Mechanism of Benzyladenine-Induced Stimulation of the Synthesis of 5-Aminolevulinic Acid in Greening Cucumber Cotyledons: Benzyladenine Increases Levels of Plastid tRNAGlu

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 synthesisof ALA by plastids isolated from BA-treated cotyledons was twicethat by plastids from untreated controls. Western blot analysisof stromal proteins showed that BA did not affect the levelof glutamyl-tRNA synthetase or of glutamate l-semialdehyde (GSA)aminotransferase. Analysis of free amino acids revealed thatBA did not increase the level of glutamate in the stroma. However,the amount of total plastidic RNA was doubled in BA-treatedcotyledons. Northern blot analysis showed that the level ofplastid tRNA^Glu was increased by treatment with BA to the sameextent as that of another plastid tRNA, reflecting an increasein total plastidic RNA. The rate of formation of glutamyl-tRNAwas also doubled in plastids from BA-treated cotyledons. Theresults indicate that stimulation of the synthesis of ALA byBA is due to an increased level of tRNA^Glu in plastids. (Received June 6, 1993; Accepted November 26, 1993)