced ethylene synthesis because growth inhibition was not rescued by inhibition of ethylene synthesis. Also, petiole treatment of Arabidopsis with 1-N-naphthylphthalamic acid similarly inhibited leaf growth of both wild-type plants and ethylene-insensitive ein4 mutants.

¹ This work was supported by the Minot State University Small Grant Program for Faculty Research (grant to C.P.K.), by the North Dakota Biomedical Research Infrastructure Network (grant to C.P.K.), and by the National Science Foundation (grant no. NSF DBI–0077769 to J.D.C.).

^* Corresponding author; e-mail kellerch{at}minotstateu.edu; fax 701–858–3163.

Received August 25, 2003; returned for revision October 6, 2003; accepted December 23, 2003.

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