A Methyltransferase Trio Essential for Phosphatidylcholine Biosynthesis and Growth

Abstract

Phosphatidylcholine (PC) is a primary class of membrane lipids in most eukaryotes. In plants, the primary PC biosynthetic pathway and its role in plant growth and development remain elusive due to lack of a mutant model with substantially decreased PC content. Recently, a double mutant of Arabidopsis (Arabidopsis thaliana) PHOSPHO-BASE N-METHYLTRANSFERASE 1 (PMT1) and PMT3 was reported with reduced PC content and defective plant growth. However, residual PC content as well as the nonlethal phenotype of the mutant suggests an additional enzyme contributes to PC biosynthesis. In this article, we report on the role of three PMTs in PC biosynthesis and plant development, with a focus on PMT2. PMT2 had the highest expression level among the three PMTs, and it was highly expressed in roots. The pmt1 pmt2 double mutant enhanced the defects in root growth, cell viability, and PC content of pmt1, suggesting that PMT2 functions together with PMT1 in roots. Chemical inhibition of PMT activity in wild-type roots reproduced the short root phenotype observed in pmt1 pmt2, suggesting that PMT1 and PMT2 are the major PMT isoforms in roots. In shoots, pmt1 pmt2 pmt3 enhanced the phenotype of pmt1 pmt3, showing seedling lethality and further reduced PC content without detectable de novo PC biosynthesis. These results suggest that PMTs catalyze an essential reaction step in PC biosynthesis and that the three PMTs have differential tissue-specific functions in PC biosynthesis and plant growth.

Non‐specific phospholipases C, NPC2 and NPC6, are required for root growth in Arabidopsis

“…DAG is readily converted by DAG kinase to produce PA, a representative lipid second messenger in plants, but the involvement of a specific diacylglycerol kinase (DGK) or PA signal in root growth remains unclear. However, PCho is known to be involved in root growth because knock‐out of PMT1, a primary enzyme that produces PCho from PEtn conferred reduced PCho level and root length, which were rescued by exogenous PCho supplementation (Cruz‐Ramírez et al ., ; Liu et al ., ). Thus, we tested whether the reduced root growth in the knock‐down mutant lines was rescued by exogenous PCho supplementation.…”

“…Previous reports showed that mutation in PMT1 but not PMT2 or PMT3 affected root growth (Cruz‐Ramírez et al ., ; Lee and Jez, ; Liu et al ., ), and additional mutation in PMT2 further shortens the root length of pmt1 mutant (Liu et al ., ). PCho supplementation to these mutants ( pmt1 and pmt1 pmt2 ) rescued the root length but did not affect the wild type (Liu et al ., ). The short root phenotype in our knock‐down mutant lines of NPC2 and NPC6 was also rescued by PCho; hence NPC2 and NPC6 may function in roots to hydrolyze PC to produce PCho for their growth.…”

“…Thus, we propose that two concurrent pathways catalyzed by NPCs (NPC2 and NPC6) and PMTs (PMT1 and PMT2) produce PCho to maintain root growth (Figure c). NPC2 and NPC6 may play a redundant role in the NPC pathway, whereas PMT1 may play a more dominant role than PMT2 in the PMT pathway because the pmt1 mutant showed a shorter root length and pmt2 enhanced the root phenotype only in a pmt1 background (Cruz‐Ramírez et al ., ; Liu et al ., ). We highlight an emerging function for NPC2 and NPC6 in roots to promote growth, a distinct role from what was previously assessed in gametogenesis (Ngo et al ., ).…”

“…We highlight an emerging function for NPC2 and NPC6 in roots to promote growth, a distinct role from what was previously assessed in gametogenesis (Ngo et al ., ). The role of PCho in root growth remains enigmatic (Cruz‐Ramírez et al ., ; Chen et al ., ; Liu et al ., , ; Chen et al ., ). In mammalian cells, PCho is known to promote cell proliferation, and thus is used as a molecular marker for cancer cells (Eliyahu et al ., ).…”

“…Intriguingly, the root phenotypes were rescued by exogenous supplementation of PCho. In plants, PCho is synthesized also from phosphoethanolamine (PEtn) by three copies of phospho‐base N ‐methyltransferases (PMTs), and mutation in PMT1 produces a short root phenotype that can be complemented by PCho (Cruz‐Ramírez et al ., ; Liu et al ., , ). Here, we found that only PMT1 was significantly upregulated in the knock‐down mutant lines, which was attenuated by PCho supplementation.…”