Effect of kinetin on early stages of chlorophyll biosynthesis in streptomycin-treated barley seedlings

Treatment of barley seeds (Hordeum vulgare L.) with streptomycin, an inhibitor of plastid protein synthesis, resulted in growth of the albino phenotype seedlings with ribosome-deficient undifferentiated plastids and chlorophyll (Chl) level as low as 0.1% of that in control plant leaves. A major effect of the antibiotic was almost complete suppression of the ability of plants to synthesize 5-aminolevulinic acid (ALA) intended for Chl biosynthesis. The activity of synthesis of ALA intended for heme porphyrin biosynthesis in etiolated and greening seedlings and in light-grown albinophenotype plants was insensitive to light and cytokinins. In the upper parts of leaves of streptomycin-treated plants, exhibiting 60% Chl deficit, the cells with three types of chloroplasts could be observed: normally developed chloroplasts, chloroplasts composed of single thylakoids and grana, and completely undifferentiated plastids. In this Chl-deficient tissue, ALA synthesis was found to be stimulated by kinetin but much less than in leaves of the control plants. The endogenous cytokinin content in etiolated and greening seedlings treated with streptomycin was almost the same as it was in untreated control seedlings. The cytokinin level in the white tissue of plants grown in the light was on average twice as high as that in green leaves of the control plants. The capability of kinetin to stimulate the synthesis of ALA used for Chl biosynthesis was found to correlate with the Chl content and organization of the chloroplast internal structure. This correlation confirms the hypothesis that the normally developed internal structure of plastids is essential for the adequate phytohormone response in plants.