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 chlorophyl] accumulation in a concentration-dependent manner and led to the accumulation of excessive amounts of uroporphyrin(ogen) III (URO[gen]) and to a minor extent of heptacarboxylporphyrin(ogen). When dark-grown leaves were incubated with CsCl, accumulation of URO(gen) was observed only after feeding of the tetrapyrrole precursor 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 extracts of CsCl-incubated (15 mM) versus control leaves. UROD activity was drastically decreased upon CsCl treatment in leaves incubated in the dark or in the light (44 and 86%, respectively). Selected preceding enzymes of the tetrapyrrole biosynthetic pathway, 5-aminolevulinic acid 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 incubation conditions (15 mM CsCl). Furthermore, the ALA synthesizing capacity did not differ in leaves incubated with and without Cs⁻ cations. UROD activity of crude homogenates from control plants and after partial purification was reduced to 56 and 80%, respectively, upon addition of 10 mM CsCl. Equal concentrations of KCl were not inhibitory. Enzyme 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 visible cytotoxic effect of CsCl appeared to be a selective inhibition of UROD resulting in accumulation of photosensitizing URO (gen). Consequences of the diminished UROD activity on early steps of the tetrapyrrole biosynthesis and its functional and regulatory significance for the porphyrin synthesis are discussed.