Regulation by auxins

In cell cultures, the expression of the reductase mRNA, like the tdc and stric-tosidine synthase sts) genes, was found to be induced by elicitors and down-regulated by auxins. GlOH was found to be localized in provacuolar membranes and not in the endoplasmic reticulum like many other cytochrome P450 enzymes. Interestingly, this enzyme is inhibited by the end product, alkaloid catharanthine, but not by vindoline and vinblastine. Therefore, feedback regulation may also operate in vivo, provided that the catharanthine and GlOH are within the same cellular compartment (Facchini and De Luca, 2008). 

Theologis, A. 1986. Rapid gene regulation by auxin. Ann. Rev. Plant Physiol. 37 407-438. 

There are several ways to alter effective levels of putative hormones. The first is to excise a plant tissue that is incapable of synthesizing the hormone itself, and allow the tissue to become depleted of the hormone. If this causes cessation of a particular response, and upon readdition of the compound the response is restored, there is reason to believe that the compound is a hormone controlling that process. For example, excision of sections of coleoptiles results in a marked decline in growth rate [13]. Since auxin can restore the growth rate, while none of the other hormones can substitute for auxin, the evidence that coleoptile cell elongation is regulated by auxin is strong. 

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). 

Pasquali, G., Goddijn, O. J. M., de Waal, A., Verpoorte, R., Schilperoort, R. A., Hoge, J. H. C. and Memelink, J. 1992. Coordinated regulation of two indole alkaloid biosynthetic genes from Catharanthus roseus by auxin and elicitors. Plant Molecular Biology, 18 1121-1131. 

Mathesius, U., Flavonoids induced in cells undergoing nodule organogenesis in white clover are regulators of auxin breakdown by peroxidase, J. Exp. Bot., 52, 419, 2001. 

Murphy, A., Peer, W.A., and Taiz, L., Regulation of auxin transport by aminopeptidases and endogenous flavonoids, Planta, 211, 315, 2000. 

The formation of plant cellulases has been found to be closely regulated by different growth hormones, particularly auxin (6,11), steroids (12), or ethylene gas (13). The hormones act in different tissues under different circumstances, and they seldom lead to such high cellulase activity that there is a net decline in total cellulose. Indeed, cellulose biosynthesis usually continues even while partial hydrolysis occurs, and net cellulose deposition often keeps pace with growth under all of these conditions (14). 

Efforts may now have been successful Whereas normal tobacco cells require auxin for division, sequence tagged (TDNA) lines encoding an adenylyl cyclase were obtained which were auxin-independent but cAMP-dependent. From one line (axi 141), a complementary DNA encoding adenylyl cyclase has been isolated with characteristic leucine repeats and similarity to yeast adenylyl cyclase (Ichikawa et al., 1997). The result seems not to be the expression of an alternative division pathway from the normal auxin-driven division since it is blocked by auxin inhibitors and is activated by cAMP and the cyclase activator forskolin. Perhaps a link to G-protein at the membrane will now bring plant growth regulation even closer to that of animals. 

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