Expression of genes for chloroplast proteins in pine yellow cotyledons grown at low temperature

Abstract

Cotyledons of most gymnosperms synthesize chlorophylls in complete darkness at 25°C under standard germination conditions and differentiated stromal and granal lamellae are evident in their chloroplasts. The protein components of the two photosystems and the Cyt b/f complex are expressed and accumulate in the thylakoids of dark-grown cotyledons but the O2-evolving enzyme remains latent. Genes for chloroplast proteins are expressed in the dark-grown cotyledons of gymnosperms. Cotyledons of Japanese black pine (Pinus thunbergii) were yellow when they developed in darkness at 8°C since the light-independent synthesis of chlorophyll was almost completely inhibited in these cotyledons. The level of chlorophyll in dark-grown cotyledons was less than one-twentieth of that in light-grown cotyledons at the same temperature. In the yellow cotyledons, levels of transcripts of cab, rbcS, rbcL and psbA genes were quite high. The large and small subunits of ribulose-1,5-bisphosphate carboxylase/oxygenase were also detected at relatively high levels in yellow cotyledons. However, the accumulation of the two apoproteins of the light-harvesting chlorophyll a/b-binding protein of PSII was limited because of the limited supply of chlorophyll. In conclusion, light and chlorophyll appear not to be necessary for the expression of some members of cab and rbcS in the nuclear genome and the expression of rbcL and psbA in the chloroplast genome in pine cotyledons