Revisiting the early light-induced protein hypothesis in the sustained thermal dissipation mechanism in yew leaves

Overwintering evergreen trees in boreal regions continuously convert absorbed light energy into heat through a process known as 'sustained thermal dissipation'. To better understand this mechanism, this study examined the alterations in the photosynthetic apparatus and transcriptomes of yew (Taxus cuspidata) leaves throughout the year, comparing sun-exposed and shaded leaves. The Y(II) parameter, conventionally used to estimate the quantum yield of photosystem II (PSII), suggests the occurrence of temperature-dependent thermal dissipation during winter. On the other hand, the levels of photosystem subunits, including the D1 subunit of the PSII reaction center, remain relatively stable year-round, suggesting that typical photoinhibition is unlikely to occur. Time-resolved chlorophyll fluorescence analysis revealed that heat dissipation at the PSII antenna is prominent in winter. Winter transcriptomes are notably characterized by a predominance of Elip transcripts encoding early light-induced protein (ELIP), which constitute 20% of the total transcripts, as deduced from RNA-seq analysis. Furthermore, ELIP protein concentration increases to nearly half that of the major light-harvesting complexes. The predicted structure of ELIP includes potential chlorophyll a and carotenoid binding sites. Considering a previous report showing ELIP's capacity for energy dissipation, these findings lead to a reevaluation of its significant role in sustained thermal dissipation.