Auxin growth

Auxins. Growth reactions of various plant organs, principally from Avena or Triticum coleoptiles (oat or wheat coleoptiles) are used for bioassay of the auxin activity of a substance. 

The mode of action is by inhibiting 5-enolpymvyl-shikimate-3-phosphate synthase. Roundup shuts down the production of the aromatic amino acids phenylalanine, tyrosine, and tryptophane (30). Whereas all these amino acids are essential to the survival of the plant, tryptophane is especially important because it is the progenitor for indole-3-acetic acid, or auxin, which plays an important role in growth and development, and controls cell extension and organogenesis. 

A. C. Leopold, Auxins and Plant Growth, University of California Press, Berkeley, 1955. 

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. 

In addition to inhibitory chemicals which enter the plant from the external environment, many endogenous inhibitors appear to function as regulators of seed germination and plant growth and development. The interrelationships between endogenous inhibitors and growth promoters such as the auxins, gibberellins, and kinins remain to be elucidated. 

Relatively few sites of growth-modifying or growth-inhibitory action have been identified at the cellular and molecular level. Consequently, the exact action of, and relationships between, auxins, gib-berellins, kinins, and growth inhibitors remain to be elucidated. 

Micellaneous Vitamins, plant growth regulators (auxin.s, cytokinins, gib-berellins), alkyl sulfides, ethanol, H. K nitrate, phosphate, HCO,... 

Acylanilides (abbreviated as anilides in this article) are generally used as selective pre- and/or post-emergence herbicides in paddy rice fields. The herbicidal activity of the anilides is similar to those exhibited by the auxin-like plant growth regulators. 

Auxins Derivative of tryptophan - e.g. indole acetic acid. Controls H+/K+ balance and growth... 

Nevertheless, the avena coleoptile exhibits a curvature to unilateral UV-illumina-tion with a satisfactory log-linear response/time relationship38) (the bending mode is similar to that observed for the second positive curvature which develops from the coleoptile base cf. 2.2). Fig. 5 338) shows that the double-peaked action spectrum does not match neither flavin (Fig. 5 5,16S)) nor carotenoid absorption (Fig. 5 4,183)), most likely excluding both as photoreceptors. The growth hormone auxin (cf. 2.4 and Scheme 1) has been discussed to be a possible photoreceptor. However, in this case, this is not supported by the action spectrum either. 

Phycocymes shows a positive UV-induced light-growth-response (Fig. 3 3) as well as a negative phototropic curvature, as depicted in Fig. 5 1. The exceptionally good fit of this action spectrum and auxin absorption (Fig. 5 2) might indicate that auxin is the UV-photoreceptor. 

Table 1. Bluelight induced bending of decapitated avena coleoptiles with different fillings, with and without application of the growth-hormone auxin (cf. Sect. 2.4). Clearly the quantity of bending depends on the intrinsic light gradient (after v. Guttenberg75)).

Scheme 1. Molecular structure of the plant growth hormone auxin (indoleacetic acid, IAA). Extremely small amounts (nanomolar) can be detected by the auxin standard test 5 mm long segments of pea shoots elongate faster in the presence of exogenous auxin, which can be taken as a sensitive assay...

HAYASHI, H. CZAJA, I, LUBENOW, H SCHELL, J WALDEN, R Activation of a plant gene by T-DNA tagging auxin-independent growth in vitro, Science, 1992, 258, 1350-1353. 

Next to the amount of P, the chemical form of this nutrient (Lambers et al. 2002 Shu et al. 2005 Shane et al. 2008) and the availability of other nutrients, especially nitrogen, potassium, and iron (Shane and Lambers 2005) affects the formation of cluster roots. It seems to be regulated by several plant hormones. Thus, application of auxin led to the production of cluster roots in white lupin at P concentrations that normally suppress cluster roots (Gilbert etal. 2000 Neumann et al. 2000). Cytokinines might also play a role, as kinetin applied to the growth medium of P-deficient white lupin inhibited the formation of cluster roots (Neumann et al. 2000). 

Indole-3-acetic acid is a natural plant auxin and is used as a control in research on plant growth. 

Dicamba (3,6-dichloro-2-methoxybenzoic acid) is primarily used as a postemergence broadleaf herbicide, which interferes with normal plant auxin function, subsequently causing uncontrolled growth and the inhibition of the phototropic and geotropic function. Cumulative response results in plant death. The success of auxinic analogues such as Dicamba and 2,4-dichloropheno-xyacetic acid in weed control has led to widespread manufacturing and use. Estimated U.S. production for Dicamba was 5 million kg in 1990 [391]. 

---