The authors have developed a sensitive bioassay that can be used to detect auxins as well as cytokinins. The bioassay is based on the expression in transformed tobacco (Nicotiana tabacum) mesophyll protoplasts of a chimeric gene, consisting of the upstream sequences of the Agrobacterium tumefaciens gene 5, coupled to the coding sequence of the beta-glucuronidase. The expression of this gene is induced by the presence of both auxin and cytokinin in the culture medium. Using this assay, indole-3-acetic acid was detected at 5 x 10(-8) molar, whereas trans-zeatin could be detected at 5 x 10(-11) molar. The assay can be performed in microtiter plates, allowing numerous samples to be analyzed simultaneously. Only 2.5 x 10(5) protoplasts are required for one individual assay in 250 microliters of culture medium and for qualitative results, the reaction is readily visualized by ultraviolet light.
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Boerjan et al. (1992) studied this question.