Effects of gabaculine on 5-aminolevulinic acid dehydratase during development of bean chloroplasts

Abstract

Etiolated bean seedlings ( Phaseolus vulgaris, L., 8-day-old), were treated with 0, 10, 15 and 30 μM gabaculine (GAB) for 10 h and grown under light or dark conditions to study the changes in 5-aminolevulinic dehydratase (ALA D) activity of leaves and plastids. The activity of ALA D in bean leaves developed rapidly in light to reach a maximum of about 2.7-fold of that obtained in the dark, after 60 h of illumination. The corresponding activity in chloroplasts or etioplasts (about 50% of that in the leaves) was clearly reduced upon GAB treatment and correlated with the concentrations used. Inhibition amounting to 50% and 30% of the control values was recorded at 15 μM GAB after two days in light and dark, respectively. The effect of 25 h of treatment with 15 μM GAB in light in combination with 5 mM ALA, using 7- and 9-day-old etiolated seedlings, resulted in a clear reduction of the inhibitory effect of GAB on ALA D activity in leaves. ALA accumulation was also increased compared with the untreated control. These changes were not observed when 4 mM glutamate-1-semialdehyde (GSA) or 2 mM 4,5-dioxovalerate (DOVA) were used instead of ALA. In most cases, however, less pronounced inhibitory effects of GAB on both parameters were obtained with older leaves. The ultrastructure of the chloroplasts was clearly affected by GAB especially at 30 pM resulting in a more rounded organelle with both stroma and grana appearing as a part of a circular arrangement. Also number and size of grana were greatly reduced at 10 and 30 μM GAB. Additionally, the thylakoid membranes at 10 μM GAB appeared to be of the stroma type and were generally reduced in number. These results indicate that GAB inhibits the generation of ALA D activity in addition to its role in limiting ALA synthesis.