De-etiolation is Almost Color Blind: The Study of Photosynthesis Awakening under Blue and Red Light

The synthesis and assembly of functioning photosynthetic complexes in chloroplasts developing from etioplasts during the de-etiolation of angiosperm seedlings are imperative for the plant's autotrophic lifestyle. This study compared the de-etiolation process under monochromatic red or blue light of equal photon flux density during a 24-h illumination period of etiolated Arabidopsis seedlings. The aim was to elucidate the impact of these light wavelengths on the etioplast-to-chloroplast transformation and the initiation of light-dependent photosynthetic reactions. Both treatments led to the formation of functional young chloroplasts; however, the etioplast-to-chloroplast transition and the assembly of photosynthetic complexes occurred unevenly, with individual steps tuned by red or blue light. Ultrastructural analysis suggested faster prolamellar body's disassembly under blue light, while low-temperature fluorescence studies indicated a slower transformation of protochlorophyllide to chlorophyllide, and chlorophyll a, under these conditions. The red light further promoted the synthesis of chlorophyll b and LHCII antenna proteins. However, the efficiency of antennae in dissipating excess absorbed energy was higher for seedlings de-etiolated under blue light; the maximum quantum yield of the photosystem II reached 0.81 after 24-h de-etiolation, equivalent to mature plants. Blue light seemed to enhance the development of well-functioning photosystems (I and II) and antennae. These findings are important for gaining a deeper understanding of photoreceptor regulation of de-etiolation and for utilizing selected light regimes to improve crop yield.