We used coleoptile sections ofAvena sativa, Sorghum bicolor, andZea maysseedlings to examine interactions between epidermal growth factor (EGF) and indole-3-acetic acid (IAA) that may affect plant growth and development. Our 24-h bioassays employed three controls ranging in dilution from 10−4to 10−8g ml−1: (1) 50 mM potassium-phosphate buffer solution (pH=6.0), (2) bovine serum albumin, a nonspecific protein; and (3) IAA; plus two treatments: (1) mouse epidermal growth factor (EGF) ranging from 10−6to 10−10gml−1, and (2) EGF+IAA. In all three species growth in IAA, EGF, and EGF+IAA treatments showed significant increases over controls; EGF+IAA showed significant increases in growth over IAA alone. As the concentrations of IAA decreased, the EGF and IAA interaction became more pronounced. At the highest IAA concentrations, EGF+IAA increased growth rates ca. 2to 39, whereas at lower IAA concentrations EGF+IAA promoted growth as much as 121, thereby lowering the normal IAA physiological set point up to three or four orders of magnitude. Our data suggest that an interaction between EGF and IAA may allow plants to recognize and respond to animal biochemical messengers, resulting in changes in plant cell elongation that ultimately may alter plant grow
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