Monochromatic green light induces an aberrant accumulation of geranylgeranyled chlorophylls in plants.

ABSTRACT

Light quality is an important environmental factor affecting the biosynthesis of photosynthetic pigments whose production seems to be affected not only quantitatively but also qualitatively. In this work, we set out to identify unusual pigment detected in leaves of barley (Hordeum vulgare L.) and explain its presence in plants grown under monochromatic green light (GL; 500-590 nm). The chromatographic analysis (HPLC-DAD) revealed that a peak belonging to this unknown pigment is eluted between chlorophyll (Chl) a and b. This pigment exhibited the same absorption spectrum and fluorescence excitation and emission spectra as Chl a. It was negligible in control plants cultivated under white light of the same irradiance (photosynthetic photon flux density of 240 µmol m-2 s-1). Mass spectrometry analysis of this pigment (ions m/z = 889 [M-H]-; m/z = 949 [M+acetic acid-H]-) indicates that it is Chl a with a tetrahydrogengeranylgeraniol side chain (containing two double bonds in a phytyl side chain; Chl aTHGG), which is an intermediate in Chl a synthesis. In plants grown under GL, the proportion of Chl aTHGG to total Chl content rose to approximately 8% and 16% after 7 and 14 days of cultivation, respectively. Surprisingly, plants cultivated under GL exhibited drastically increased concentration of the enzyme geranylgeranyl reductase, which is responsible for the reduction of phytyl chain double bonds in the Chl synthesis pathway. This indicates impaired activity of this enzyme in GL-grown plants. A similar effect of GL on Chl synthesis was observed for distinct higher plant species.