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Characterization of LI818-like genes under various stress conditions in the marine diatom Thalassiosira pseudonana Zhu, Songhua

Abstract

The diatom Thalassiosira pseudonana has genes for 32 members of the Light-harvesting Complex (LHC) superfamily. Within this superfamily, Lhcx1, Lhcx2, Lhcx4, Lhcx5 and Lhcx6 are found to be highly related to LI818 genes in green algae. Since the gene for PsbS is missing, which is crucial for non-photochemical quenching (NPQ) in higher plants, I am investigating the possibility that one or more of the five LI818-like proteins could be substituting for PsbS in responding to high light (HL) in T. pseudonana. Four LI818-like transcripts (Lhcx1/2/4/6) were transiently accumulated upon HL, suggesting that they were high light inducible genes. However, the level of Lhcx1 protein doubled after 1 h of HL and remained at elevated levels once induced. In parallel, the effect of HL on photophysiological parameters was also examined. After exposure to HL, NPQ was induced rapidly, and reached a maximum after 1 h, and stayed at the constant high level over the rest of HL period. There was an abrupt rise in diatoxanthin within few minutes, followed by a continuous accumulation over the remainder of HL period, suggesting its potential photoprotection role during HL stress. Altogether, the high level of NPQ is accompanied with upregulated Lhcx1 protein and a continuous increase of diatoxanthin after 1 h of HL, suggesting that Lhcx1 may play a role in thermal energy dissipation (NPQ) or it could provide increased stability to the thylakoid membrane assembly during HL. The abundances of most of LI818-like transcripts were down-regulated under iron deficiency. In contrast, Lhcx1 protein was upregulated under iron deficiency, suggesting this gene is independently transcriptionally and translationally regulated. However, copper starvation had less effect on the expression of all Lhc genes relative to iron deprivation, suggesting that iron plays a key role in regulating the expression of Lhc genes. Unlike D1 protein, several PSI subunits were substantially reduced under iron deficiency, demonstrating that PSI reaction center was more affected by iron limitation. My data revealed that the accumulation of Lhcx1 is accompanied by the degradation of PSI proteins under iron limitation, suggesting that Lhcx1 could be involved in the remodeling of PSI.

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