Interactions of auxinic compounds on a Ca2+ signaling and root growth in Arabidopsis thaliana

Authors: Teaster, Neal; SPARKS, JEFFREY - Samuel Roberts Noble Foundation, Inc; BLANCAFLOR, ELISON - Samuel Roberts Noble Foundation, Inc; Hoagland, Robert

Submitted to: American Journal of Plant Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/30/2015
Publication Date: 12/3/2015
Citation: Teaster, N.D., Sparks, J.A., Blancaflor, E.B., Hoagland, R.E. 2015. Interactions of auxinic compounds on a Ca2+ signaling and root growth in Arabidopsis thaliana. American Journal of Plant Sciences. 6:2989-3000.

Interpretive Summary: Auxin, identified as a growth hormone in plants in the 1930’s, affects virtually all aspects of plant growth. In the 1940’s, 2,4-D was synthesized, shown to affect plant growth via auxin-like activity, and became a major herbicide. Several synthetic auxinic chemicals are used as herbicides, and crop seeds (corn, soybean and cotton) with resistance to some of these compounds are being commercialized. Calcium is a well-known cellular messenger that influences numerous processes (metabolism, stomatal opening, ion transport, etc.) and correlations of auxin and calcium signaling have been reported. Scientists in the USDA-ARS Crop Production Systems Research Unit, Stoneville, MS and The Samuel Roberts Noble Foundation, Inc., Ardmore, OK, studied free cytoplasmic calcium flux in live root tissue of transformed (YC 3.60 calcium reporter) Arabidopsis plants in response to auxinic compound application via measuring changes in fluorescence resonance energy transfer with confocal microscopy and cutting-edge software. Auxin, auxin-like compounds and two auxinic herbicides elicited specific calcium response signatures within seconds after application, and root elongation was reduced within minutes. After 96 hours, root growth was altered by most compounds, but the herbicides 2,4-D and mecoprop caused the most potent effects (increased lateral roots; stunted, malformed root tips; aggregates of root hairs). The rapid interaction of auxinic compounds with calcium signaling responses suggests this model plant system may be a useful tool to glean unique information about herbicide mode of action, secondary herbicide effects and mechanisms of herbicide resistance in weeds.

Technical Abstract: Auxinic-like compounds have been widely used as weed control agents. Over the years, the mode of action of auxinic herbicides have been elucidated, but most studies thus far have focused on their effects on later stages of plant growth. Here, we show that some select auxins and auxinic-like herbicides trigger a rapid elevation in root cytosolic calcium levels within seconds of application. Arabidopsis thaliana plants expressing the Yellow-Cameleon (YC) 3.60 calcium reporter were treated with indole-3-acetic acid (IAA), indole-3-butyric acid (IBA), 1-naphthalene acetic acid (NAA), and two synthetic herbicides, 2,4-dichlorophenoxyacetic acid (2,4-D) and mecoprop [2-(4-chloro-2-methylphenoxy) propanoic acid], followed by monitoring cytosolic calcium changes over a 10 minute time course. Seconds after application of compounds to roots, the Ca2+ signaling-mediated pathway was triggered, initiating the plant response to these compounds as monitored and recorded using Fluorescence Resonance Energy Transfer (FRET)-sensitized emission imaging. Each compound elicited a specific and unique cytosolic calcium signature. Also primary root development and elongation was greatly reduced or altered when exposed at two concentrations (0.10 and 1.0 µM) of each compound. Within 20 to 25 min after triggering of the Ca2+ signal, root growth inhibition could be detected. We speculate that differences in calcium signature among the tested auxins and auxinic herbicides might correlate with their variation and potency with regard to root growth inhibition.