Somatic embryogenesis of the carrot (Daucus carota L.) depends on a set of factors, some of which accumulate in culture medium (conditioned medium, CM). When embryogenic cell clusters were transferred to an embryo-inducing medium, addition of CM derived from somatic embryo culture markedly stimulated somatic embryo formation. The active principles were purified using a simple bioassay system and identified to be phytosulfokines (PSKs), sulfated oligopeptide growth factors originally isolated from a CM derived from asparagus (Asparagus officinalis L.) mesophyll culture. Quantification studies using a competition ELISA system employing an anti-PSK-alpha polyclonal antibody showed that PSK production might be related to growth of cells, rather than development of somatic embryos. Thus the stimulatory effect of PSK on somatic embryo formation might be due to promotion of cell proliferation.

No abstract

An in vitro assay system to detect tyrosylprotein sulfotransferase (TPST) activity of higher plant cells was established, using synthetic oligopeptides based on the deduced amino acid sequence of a phytosulfokine-K K (PSK-K K) precursor. TPST activity was found in microsomal membrane fractions of rice, asparagus and carrot cells and it was confirmed that acidic amino acid residues adjacent to the tyrosine residues of acceptor peptides were essential to the sulfation reaction. The asparagus TPST exhibited a broad pH optimum of 7.0^8.5, required manganese ions for maximal activity and appeared to be a membrane-bound protein localized in the Golgi apparatus. These enzymes should be defined as a new class of plant sulfotransferases that catalyze tyrosine O-sulfation of a PSK-K K precursor and other unknown proteins.z 2000 Federation of European Biochemical Societies.

No abstract

Phytosulfokine-alpha, a sulfated pentapeptide growth factor universally found in both monocotyledons and dicotyledons, strongly promotes proliferation of plant cells in culture. It is similar to animal polypeptide hormones in that it is processed from a larger precursor, preprophytosulfokine, although the putative processing sites do not conform to consensus sequences for endoproteolytic processing sites flanking animal prohormones. Like the animal preprohormones, preprophytosulfokine also has a signal peptide at the N-terminus for targeting to secretory pathways. The preprophytosulfokine gene has been confirmed to be expressed in vivo as well as in vitro.