Photoreduction of protochlorophyllide and its relationship to δ-aminolaevulinic acid synthesis in the leaves of dark-grown barley (Hordeum vulgare) seedlings

The photoreduction of protochlorophyllide (Pchl) in dark-grown leaves of barley (Hordeum vulgare) brings about the synthesis of delta-aminolaevulinic acid (AmLev). Manipulation of the Pchl level in the leaves by incubation in AmLev indicated that the production of AmLev was intimately related to the state of the Pchl reductase ternary complex. Free Pchl reductase that is unassociated with substrate/product appeared at first to be essential for the photoinduction of AmLev synthesis. Experiments on the photoreduction of Pchl in dark-grown leaves exposed to low-energy red-light, however, showed that photoreduction and AmLev synthesis would occur when the Pchl reductase, together with substrate, was maintained at relatively high endogenous concentration. Under such conditions the availability of free reductase protein would be negligible. An alternative scheme is presented, therefore, that can explain many, if not all, of the observations on AMLev synthesis and its close relationship to Pchl reduction, and which is based on a common supply of NADPH for the reduction of glutamate to AmLev and the synthesis of chorophyll(-ide).