Auxin Inhibitor Herbicides

Auxin inhibitor herbicides include the so-called wild oat herbicides shown in Figure 5.22. Their classification as auxin inhibitors is based on their inhibition of auxin-induced responses in auxin bioassays and their antagonism of auxin herbicides. " This anti-auxin activity of diclofop-methyl is almost certainly secondary in importance to its inhibition of acetyl-CoA carboxylase (see Chapter 3). 

Other auxin-like herbicides (2,48) include the chlorobenzoic acids, eg, dicamba and chloramben, and miscellaneous compounds such as picloram, a substituted picolinic acid, and naptalam (see Table 1). Naptalam is not halogenated and is reported to function as an antiauxin, competitively blocking lAA action (199). TIBA is an antiauxin used in receptor site and other plant growth studies at the molecular level (201). Diclofop-methyl and diclofop are also potent, rapid inhibitors of auxin-stimulated response in monocots (93,94). Diclofop is reported to act as a proton ionophore, dissipating cell membrane potential and perturbing membrane functions. 

Hall et al. (1998) reported that an ALS-resistant biotype of false cleavers was cross-resistant to a broad range of ALS inhibitors, as well as to an auxin-type herbicide, quinclorac, which had never before been applied to these fields. A similar case of quinclorac multiple resistance in smooth crabgrass has been reported in California when plants were previously treated with ACCase herbicides. Data suggest a target site-based mechanism of resistance involving the accumulation of cyanide derived from stimulated ACC synthesis, which is a precursor of ethylene (Abdallah et al., 2004). 

We proposed a metabohc pathway of phosphonates with structure lo in plants when these compounds lo were initially designed. lo might be metabolized to substituted phenoxyacetic acids (lo-a) as auxin-type herbicide and acylphospho-nates (lo-c) as PDHc El inhibitors after the hydrolysis and oxidation by esterases and oxidases took place in plants (Scheme 1.21). 

None of these had pronounced herbicidal activity but rather PGR activity. Compound % for example, has been shown to be an auxin transport inhibitor, a property probably shared by the other members of this class also. A structure activity analysis for this group of compounds has been reported by Katekar ( ) ... 

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.

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

The first study relates to the identification and design of novel herbicides and plant growth regulators that act by inhibiting polar transport of the plant hormone auxin (100). Previous studies (101) had identified seven auxin transport inhibitors, from which it was possible to describe a three-component... 

There are no commercial herbicide resistant crops that function by increased expression of the protein target, although some level of plant resistance has been reported for glyphosate, glufosinate, some DPEs and inhibitors of hydroxy-phenylpyruvate dioxygenase. Similarly, cellular sequestration of the herbicide from the target has been reported with some DPEs, auxins and photosystem I inhibitors, but none have been developed commercially [3]. 

Growth regulators organic compounds, which in small quantities inhibit, accelerate or in some way influence physiological processes in plants. G.r. include natural (endogenous) substances, e.g. Phytohormones (see) and native inhibitors and many synthetic compounds, especially herbicides and Grot retardants (see). ITie Auxins (see) include natural and synthetic compounds. 

The physical properties of pyridine-containing acids make them particularly attractive starting points for combinatorial libraries for herbicide screens. Pyridines are relatively polar (log P = 0.60 for pyridine vs. 2.13 for benzene) (4) and the sinq>le unsubstituted acids are weakly acidic (nicotinic acid pKa 4.8). (5) Perhaps for these reasons, pyridine-containing acids have found considerable utility as herbicides. Representative examples of commercially successful, phloem mobile, pyridine acids (or dieir derivatives) include picloram, clopyralid, and triclopyr (synthetic auxins) imazethapyr and nicosulfuron (ALS inhibitors) clodinafop and haloxyfop (ACCase inhibitors). Each has a direct counterpart in the conesponding benzene series but, in these instances, the pyridine analog is invariably the superior herbicide. 

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