Auxins herbicides

It is not the purpose of this section to repeat previous reviews on auxins and auxin herbicides. A brief summary will be presented on the current views on the target site(s) for the auxin herbicides and recent developments concerning mode of action. 

The reason for the phytotoxic action of the auxin herbicides is proposed to be an aberrant nucleic acid metabolism induced by a hormonal imbalance in treated tissues.  

It is necessary to briefly consider the current view on the mode of action of auxins in order to identify the basis of phytotoxicity of the auxin herbicides. However, in spite of 40 years of extensive research, aided by the use of the auxin herbicides, there is no universal view on the auxin binding sites (receptors) and the events following binding. 

The existence of auxin receptors/binding proteins, the likely target sites of auxin herbicides, has been the subject of considerable research in recent years (for reviews, see Refs. 4, 173, and 179-181). There seems little 

In conclusion, the auxin herbicides are clearly auxin mimics however, despite many years of research, their target site(s) and detailed mechanisms of action remain to be clearly identified. 

---

F. Scheltrup and K. Grossmann, Abscisic Acid is a Causative Factor in the Mode of Action of the Auxinic Herbicide, Quinmerac, in Cleaver (Galium aparine L.) , J. Plant Physiol., 1996,147, 118-126,. 

The notion of using exogenous chemicals to control plant growth, and hence productivity, is not new. Many of the first commercial herbicides (auxin herbicides) were developed as a direct consequence of plant growth regulator research (see Chapter 1, Section 3.3). 

Grossmann, K. (2000) Auxin herbicides current status of mechanism and mode of action. Pest Manage Sci, 66, 113-120. 

Since the same effects occurred with the non-herbicidal (2,6-dichloro-phenoxy) acetic acid as with toxic compounds, glucose metabolic changes are not the important singular effects of auxin herbicides. 

Indole-3-acetic acid is rather readily oxidized by peroxidases and is, in fact, probably not present in the plant in the free form to any appreciable extent. The nature of the complexing groups is not clear. The inherent instability of the compound in living tissue has made experimental observations difficult, and (the more stable) 1-naphthaleneacetic acid has often been used instead, although it is by no means certain that the biological activities are comparable. One view held is that auxin herbicides are effective either because they do not readily form conjugate systems, or because the conjugate retains the phytotoxic properties. 

It has long been known that detergents increase the effectiveness of auxin herbicides. This may now be interpreted as perhaps not due entirely to the wetting properties of the detergents and it would be of interest to compare fatty acid ester detergents with those not containing fatty acid groups in herbicide trials. 

A new class of plant auxin herbicides are the phytotropins, exemplified by the semicarbazone diflufenzopyr [148], Diflufenzopyr is a systemic herbicide for the selective control of broadleaf and perennial weeds in corn. Unlike the early auxin herbicides, such as 2,4-d and dicamba, which acted as mimics of indole-3-acetic acid, phytotropins prevent polar auxin transport in sensitive plants, causing stunting and loss of tropic response [149],... 

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

Various chemical classes of auxin herbicides with different weed spectra and types of selectivity have been synthesized and commercially introduced over the years. They include phenoxycarboxylic acids, benzoic acids, pyridine carboxylic acids, aromatic carboxymethyl derivatives and a relatively new category, the quinolinecarboxylic acids (Figure 1). These compounds basically act as synthetic mimics of indole-3-acetic acid (lAA), which is the principal natural auxin in higher plants [2,3,5,7]. 

Auxin Herbicides Synthetic mimics of natural auxin - Indole -acetic-acid... 

Figure 1. Structures of natural auxin, indole-3-acetic add, and synthetic auxin herbicides and auxin transport inhibitors (phytotropins) belonging to the different chemical families.

Auxin Herbicide Action in Dicots Mimic of Auxin Overdose... 

Figure 2. Three-phase-process of auxin herbicide action exemplified for the dicot weed Galium aparine after root-uptake and translocation in the plant (shown in autoradiographs using labelled compound).

In this context, we recently discovered close correlations between auxin-induced ethylene synthesis, massive accumulations of ABA in root md even more in shoot tissue and inhibition of their growth. This phenomenon, which has been reported for the first time, was demonstrated for auxin herbicides from different chemical classes (Figure 1), such as quinmerac, quinclorac, MCPA, naphthalene acetic acid, dicamba and picloram, and lAA at high concentration, in a variety of dicot species, including members of the Sola-naceae, Umbelliferae, Fabaceae, Scrophulariaceae and Brassicaceae [35, 39, 40]. ABA accumulation and concomitant epinastic effects and reduced shoot growth were also... 

Figure 3. Model of the mode of action of auxin herbicides in the induction of growth inhibition, tissue damage and senescence in dicot plant species, as illustrated for cleavers Galium aparine). ABA, abscisic acid ACC, 1 -aminocyclopropane-1 -carboxylic acid ROS, reactive oxygen species SAM, S-adenosylmethionine. Modified fiom [20,41],...

Synergistic Cross-Talk between Auxin Transport Inhibitor and Auxin Herbicide... 

Auxin Herbicide Action in Grasses The Role of Cyanide... 

The final example of our chemical genetic approach to dissect herbicide action involves auxin biology. A subset of our bioactive compound library that exhibited auxinic herbicidal symptoms was tested for inhibition of root growth of Arabidopsis seedlings. The most potent of these compounds was a novel picolinate auxin DAS534. Previous mutational studies with 2,4-D have elucidated an auxin signal-... 

---