Brassinolide-auxin

Nakamura, A., Higuchi, K., Goda, H., Fujiwara, M. T., Sawa, S., Koshiba,T., Shimada, Y., and Yoshida, S. (2003). Brassinolide induces IAA5, IAA19, and DR5, a synthetic auxin response element in Arabidopsis, implying a cross talk point of brassinosteroid and auxin signalhng. Plant Physiol. 133,1843-1853. 

J.H. Yopp, Southern Illinois University, Carbondale, became interested in the brassins project in its early stages (36) and he conducted cooperative brassins studies with both Mitchell and Mandava. Yopp et al. (37, 38) evaluated brassins responses in a number of auxin, gibberellin, and cytokinin bioassays and showed that brassins responded similarly to some of the known hormones in certain systems but not in others. These studies led to more cooperative investigation of brassinolide and other brassinosteroids (39, 40). 

Specificity. Using the first member of the family, brassinolide (BR), an extensive survey of its effects in 17 bioassays, which varied in their responses to gibberellins, auxins and cytokinins, showed that BR did not behave exclusively as any one of those hormones. In some supposedly specific bioassays BR was as effective, or more so, as the hormone the assay was supposed to detect (9,10). This also applies to the rice lamina inclination assay (11), which is now frequently used. 

It has been reported that brassinolide (BR) has some actions similar to auxin and cytokinin (7). In order to compare the physiological effects of BR in these respects and to determine if this is so, we studied three aspects as follows ... 

The three experiments with brassinolide showed that the physiological effect of brassinolide seems to be that of both an auxin and cytokinin. The twining growth effect, however, is the characteristic response evoked by brassi-nosteroids, and it was thought that the twining movement of the tendril and the twiner (climber) reported for auxin, or abscisic acid (ABA), or ethylene earlier is doubtful. In other words, brassinolide as a steroid may act on membrane systems. 

Until the discovery of brassinolide by USDA scientists in 1979, it was thought that only five groups (indole auxins, gibberellins, cytokinins, abscisic acid, and ethylene) of hormones were responsible for regulating plant growth and development. Following this discovery, a number of compounds similar to brassinolide both in structure and physiological activity were isolated from different parts of plants. On the basis of published... 

Fitting s attempt to identify his "Pollen Hormone" chemically was unsuccessful. Today, however, we know that pollen tissues are the source of the known plant hormones (auxin, gibberellins, cytokinin), as well as "brassinolide."... 

Brassinolide was tested on 17 bioassays for growth substance. The results led to claims that brassinolide possesses a broad spectrum of biological activity, including gibberellin-, auxin- and cytokinin-like activity (32,34). These claims must be treated with some caution however, since the claimed "specificity of some of the bioassays selected is questionable. At present three bioassay techniques (35,36,37) are used routinely for the detection of brassinolide activity. All three assays are sensitive to auxin, which is a prerequisite for the detection of brassinolide-like compounds. This is not to say that brassinolide has auxin-like activity, but rather there seems to be an interaction of cooperative action between auxin and BR. 

Biologically active brassinosteroids by themselves cause little curvature however, if a brassinolide treatment is co-applied with auxin the ensuing curvature is greater than the sum total of lAA and brassinolide applied separately (Figure 3). 

Figure 6 shows that by pretreating lAA-sensitive tissues with as little as 10 pmol of lAA reduces the sensitivity of the tissues to subsequent auxin applications and that an application of 1 nmol of lAA desensitizes the tissues completely. This attenuation of lAA-induced growth after a chronic stimulation is prevented by pretreating the tissues with as little as 100 pmol of brassinolide. One possible explanation of these results is that the initial application of lAA reduced the ability of tissues to take up additional lAA by blocking putative transport channels. 

Analysis of tissues for lAA as determined by reverse isotope dilution assays reveals however, that brassinolide does not affect auxin uptake or auxin movement within the tissues (41). In this same study we also observed that bean internode sections treated with brassinolide contained significantly less lAA after 2 hours than tissues treated with lAA alone, even though BR potentiated lAA-induced growth by more than 400%. 

Maeda E (1965) Rate of lamina inclination in excised rice leaves. Physiol Plant 18 813-827 Takeno K, Pharis RP (1982) Brassinosteroid-induced bending of the leaf lamina of dwarf rice seedlings an auxin-mediated phenomenon. Plant Cell Physiol 23 1275-1281 54. Wada K, Marumo S, Ikekawa N, Morisaki M, Mori K (1981) Brassinolide and homobras-sinolide promotion of lamina inclination of rice seedlings. Plant Cell Physiol 22 323-325 Wada K, Marumo S, Abe H, Morishita T, Nakamura K, Mori K (1984) A rice lamina inclination test - a microquantitative bioassay for brassinosteroids. Agric Biol Chem 48 719-726... 

Yopp JH, Mandava NB, Sasse JM (1981) Brassinolide, a growth-promoting steroidal lactone. I. Activity in selected auxin bioassay. Physiol Plant 53 445-452... 

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