Auxin-cytokinin interactions in transgenic plants expressing the A. tumefaciens ipt, iaaM and iaaH genes
1996
Ekloef, S. (SLU, Umeaa (Sweden). Inst. foer Skoglig Genetik och Vaextfysiologi)
In this study a promoter-trap-ipt vector was used to produce fertile cytokinin-overproducing plants with very weak expression of the Agrobacterium tumefaciens ipt gene. Key aspect of cytokinin metabolism in these moderately cytokinin-overproducing tobacco plants was investigated. It was concluded that the conjugation of glucose to the 7-position of zeatin (Z) is important in maintaining Z homeostasis, while the importance of the cytokinin oxidase activity is not as significant in planta as a number of in vitro studies have suggested. The effect of endogenous cytokinin-overproduction on the levels and metabolism of the auxin, indole-3-acetic acid (IAA), and vice versa were characterised. It was demonstrated that these two hormones each exert a negative effect on the other hormone's concentration. IAA supressing the synthesis and/or transport of cytokinins and cytokinins depressing IAA synthesis. A cross between transgenic plants that each overproduced one of these hormones, simultaneously displayed phenotypic traits associated with overproduction of both auxin and cytokinin. In these plants, which expressed the ipt and iaaH + iaaM genes, the steady-state levels of IAA and (zeatin nucleoside + -nucleotide) were similar to those in wild-type plants. These observations suggest, contrary to the prevailing theory in the field, that release of lateral shoot, adventitious root formation and a varity of other phenotypic traits are not governed by the ratio of auxin to cytokinin. Physiological consequences of these findings are discussed. The development of a microscale technique for IAA quantification has allowed us to perform detailed studies of the IAA distribution in leaves of wild-type and IAA-overproducing tobacco. The discovery of concentration-gradients in the distribution of IAA over leaf-tissues with different physiological status constitutes a start-point for further studies of the influence of fields of growth substances in the regulation of plant growth.
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