Elicitor Induction of Cytochrome P-450

Elicitor Induction of Cytochrome P-450 Monooxygenases in Cell Suspension Cultures of Chickpea (Cicer arietinum L.) and Their Involvement in Pterocarpan Phytoalexin Biosynthesis Werner Gunia, Walter Hinderer*, Uta Wittkampf, and Wolfgang Barz Lehrstuhl für Biochemie der Pflanzen, Westfälische Wilhelms-Universität Münster, Hindenburgplatz 55, D-4400 Münster, Bundesrepublik Deutschland * Biotest Pharma, D-6072 Dreieich, Bundesrepublik Deutschland Z. Naturforsch. 46c, 58-66 (1991); received October 29, 1990 Cicer arietinum, Yeast Elicitor, Cytochrome P-450 Monooxygenases, Pterocarpan Phytoalexins, Microsomes A yeast glucan elicitor causes the accumulation of the pterocarpan phytoalexins medicarpin and maackiain in chickpea (Cicer arietinum) cell suspension cultures established from seeds. A cell culture line from a chickpea cultivar resistant against its main fungal pathogen Asco- chyta rabiei accumulates large amounts (944 nmol/g fr. wt.) whereas a cell culture line from a susceptible cultivar accumulates only low amounts (38 nmol/g fr. wt.) of the phytoalexins. This is consistent with differential accumulation of pterocarpan phytoalexins in intact plants [1], The first reactions in the pterocarpan-specific branch of biosynthesis are hydroxylation of the isoflavone intermediate formononetin in position 2' or 3', catalyzed by microsomal cyto chrome P-450 monooxygenases. Upon elicitation formononetin 2'-hydroxylase undergoes a strong transient induction in the cell suspension culture of the resistant cultivar, whereas in the cell culture from the susceptible cultivar it is only slightly induced. In both cell suspension cul tures the induction of cinnamic acid 4-hydroxylase and of formononetin 3'-hydroxylase does not show a clear correlation with phytoalexin accumulation. Experiments with different elici tor concentrations confirm that formononetin 2'-hydroxylase is much more induced in cell cul tures from the resistant cultivar than from the susceptible one. It is concluded that the massive difference in phytoalexin accumulation between cell suspension cultures from the resistant and susceptible cultivar is determined mainly by the differential induction of formononetin 2'-hydroxylase activity. Introduction biosynthesis [8], co-hydroxylation of fatty acids [9] Cytochrome P-450 monooxygenases (EC as well as in pesticide and herbicide metabolism 1.14.14.1) are widely distributed in animals, plants [10, 11]. In plant secondary product metabolism and microorganisms. In eukaryots these enzymes they catalyze reactions in the general phenylpropa- are localized in microsomes where these mem noid pathway [12] and in the biosynthesis of car- brane proteins occur together with cytochrome denolides [13], monoterpene alkaloids [14, 15] and P-450 reductases (EC 1.6.2.4). The structure of the flavonoids [16, 17], plant cytochrome P-450 system is highly analo In addition, a number of cytochrome P-450 gous to the enzymes from animals [2, 3], However, monooxygenases are involved in phytoalexin bio the plant cytochrome P-450 monooxygenases do synthesis. Examples are psoralen synthase known not possess such a broad substrate specifity as has from the biosynthesis of furanocoumarins in been observed with the hepatic enzymes [4-6], Petroselinum hortense and Ammi majus [18, 19], the dihydroxypterocarpan 6a-hydroxylase in Plant cytochrome P-450 monooxygenases are involved in a number of biochemical processes volved in the biosynthesis of the glyceollins in soy such as biosynthesis of gibberellic acid [7], sterol bean [20] and isoflavone 2'- and 3'-hydroxylases demonstrated in the formation of the pterocarpans medicarpin and maackiain in chickpea (Cicer arie Abbreviations: C4H, cinnamic acid 4-hydroxylase; CO, tinum) [21]. Phytoalexins accumulate in plants carbon monoxide; F2H, formononetin 2'-hydroxylase; F3H, formononetin 3'-hydroxylase; PAL, phenylala after stress, wounding or microbial infection and nine ammonia lyase. are an integral part of plant antimicrobial defence Reprint requests to Prof. Dr. W. Barz. reactions [22, 23]. Verlag der Zeitschrift für Naturforschung, D-7400 Tübingen Medicarpin and maackiain (Fig. 1) are synthe 0939-5075/91/0100-0058 $01.30/0 sized in chickpea via the general phenylpropanoid W. G unia et al. ■ Elicitor Induction of Cytochrome P-450 Monooxygenases in Chickpea 59 OCH- 2'-Hydroxyformononetin 3'-Hydroxy formononetin I Fig. 1. Hydroxylation reactions of the isofla vone intermediate formononetin in the posi tions 2' and 3', catalyzed by microsomal cyto OCH- chrome P-450 monooxygenases as involved in Medicarpin Maackiain medicarpin and maackiain biosynthesis. pathway with the isoflavone formononetin as an ble (ILC 1929) cultivars of chickpea [31] were intermediate [24]. The subsequent enzymes of the grown on a PRL-4c medium [32] with the addition pterocarpan specific branch of phytoalexin forma of 2 mg/ml glycine instead of yeast extract. The tion have been recently detected with isoflavone 2'- culture conditions were those of Keßmann and and 3'-hydroxylase [21], NADPH: isoflavone oxi- Barz [31]. doreductase [25] and pterocarpan synthase [26], respectively. Preparation of elicitor Up to now only limited knowledge is available The preparation of a yeast glucan elicitor was concerning quantitative differences in the induc performed according to Schumacher et al. [33]. tion of phytoalexin biosynthetic enzymes ex The final preparation was lyophilized and stored pressed by plants which are either susceptible or as a dry powder until use. The glucose content of resistant against their natural microbial pathogens the elicitor preparation was determined with the [27, 28]. Earlier investigations had indicated [29, anthrone method [34]. For elicitation an appro 30] that the chickpea cultivars greatly differed in priate amount of this powder was dissolved in the expression of the isoflavone 2'-hydroxylase. water (2 ml), autoclaved for 20 min and applied Therefore, we now report on the differential elici under sterile conditions to the cell cultures at day 3 tor induction of cytochrome P-450 monooxygen of the culture period. ases (i.e. cinnamic acid 4-hydroxylase, formonone tin 2'- and 3'-hydroxylase) in chickpea cell suspen Extraction of phytoalexins sion cultures established from different cultivars which are resistant and susceptible against the Phenolic compounds containing medicarpin natural fungal pathogen Ascochyta rabiei [31]. and maackiain were extracted from the culture me dium [31] and the cells [35] with ethyl acetate. Materials and Methods Preparation of microsomes Cell suspension cultures Microsomes were prepared from cell suspension Cell suspension cultures, derived from seeds of cultures of chickpea according to Hinderer et al. Ascochyta rabiei resistant (ILC 3279) and suscepti [21]. 60 W. G unia et al. • Elicitor Induction of Cytochrome P-450 Monooxygenases in Chickpea Enzyme assays Results Cinnamic acid 4-hydroxylase was assayed in a Upon elicitation cell suspension cultures of total volume of 1 ml and incubated for 30 min at chickpea accumulate the pterocarpan phytoalexins 25 °C. The assay mixture contained 25 to 100 |il medicarpin and maackiain in essentially the same microsomal preparations (0.1 to 0.3 mg protein), manner as intact plants when infected with the 50 (im cinnamic acid (in 25 |il MeOH; Roth, Karls fungal pathogen Ascochyta rabiei [1, 31]. There ruhe), 1 mM NADPH and buffer (100 m M potas fore we used such chickpea cell suspension cultures sium phosphate, pH 7.0, with 400 m M sucrose). to determine biochemical differences between The reaction was terminated and the products resistant and susceptible cultivars. were extracted with ethyl acetate (3><2 ml). The Upon elicitation with yeast extract cell suspen organic phases were collected, dried with a rotary sion cultures from the resistant cultivar ILC 3279 evaporator (30 °C) and redissolved in 150 (il accumulated large amounts of the phytoalexins MeOH. Aliquots of 20 (il were analyzed by HPLC and concomitantly formononetin 2'- and formo methods. Formononetin 2'- and 3'-hydroxylase nonetin 3'-hydroxylase activities were strongly in were determined according to Hinderer et al. [21] duced [21]. In our present study we changed to a with the above mentioned modifications. yeast glucan preparation as a more defined source of elicitor which mainly consists of glucose and mannose residues [33, 38, 39]. The glucose content Chromatographic analyses of our elicitor preparation was determined to be Phytoalexins were separated on a Merck 450 nmol glucose residues per mg dry powder. LiChrosorb RP select B HPLC column (5 |im, In Fig. 2 A the effect of this elicitor preparation 250 x 4 mm i.d.) according to Köster et al. [35, 36]. on phytoalexin accumulation is demonstrated. Separation of substrate and product from the The cell suspension culture from the resistant culti assays for cinnamic acid 4-hydroxylase was var accumulated phytoalexins with a maximum of achieved on a Merck LiChrosher 100 RP select B 944 nmol/g fresh weight (in cells and medium) 12 h HPLC column (5 |im, 250 x 4 mm i.d.). A linear after elicitation. Medicarpin was the predominant gradient was run from 25% to 65% eluent B in phytoalexin as has been previously observed with 12 min (eluent A: 1.5% phosphoric acid; eluent B: an elicitor preparation from the natural pathogen 84% acetonitrile in water) with a flow rate of Ascochyta rabiei [31]. In contrast, cell suspension 0.8 ml/min at room temperature. Cinnamic acid cultures from the susceptible cultivar ILC 1929

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