INFLUENCE OF CESIUM ON TETRAPYRROLE BIOSYNTHESIS IN ETIOLATED AND GREENING BARLEY LEAVES

Cesium chloride (CsCl) treatment of greening primary leaves of barley for 8 h inhibited chlorophyll accumulation in a concentration-dependen t manner and led to the accumulation of excessive amounts of uroporphy rin(ogen) III (URO[gen]) and to a minor extent of heptacarboxylporphyr in(ogen). When dark-grown leaves were incubated with CsCl, accumulatio n of URO(gen) was observed only after feeding of the tetrapyrrole prec ursor 5-aminolevulinic acid. Western blot analysis showed no apparent difference in content of uroporphyrinogen decarboxylase (EC 4.1.1.37, UROD) or selected proteins involved in tetrapyrrole biosynthesis in ex tracts of CsCl-incubated (15 mM) versus control leaves. UROD activity was drastically decreased upon CsCl treatment in leaves incubated in t he dark or in the light (44 and 86%, respectively). Selected preceding enzymes of the tetrapyrrole biosynthetic pathway, 5-aminolevulinic ac id dehydratase (EC 4.2.1.24, ALAD) and porphobilinogen deaminase (EC 4 .3.1.8, PBGD), were influenced only to a minor extent under standard i ncubation conditions (15 mM CsCl). Furthermore, the ALA synthesizing c apacity did not differ in leaves incubated with and without Cs+ cation s. UROD activity of crude homogenates from control plants and after pa rtial purification was reduced to 56 and 80%, respectively, upon addit ion of 10 mM CsCl. Equal concentrations of KCl were not inhibitory. En zyme assays of the same barley extract in the presence of CsCl yielded no effect on ALAD and a minor loss of PBGD activity. The initial visi ble cytotoxic effect of CsCl appeared to be a selective inhibition of UROD resulting in accumulation of photosensitizing URO(gen). Consequen ces of the diminished UROD activity on early steps of the tetrapyrrole biosynthesis and its functional and regulatory significance for the p orphyrin synthesis are discussed.