Abstract
The perception pathway for endogenous auxin has been well described, yet the mode of action of synthetic auxin herbicides, used for >70 years, remains uncharacterized. We utilized transcriptomics and targeted physiological studies to investigate the unknown rapid response to synthetic auxin herbicides in the globally problematic weed species Erigeron canadensis. Synthetic auxin herbicide application consistently and rapidly down-regulated the photosynthetic machinery. At the same time, there was considerable perturbation to the expression of many genes related to phytohormone metabolism and perception. In particular, auxin herbicide application enhanced the expression of the key abscisic acid biosynthetic gene, 9-cis-epoxycarotenoid deoxygenase (NCED). The increase in NCED expression following auxin herbicide application led to a rapid biosynthesis of abscisic acid (ABA). This increase in ABA levels was independent of a loss of cell turgor or an increase in ethylene levels, both proposed triggers for rapid ABA biosynthesis. The levels of ABA in the leaf after auxin herbicide application continued to increase as plants approached death, up to >3-fold higher than in the leaves of plants that were drought stressed. We propose a new model in which synthetic auxin herbicides trigger plant death by the whole-scale, rapid, down-regulation of photosynthetic processes and an increase in ABA levels through up-regulation of NCED expression, independent of ethylene levels or a loss of cell turgor.
Highlights
Auxin is essential for plant growth and development, and the perception and signaling pathway for this hormone is well described (Hagen and Guilfoyle, 2002; Dharmasiri et al, 2005a; Kepinski and Leyser,2005;Tan et al.,2007).In contrast,synthetic auxins have been used as herbicides for >70 years, yet, unlike endogenous auxin, the precise mode of action leading to plant death by these compounds remains unknown
We propose a new model in which synthetic auxin herbicides trigger plant death by the whole-scale, rapid, down-regulation of photosynthetic processes and an increase in abscisic acid (ABA) levels through up-regulation of NCED expression, independent of ethylene levels or a loss of cell turgor
At 1, 6, and 24 herbicide treatment (HAT), a 5 ml sample of gas was taken from each treatment vial and ethylene was measured with a gas chromatograph (Agilent 7890, Agilent Technologies) equipped with a flame ionization detector and a HayeSep N 80/100 mesh silcosteel 1.22 m×1.58 mm column (Agilent Technologies).The injector temperature was 126 °C and helium carrier gas flow was set to 40 ml min−1
Summary
Auxin is essential for plant growth and development, and the perception and signaling pathway for this hormone is well described (Hagen and Guilfoyle, 2002; Dharmasiri et al, 2005a; Kepinski and Leyser,2005;Tan et al.,2007).In contrast,synthetic auxins have been used as herbicides for >70 years, yet, unlike endogenous auxin, the precise mode of action leading to plant death by these compounds remains unknown. Attempts to demystify the herbicidal mode of action of auxins have focused intensively on the auxin-binding TRANSPORT INHIBITOR RESPONSE 1 (TIR1)/AUXIN SIGNALING F-BOX (AFB) receptors in Arabidopsis thaliana L. Analysis of A. thaliana mutant lines has confirmed that these receptor genes are essential for the plant perception and specificity of auxin herbicides. While this research has been instrumental in demonstrating the perception pathway of synthetic auxins, it has not revealed the specific physiological mechanisms leading to plant death that are activated by the perception of these exogenous compounds
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