Evidence of Plastid Control of Abscisic Acid Accumulation in Barley (Hordeum vulgare L.)

Two treatments affecting the functioning of barley (Hordeum vulgare L.) plastids were used to study the localisation and control of ABA accumulation in response to partial dehydration. The X-ray-induced barley mutant albostrians was used to compare ABA accumulation in green, white and striped mutant leaves, and in green leaves of albostrians which had been treated with the herbicide norflurazon. NF inhibits carotenoid synthesis and in high light results in photo-bleaching of chlorophyll.

ABA accumulation was severely inhibited in completely white mutant leaves, in white portions of striped leaves, and in NF-treated leaves which had been photo-bleached. These leaves also had much lower stomatal conductances than those of green leaves, and plastid ribosomes were completely absent. NF had no effect on ABA accumulation in leaves grown in the dark, in low light (where NF reduced carotenoid but not chlorophyll levels) and in high light prior to the onset of photo-bleaching. In all these treatments plastid ribosomes were still present. Although hardly any ABA accumulation took place in white leaves, the presence of low levels of ABA in these leaves was indicated by UV isomerisation of assay samples.

It was concluded firstly, that protein synthesis on plastid ribosomes was necessary at some stage for drought stress to elicit a rise in ABA synthesis, and secondly, that enzymes for every step in the synthesis of ABA are encoded on nuclear DNA.