Developmental and circadian control of the capacity for delta-aminolevulinic acid synthesis in green barley

The synthesis of delta-aminolevulinic acid (delta-ALA) is a key step in the regulation of tetrapyrrole synthesis. To study the developmentally and circadian-clock controlled mechanism that co-ordinates synthesis of chlorophylls and chlorophyll-binding proteins, delta-ALA-synthesising capacity was analysed in barley (Hordeum vulgare L.) primary leaves grown under dark/light or constant light conditions. The delta-ALA-forming activity oscillated within 24 h with a aximum at the transition of dark to light and a minimum 12 h later, indicating the involvement of the circadian oscillator during development. The capacity for delta-ALA synthesis increased transiently in the middle of barley primary leaves. The delta-ALA-forming-activity correlated well with the previously published stead-state level of mRNA for light-harvesting chlorophyll-binding proteins in space and time; this supports the view of a co-ordinate synthesis of chlorophyll and pigment-binding proteins. Steady-state levels of mRNAs encoding the three enzymes of the delta-ALA-synthesising pathway and of proteins for glutamyl-tRNA reductase (GluTR) and glutamate 1-semialdehyde aminotransferase (GSA AT; EC 5.4.3.8) were analysed for their developmental and circadian expression in barley leaves. The contents of GluTR and mRNA and protein cycled parallel to the changes in delta-ALA-forming activity. The levels of GSA AT mRNA oscillated in an opposite phase, but the protein content did not show substantial oscillation under diurnal and circadian growth conditions. No circadian oscillation was detected for glytamyl tRNA synthase (GluRS; EC 6.1.1.17). Maximal GluTR mRNA content and protein was observed in the middle (segments 3 and 4) of the barley primary leaves. The developmentally controlled expression of GluTR therefore differs from that of GSA AT and GluRS, but resembles the capacity for d delta-ALA synthesis in a barley leaf gradient. These data indicate that the oscillating, light-dependent and spatial expression of GluTR mRNA might contribute to the regulated formation of the chlorophyll precursor delta-ALA.