Specific induction of TaAAPT1, an ER- and Golgi-localized ECPT-type aminoalcoholphosphotransferase, results in preferential accumulation of the phosphatidylethanolamine membrane phospholipid during cold acclimation in wheat

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Abstract
Cold acclimation requires substantial alteration in membrane property. In contrast to well-documented fatty acid unsaturation during cold acclimation, changes in phospholipid biosynthesis during cold acclimation are less understood. Here, we isolated and characterized two aminoalcoholphosphotransferase (AAPT) cDNAs, TaAAPT1 and TaAAPT2, from wheat. AAPTs utilize diacylglycerols and CDP-choline/ethanolamine as substrates and catalyze the final step of the CDP-choline/ethanolamine pathway for phosphatidylcholine (PC)/phosphatidylethanolamine (PE) synthesis, respectively. Functionality of TaAAPT1 and TaAAPT2 was demonstrated by heterologous expression in a yeast cpt1Δ ept1Δ double mutant that lacks both AAPT activities. Detailed characterization of AAPT activities from the transformed mutant cells indicated that TaAAPT1 is an ECPT-type enzyme with higher ethanolamine phosphotransferase (EPT) activity than choline phosphotransferase (CPT) activity, while TaAAPT2 is a CEPT-type with the opposite substrate preference. Transient expression of GFP-fused TaAAPT1 and TaAAPT2 proteins in wheat and onion cells indicated they are localized to both the endoplasmic reticulum and Golgi apparatus, suggesting that the final synthesis of PE and PC via the CDP-choline/ethanolamine pathway occurs in these organella. Quantitative PCR analyses revealed that TaAAPT1 expression is strongly induced by cold, while TaAAPT2 was constitutively expressed at lower levels. Measurement of phospholipid content in wheat leaves indicated that PE is more prominently increased in response to cold than PC and accordingly PE/PC ratio increased from 0.385 to 0.530 during 14 days of cold acclimation. Together, these data suggested that an increase in the PE/PC ratio during cold acclimation is regulated at the final step of the biosynthetic pathway.

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