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Background: The initial stage of the biosynthesis of steroid hormones in animals occurs in the mitochondria of steroidogenic tissues, where cytochrome P450SCC (CYP11A1) encoded by the CYP11A1 gene catalyzes the conversion of cholesterol into pregnenolone – the general precursor of all the steroid hormones, starting with progesterone. This stage is missing in plants where mitochondrial cytochromes P450 (the mito CYP clan) have not been found. Generating transgenic plants with a mitochondrial type P450 from animals would offer an interesting option to verify whether plant mitochondria could serve as another site of P450 monooxygenase reaction for the steroid hormones biosynthesis. Results: For a more detailed comparison of steroidogenic systems of Plantae and Animalia, we have created and studied transgenic tobacco and tomato plants efficiently expressing mammalian CYP11A1 cDNA. The detailed phenotypic characterization of plants obtained has shown that through four generations studied, the transgenic tobacco plants have reduced a period of vegetative development (early flowering and maturation of bolls), enlarged biomass and increased productivity (quantity and quality of seeds) as compared to the only empty-vector containing or wild type plants. Moreover, the CYP11A1 transgenic plants show resistance to such fungal pathogen as Botrytis cinerea. Similar valuable phenotypes (the accelerated course of ontogenesis and/or stress resistance) are also visible in two clearly distinct transgenic tomato lines expressing CYP11A1 cDNA: one line (No. 4) has an accelerated rate of vegetative development, while the other (No. 7) has enhanced immunity to abiotic and biotic stresses. The progesterone level in transgenic tobacco and tomato leaves is 3-5 times higher than in the control plants of the wild type. Conclusions: For the first time, we could show the compatibility in vivo of even themost specific components of the systems of biosynthesis of steroid hormones in Plantae and Animalia. The hypothesis is proposed and substantiated that the formation of the above-noted special phenotypes of transgenic plants expressing mammalian CYP11A1 cDNA is due to the increased biosynthesis of progesterone that can be considered as a very ancient bioregulator of plant cells and the first real hormone common to plants and animals. [ABSTRACT FROM AUTHOR]