Research School of Biology, The Australian National University, Canberra ACT 2601, Australia
Phosphatidylcholine is a major phospholipid of cellular membranes. Its biosynthetic pathway is complex but in plants the sole entry point for de novo phosphatidylcholine synthesis is thought to be the conversion of phosphoethanolamine to phosphomethylethanolamine catalysed by phosphoethanolamine N-methyltransferase (NMT). The Arabidopsis genome is predicted to encode three NMTs. One of these enzymes (NMT1) has been characterized1 and confirmed to catalyse all three methylations from phosphoethanolamine to phosphocholine, while a second one was reported to only catalyse the last two methylations from phosphomethylethanolamine to phosphocholine, and hence re-annotated as a PMEAMT2. We will report on the biochemical and in planta function of the third enzyme (NMT3), which was so far unknown. Similar to NMT1, NMT3 restores the growth of a yeast choline-auxotroph strain, and suppresses the root defects of the Arabidopsis xipot1/nmt1 loss-of-function mutant, when driven by NMT1 promoter. However, contrary to that mutant, the nmt3 knock-out mutant is phenotypically similar to wild-type under normal growth conditions. In stark contrast, the nmt1 nmt3 double mutant shows drastically reduced growth of both root and rosette, delayed bolting and a bushy, ball-like inflorescence, with dramatically reduced fertility. Loss of both NMTs impacts on cell proliferation and expansion, impairs the development of male and female floral organs, and also affects embryogenesis. Our results demonstrate unequal contributions of the two NMTs in these phenotypes, with gene dosage effects, and also specificity brought about by non-totally overlapping expression patterns. They highlight the physiological importance of the methylation pathway for sustaining plant growth and reproduction.
References: 1. Cruz-Ramírez et al. (2004), Plant Cell: 16, 2020-2034. 2. BeGora et al. (2010) JBC, 285: 29147-29155.