Phylogenetic analysis and seasonal cold acclimation-associated expression of early light-induced protein genes of Rhododendron catawbiense

The early light-induced proteins (ELIPs) are nuclear-encoded, light stress-induced proteins located in thylakoid membranes and related to light-harvesting Chl a/b-binding proteins. Recent evidence from physiological and genetic (mutant) studies supports a photoprotective function for ELIPs, particularly when green tissues are exposed to high light intensities at suboptimal temperatures. Broad-leaved evergreens belonging to genus Rhododendron are often exposed to a combination of low temperatures and high light in their natural habitat as the understory plants in deciduous forests and, therefore, are expected to employ photoprotective strategies during overwintering phase. Here we report analysis and characterization of previously identified ELIP expressed sequence tags (ESTs) from winter-collected Rhododendron catawbiense leaves. 5' or 3' rapid amplification of complementary DNA ends (RACEs) coupled with bioinformatic analyses were used to identify seven unique ELIPs from the 40 ESTs and were designated as RcELIP1-RcELIP7. Phylogenetic analysis revealed separate clustering of ELIP homologs from lower plants, monocots and eudicots (including RcELIPs) and further indicated an evolutionary divergence of ELIPs among angiosperms and gymnosperms. To gain insights into the cold acclimation (CA) physiology of rhododendrons, relative and absolute quantitative expression of RcELIPs was examined during seasonal CA of R. catawbiense leaves using real time reverse transcriptase-polymerase chain reaction. All seven RcELIPs were distinctly upregulated during the CA. It is postulated that RcELIPs expression constitutes an adaptive response to cold and high light in winter-adapted rhododendron leaves and perhaps plays a key role in the protection of photosynthetic apparatus from these stresses.