Auxin Transport Inhibitors

Naptalam seems to specifically inhibit auxin transport, and it must be concluded that different sites exist for auxin transport and other auxin-induced responses. 

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A. M. Hirsch, R. V. Bhuvaneswari, J. G. Torrey and T. Bisseling, Early nodulin genes are induced in alfalfa root outgrowths elicited by auxin transport inhibitors. Proc. Natl. Acad. Sci. U.S.A. 86 1244-1248 (1989). 

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 ( ) ... 

Bailly A, Sovero V, Vincenzetti V, Santelia D, Bartnik D, Koenig BW, Mancuso S, Martinoia E, Geisler. 2008. Modulation of P-glycoproteins by auxin transport inhibitors is mediated by interaction with immunophilins. J Biol Chem 283 21817— 21826. 

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... 

Mode of action Chitin synthesis inhibitor Electron transport inhibitor Rapid excitation of nervous system by persistent activation of nicotinic acetylcholine receptors Auxin transport inhibitor... 

Defoliation. Interest in defoliation has been low in recent years. One relatively new development is the "wiltant which is applied only shortly before harvest (51). As an outgrowth of some basic studies, several auxin transport inhibitors, TIBA, DPX-1840, Alanap (N-l-naph-thylphthalamate), and morphactins (2-chloro-9-hydroxyfluorene-9-car-boxylic acid), were shown to promote ethylene- and ethephon-mediated leaf abscission (52, 53). Subsequently, CA3 was found to be even more active in promotion of ethylene-induced abscission (54). It now appears that the CA3 counteracts the inhibitory effect of auxin on ethylene-induced leaf abscission (55) thus, CA3 might improve the performance of any defoliant that achieves part of its action by stimulating stress-induced ethylene production and lowering the natural auxin content of the dam-... 

In some cases it might be desirable to inhibit ethylene synthesis chemically to prevent responses mediated by naturally produced ethylene or stress-produced ethylene. Although some substances do inhibit ethylene production modestly—e.g. TIBA (69)—no outstanding regulator of this nature has been discovered. Another possibility is to promote or inhibit ethylene action. Promotion can be accomplished by auxin transport inhibitors and GA in cases where auxins and ethylene have opposite effects 52, 53, 54, 55). Recently, silver ion was found to be a potent inhibitor of ethylene action (70). Ethylene action also can be inhibited by lowering the temperature and O2 level or increasing the CO2 level... 

Triiodo Benzoic Acid (TIBA) 10-6 is unique. It can act as an auxin, anti-auxin and auxin transport inhibitor. It can inhibit in vitro auxin transport before binding to the NPA receptor is observed. It need not act by the same mechanism as phytotropins [96]. 

It is obvious that no single structure-activity correlation can encompass all molecules which can bind to the NPA receptor. It has been suggested that the receptor may be multifaceted [106], but there may also be multiple receptors. Compounds which interact with the NPA receptor(s) inhibit auxin transport and it can be classified as auxin transport inhibitors , but auxin transport inhibition need not be the, or the only, function of the receptor(s). Flavonoids have been proposed as the natural ligands, but there may be others and these need not act in the same way as the synthetic molecules [94]. There is much that is unknown about the role and function of NPA receptors. 

The formation of root nodules associated with nitrogen fixation in legumes is an area of active study. Hormonal involvement in the developmental modifications needed to establish the nodule, has been reviewed [186]. Studies on the molecular characterization of nodulation have shown that early nodulation genes are induced by auxin transport inhibitors, and inhibitor treated roots produce pseudonodules [187]. Rhizobium species appear to produce lAA by a pathway involving the conversion of tryptophan to indole-... 

Fig. 2. MTs in cells of ev. Alarik maize roots after their treatments either with the auxin transport inhibitor naphthylphthalamic acid (NPA) (100 pM, 6 h) (a-g) and with lAA (100 pM, 24 h) (h-j). After NPA treatment, periclinal divisions were induced in the outer cortex (a, thin arrows indicate young cell walls, thick arrow indicates pre-prophase band of MTs). Cells of the epidermis preserved well-ordered transverse CMTs both near the root apex (b) and in the transition zone (g). On the other hand, cells of the root cap (c), outer cortex (d), inner cortex (e) and of the stele parenchyma (f) showed disturbances to their general appearance and ordering (compare with Figs. 3c, d, g, h). Similarly to NPA treatment, relatively well-ordered transverse CMT arrays are still preserved in postmitotic cells of the epidermis (h) as well as in all cells of the former meristem (i,j) after the auxin treatment. Abbreviations EP, epidermis IC, inner cortex OC, outer cortex RC, root cap.

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... 

At present we know very much more about auxin binding to membrane proteins than to soluble proteins, and most of this information is from work with one species, maize Zea mays L.). It was with maize membranes that the first convincing binding of a PGR, the auxin transport inhibitor NPA, was detected [10], and similar membrane preparations were soon thereafter shown to exhibit saturable binding of the auxin, NAA [5]. 

Powerful synthetic Polar Auxin Transport Inhibitors (PATIS) have been available for many years and have proved valuable investigative tools [35]. They... 

NPA and other synthetic auxin transport inhibitors can block PAT when they are incorporated into the receiver blocks in a classical donor-to-receiver polar transport assay. Figure 4 shows that quercetin in receiver blocks can inhibit lAA arrival, but only for the first 2 h of the transport test. Thereafter, the radioactivity delivered to the receiver blocks is increased compared to controls. In some experiments lAA (or NAA) arrival in quercetin-containing receiver blocks is increased at earlier times. A crossover is never observed when NPA is used. Quercetin and other flavonoids bind strongly to cellulose [31 ] and, unlike NPA, do not diffuse away... 

Beyer EM, Johnson AL, Sweetser PB (1976) A new class of synthetic auxin transport inhibitors. Plant Physiol 57 839-841... 

Katekar GF, Geissler AE (1977a) Auxin transport inhibitors II. 2-(l-Pyrenoyl)benzoic acid, a potent inhibitor of polar auxin transport. Aust J Plant Physiol 4 321-325 Katekar GF, Geissler AF (1977b) Auxin transport inhibitors III. Chemical requirements of a class of auxin transport inhibitors. Plant Physiol 60 826-829 Kato J (1958) Nonpolar transport of gibberellin through pea stem and a method for its determination. Science 128 1008-1009... 

The Discovery of Novel Auxin Transport Inhibitors by Molecular Modeling and Three-Dimensional Pattern Analysis. 

Dhonukshe, P., Grigoriev, I., Fischer, R. et al. (2008). Auxin transport inhibitors impair vesicle motility and actin cytoskeleton dynamics in diverse eukaryotes. Proceedings of the National Academy of Sciences, 105,4489-4494. 

Geldner, N., Friml, J., Stierhof, Y.-D., Jiirgens, G. Pahne, K. (2001). Auxin transport inhibitors block PlNl cycling and vesicle trafficking. Nature, 413, 425-428. 

Bures, M.G. Black-Schaefer, C. Gardner, G. The Discovery of Novel Auxin Transport Inhibitors by Molecular Modelling and Three-dimensional Pattern Analysis. J. Compute Aided Mol. Des. 1991, 5, 323-334. 

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