Transformation of protochlorophyllide to chlorophyllide in wheat under heavy metal stress

摘要

Among various heavy-metal pollutants the most phytotoxic agents are Cd, Cr, Fe, Pb, Cu and Zn. Ions of these metals present in the environment can be the reason of a reduced plant growth, permanent damage of cells, tissues and organs and they can eventually lead to death. Heavy metals affect biosynthesis of photosynthetic pigments and assembly of the photosynthetic apparatus, which frequently results in chlorosis. There is growing evidence on the influence of heavy metals on particular steps of chlorophyll (Chl) biosynthesis. However, relatively little is known about their effect on protochlorophyllide (Pchlide) to chlorophyllide (Chlide) photoreduction. In angiosperms growing in darkness, biosynthesis of Chl is halted at the step of Pchlide formation (Sundqvist and Dahlin 1997; Schoefs 1999). The reduction of Pchlide to Chlide is triggered by light. This reaction is catalysed by the nuclear-encoded enzyme NADPH:protochlorophyllide oxidoreductase (POR). In etiolated plants, Pchlide is accumulatedin the etioplast inner membranes (EPIM), as a spectrally inhomogeneous pool. Four spectral forms of Pchlide were found having absorption maxima at 630, 645, 650, 670 nm (Boddi et al. 1992). Only the Pchlide associated with POR, having the 77 K fluorescence maximum at 655-657 nm can be reduced upon light. The photoinactive Pchlide has a fluorescence maximum at 633 nm.