Auxin sensitivity

Our improvement of this test is based on the incubation of internode sections in the inversed position (9). Under these conditions the substances undergoing test are applied to young auxin-sensitive apical internode tissues and auxin is translocated basipetally toward the zone of curvature (Figure 1). Using this simple improvement the sensitivity of the test to IAA was increased 1,000 times as little as 10 fmoles of IAA can be detected. 

Figure 1. The effect of inversion of bean first internode sections on distribution of 14C-IAA and its accumulation in auxin-sensitive zone. (Reproduced with permission from reference 9. Copyright 1985.)...

Kulaeva et al. (29) have demonstrated modified gene expression in leaf tissue after administration of BR, with several new proteins being induced, and evidence for concentration effects. Also, Clouse et al (30) have reported new polypeptides, and up and down regulation in auxin-sensitive soybean stem segments treated with BR. 

Borkird and Sung [1] isolated seven ABA-insensitive cell lines in a somatic embryo culture of carrot in which ABA failed to arrest the development of torpedo-stage embryos into plantlets, as occurs in wild type when ABA is added. All lines also showed reduced auxin sensitivity. Characterization of three lines showed lower levels of ABA uptake as a possible cause of ABA insensitivity. However, the uptake of 2,4-D was higher than in wild-type embryos. In tobacco, cell-lines resistant to inhibition of growth by ABA were also isolated [25], and cell proliferation continued in the presence of what would be growth-inhibiting ABA concentrations for normal cell lines. 

The morphology and auxin sensitivity of the variant lines suggest increased auxin production, but Southern blot analysis of DNA isolated from TA66-D1 showed that the tms locus was still mutated [21]. Although the variants mimicked the phenotype of the A6-transformed line TA6-5, RIA analysis showed that lAA levels in the variants were much lower than in TA6-5 and similar to lAA levels in the teratoma line TA66C3-78 (Table 2). The variants and the teratoma line also had similar ACC contents, which were much lower than ACC levels in TA6-5 and ACC levels in the variants increased markedly with auxin treatment [21]. Thus, like N. glutinosa, these variant lines of N. tabacum compensated for a mutant tms locus by some mechanism other than increased auxin accumulation. 

Auxin Autonomy and Auxin Sensitivity Can Be Separate Phenomena... 

The Physiological Basis for Auxin Sensitivity in Crown Gall... 

We also found that auxin-sensitive variants accumulated higher levels of ACC in response to auxin than did the auxin-resistant teratoma [21]. Analysis of the accumulation of MACC showed this difference to result from variation in the... 

Analysis of several clones of A66-transformed A. tabacum showed that cells exhibiting a high degree of auxin autonomy could be either auxin sensitive or auxin resistant. Thus, auxin autonomy and auxin sensitivity can be separate phenomena and are likely controlled by different mechanisms. Gibberellin A3 can substitute for auxin and support the growth of auxin-dependent teratoma cells, and therefore GAs may be the shoot-derived growth factors accounting for hormone independence of complex teratomas in culture. 

Auxin sensitivity, on the other hand, appears to be mediated by ACC, but not by ethylene. Thus, ACC may play a role in plant development independent from its role in ethylene biosynthesis. The sensitivity of cell lines to ACC was related to... 

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...

Russell, L., Stokes, A. R., Macdonald, H., Muscolo, A., and Nardi, S. (2006). Stomatal responses to humic substances and auxin are sensitive to inhibitors of phospholipase A2. Plant Soil 283,175-185. 

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],... 

Shen, W. H., Petit, A., Guern, J., and Tempe, J. 1988. Hairy roots are more sensitive to auxin than normal roots. Proc. Natl. Acad. Sci. USA 85, 3417-3421... 

In POLARIS (PLS) mutant, the length of the main root is shorter and a decrease in branch veins among rosette leaf veins is also observed.1000 Such phenotypes may arise from changes in sensitivity to hormones in the mutant, as it is assumed that the PLS gene is required for the maintenance of homeostasis in the cytokinin—auxin system. 

Figure 4. Proposed model of molecular interactions in the herbicidal mechanism of action of auxin herbicides in sensitive dicot and grass weeds. ABA, abscisic acid ACC, 1-aminocyclopropane-1-carboxylic acid ROS, reactive oxygen species. Modified from [7].

Ephritikhine, G., Fellner, M., Vannini, C., Lapous, D. and Barbier-Brygoo, H. (1999) The saxl dwarf mutant of Arabidosis thaliana shows altered sensitivity of growth responses to abscisic acid, auxin, gibberellins and ethylene and is partially rescued by exogenous brassinosteroid. Plant., 18,303-14. 

The LJT bioassay was developed for an auxin bioassay by Dr. Maeda of Nagoya University in 1965(. As mentioned above, the LJT is extremely sensitive and specific to BS. We established a... 

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