Quinclorac belongs to a new class of highly selective auxin herbicides

Abstract

Substituted quinolinecarboxylic acids, including quinclorac (BAS 514H), are a new class of highly selective auxin herbicides, which are chemically similar to naturally occurring compounds isolated from plants and soils. Quinclorac is used in rice to control important dicot and monocot weeds, particularly barnyardgrass. The herbicide has also been developed for application in turfgrass areas, spring wheat, and chemical fallow. Quinclorac is readily absorbed by germinating seeds, roots, and leaves and is translocated in the plant both acropetally and basipetally. By mimicking an auxin overdose, quinclorac affects the phytohormonal system in sensitive plants. The compound stimulates the induction of 1-aminocyclopropane-1-carboxylic acid (ACC) synthase activity and thus promotes ethylene biosynthesis. In susceptible dicots, increased levels of ethylene trigger an accumulation of abscisic acid (ABA), which, as part of the intrinsic auxin activity of quinclorac, plays a major role in growth inhibition and the induction of epinasty and senescence. In sensitive grasses, such as barnyardgrass species, large crabgrass, broadleaf signalgrass, and green foxtail, quinclorac leads particularly to an accumulation of tissue cyanide, formed as a co-product during increased ACC and ethylene synthesis. This causes phytotoxicity characterized by the inhibition of root and particularly shoot growth with tissue chlorosis and subsequent necrosis. These effects were not observed in tolerant rice and a resistant biotype of barnyardgrass. No significant differences in uptake, translocation, or metabolism of quinclorac between resistant and sensitive grasses were found. Hence, a target-site-based mechanism of selectivity is suggested. The induction process of the ACC synthase activity plays the primary role in the selective herbicide action of quinclorac. This is a common effect of auxin herbicides and auxins, which lead to the accumulation of cyanide and/or ABA depending on the plant species and tissues, the compound concentration in the tissue, and their biological activity.

News from Old Compounds: The Mode of Action of Auxin Herbicides

“…As shown in Galium aparine, auxin treatment via the root stimulates ethylene biosynthesis through induced ACC synthase activity in the shoot tissue within 2 h (Figure 3). This induction was inhibited by cordycepin or cycloheximide, indicating de novo enzyme synthesis [6]. Subsequently, increases in ABA and its biosynthetic precursor xanthoxal were detectable after 5-8 h [39].…”

“…The most prominent example of hormonal cross-talk in growth regulation is the stimulation of ethylene production when auxins are applied to sensitive species [5,6,7,14] or overproduced in transgenic plants [16]. This early and ubiquitous auxin response was described as early as 1935 by Zimmermann and Wilcoxon [33].…”

“…Therefore, phenomena such as epinasty of leaf lamina or petiole and stem curvature could be attributed to auxin-stimulated ethylene or auxin activity alone [14,16,35]. Although ethylene itself is not the primary agent by which auxin herbicides cause phytotoxicity [13], former investigations have made it plausible that enhanced ethylene production is involved in this process (for review see [2,5,6,7]. In a multitude of dicot species and biotypes of differing sensitivity, stimulation of ethylene formation by auxin herbicides has been found to correlate with the induced growth effects, which could be alleviated by adding inhibitors of ethylene biosynthesis [13,14,36,37,38,39].…”

“…However, conclusive evidence for a causal role of ethylene synthesis in auxin herbicide action has been presented only recently. This progress has been achieved with the identification of auxin-induced de novo synthesis of ACC synthase and plant mutants and transformants modified in ethylene biosynthesis, perception and signaling [5,6,7,16,27,34]. In experiments using transgenic tomato plants containing the ACC synthase antisense gene LE-ACS2 [38], auxinresistant or ethylene-insensitive lines ofArabidopsis, wild mustard [29] and tomato [39], herbicide-induced phenotypical alterations could be alleviated or fully reversed.…”

“…Synthetic compounds which interfere with the phytohormonal auxin system in plants include the first selective organic herbicides to be developed for agricultural and horticultural use (for review see [1,2,3,4,5,6,7]. These so-called growth regulator or auxin herbicides started a new era of weed control in modern crop production due to their ability to be translocated systemically in the plant and to exert selective action, primarily against dicot weeds in cereal crops.…”