Mechanism of Benzyladenine-Induced Stimulation of the Synthesis of 5-Aminolevulinic Acid in Greening Cucumber Cotyledons: Benzyladenine Increases Levels of Plastid tRNA<sup>Glu</sup>

The mechanism of the stimulatory effect of a cytokinin, namely, benzyladenine (BA), on the synthesis of 5-aminolevulinic acid (ALA) in cucumber cotyledons was studied. The rate of synthesis of ALA by plastids isolated from BA-treated cotyledons was twice that by plastids from untreated controls. Western blot analysis of stromal proteins showed that BA did not affect the level of glutamyl-tRNA synthetase or of glutamate l-semialdehyde (GSA) aminotransferase. Analysis of free amino acids revealed that BA did not increase the level of glutamate in the stroma. However, the amount of total plastidic RNA was doubled in BA-treated cotyledons. Northern blot analysis showed that the level of plastid tRNAGlu was increased by treatment with BA to the same extent as that of another plastid tRNA, reflecting an increase in total plastidic RNA. The rate of formation of glutamyl-tRNA was also doubled in plastids from BA-treated cotyledons. The results indicate that stimulation of the synthesis of ALA by BA is due to an increased level of tRNAGlu in plastids.