Regulation of the carotenoid content and chloroplast development by levulinic acid

Abstract

Chloroplast development and chlorophyll biosynthesis are co‐regulated. To understand the mechanism of regulation of chloroplast biogenesis by chlorophyll, development of the photosynthetic apparatus was monitored during greening of etiolated barley leaf discs in the presence of levulinic acid, an inhibitor of chlorophyll biosynthesis. Although not a direct inhibitor of carotenoid biosynthesis, treatment by levulinic acid resulted in a linear reduction in both chlorophyll and carotenoid contents. Chlorophyll biosynthesis appeared to control that of carotenes. In the presence of levulinic acid, photosystem II (PSII) activity decreased while photosystem I (PSI) activity increased when expressed on a chlorophyll basis. However, the activities of both photosystem I and II decreased when expressed on a per plastid basis. As expected, in the presence of low amounts of chlorophyll, the light‐harvesting chlorophyll‐protein complex II (LHCPII) was not visible in Coomassie‐stained gels in 20 mM levulinic acidtreated tissues, but was detected as a faint band by immunoblotting. This small amount of the LHCPII induced significant amounts of grana stacking, which was monitored as an increase in the ratio of variable to maximum fluorescence. When levulinic acid was washed from the leaf discs and the latter allowed to green in its absence, the chlorophyll and carotenoid contents and the photosynthetic activities approached the control values. Levulinic acid could be used to arrest the light‐induced chloroplast development at a desired phase of greening and removed by washing the leaves to restore the developmental process without any apparent toxic effect. Results demonstrate that biosynthesis of carotenes is regulated by that of chlorophylls and extremely low amounts of the LHCPII can induce grana stacking.