Light-induced betacyanin and flavonol accumulation in bladder cells of Mesembryanthemum crystallinum
Introduction
The halophyte Mesembryanthemum crystallinum L. (ice plant) (Aizoaceae) has been established as a model system to study osmotic stress effects (Bohnert et al., 1988, Bohnert and Jensen, 1996). Salinity and drought, but also low temperatures, initiate a complex network of hormonal and transcriptional responses leading to induction and/or repression of gene expression (Cushman, Vernon, & Bohnert, 1993). One of the most dramatic responses is the transition from C3 photosynthesis to the Crassulacean acid metabolism (Winter, 1973).
Up to now, little attention has been paid to stress-induced changes of the ice plant's secondary metabolism, which shares a unique feature with members of most families of the plant order Caryophyllales with regard to tissue pigmentation: the accumulation of betalains instead of anthocyanins (Stafford, 1994). Betalain-producing plants are unable to convert flavan-3,4-diols to anthocyanidins. Instead, they convert tyrosine via Dopa to the building blocks of the red–violet betacyanins and yellow betaxanthins (Steglich & Strack, 1990). While the function of these pigments in flower and fruit coloration might be obvious, their role in pigmentation of vegetative tissues is unknown. Fig. 1 shows the structure of betanidin and a typical anthocyanidin, cyanidin. Both are aglycones of various glycosylated structures and their acylated forms.
While there is extensive research on flavonoid biosynthesis, there is limited information on betalain biosynthesis in higher plants. Joy IV, Sugiyama, Fukuda, & Komamine (1995) describe polyphenol oxidase cDNAs of Phytolacca americana coding for tyrosinases and speculated on its possible involvement in betalain biosynthesis. A tyrosinase as part of the betalain biosynthetic pathway of Portulaca grandiflora has recently been characterized (Steiner, Schliemann, Böhm, & Strack, 1999). By using the technique of particle bombardment, Mueller, Hinz, Uzé, Sautter, & Zryd (1997) were able to show expression of a cDNA, encoding Amanita muscaria Dopa dioxygenase, in white petals of Portulaca grandiflora. Vogt, Zimmermann, Grimm, Meyer, & Strack (1997) purified betanidin glucosyltransferases from Dorotheanthus bellidiformis and discussed on the basis of substrate specificity their phylogenetic relationship with flavonoid glucosyltransferases. Finally, the formation of betacyanins acylated with hydroxycinnamic acids has been reported to be catalyzed by 1-O-hydroxycinnamoyl-β-glucose-dependent hydroxycinnamoyltransferases (Bokern, Heuer, & Strack, 1992).
The occurrence of flavonoids and betalains within the same tissue requires a close coordination of the arogenate-derived phenylalanine and tyrosine pathway branches. This should preferably be studied in intact plant systems exhibiting simultaneous induction of flavonoids and betalains in synchronously responding cells. In this respect the epidermal bladder cells of M. crystallinum are an ideal system.
With this report we introduce M. crystallinum as a model system for molecular studies on betalain biosynthesis. We describe the effects of high intensities of white light, the effect of UV-light and the application of 2-aminoindan 2-phosphonic acid (AIP) (Zon & Amrhein, 1992), an effective inhibitor of phenylalanine ammonia-lyase (PAL; EC 4.1.3.5).
Section snippets
Induction of betacyanin and flavonoid accumulation
In initial experiments with adult plants of the halophyte M. crystallinum, betacyanins, mainly betanin (betanidin 5-O-glucoside), were detected in ripening fruits, as has been shown for other Mesembryanthemum species (Piattelli & Minale, 1964). When the plants were grown in the presence of 1 M NaCl or exposed to drought, fruit ripening and pigmentation of the fruits was markedly stimulated (data not shown). No betacyanin accumulated in non-flowering young plants (6- to 10-week-old) in response
Plant material and growth conditions
Mesembryanthemum crystallinum L. (ice plant) was grown from seeds for 10 weeks in the greenhouse at a light intensity of 100–200 μE (sodium vapor lights) for a 16/8-h 25–30/21–22°C day/night cycle. Treatment of these plants with high intensities of white light was performed in a phytotron for a 18/6-h 30/20°C day/night cycle. Light sources were Osram mercury lamps (KHD 250 HD) with 500 μE or Philips Powerstar (HQI T 250/D) with 1100 μE supplemented with an additional UV-source [Osram Eversun L
Acknowledgements
The authors thank R. Schmitz (Köln, Germany) for his suggestion to use Folanorm™ and analysis of the UV-absorbing properties of this material, M. Kiess (Braunschweig, Germany) for amino acid analysis, N. Amrhein (Zürich, Switzerland) for kindly providing AIP, and H. Bohnert (Tucson, USA) for M. crystallinum seeds. This work was supported by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie.
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