At4g29530 is a phosphoethanolamine phosphatase homologous to PECP1 with a role in flowering time regulation

Abstract

SUMMARY Phosphatidylcholine (PC) and phosphatidylethanolamine (PE) are the most abundant phospholipids in membranes. The biosynthesis of phospholipids occurs mainly via the Kennedy pathway. Recent studies have shown that through this pathway, choline (Cho) moieties are synthesized through the methylation of phosphoethanolamine (PEtn) to phosphocholine (PCho) by phospho‐base N‐methyltransferase. In Arabidopsis thaliana, the phosphoethanolamine/phosphocholine phosphatase1 (PECP1) is described as an enzyme that regulates the synthesis of PCho by decreasing the PEtn level during phosphate starvation to avoid the energy‐consuming methylation step. By homology search, we identified a gene (At4g29530) encoding a putative PECP1 homolog from Arabidopsis with a currently unknown biological function in planta. We found that At4g29530 is not induced by phosphate starvation, and is mainly expressed in leaves and flowers. The analysis of null mutants and overexpression lines revealed that PEtn, rather than PCho, is the substrate in vivo, as in PECP1. Hydrophilic interaction chromatography‐coupled mass spectrometry analysis of head group metabolites shows an increased PEtn level and decreased ethanolamine level in null mutants. At4g29530 null mutants have an early flowering phenotype, which is corroborated by a higher PC/PE ratio. Furthermore, we found an increased PCho level. The choline level was not changed, so the results corroborate that the PEtn‐dependent pathway is the main route for the generation of Cho moieties. We assume that the PEtn‐hydrolyzing enzyme participates in fine‐tuning the metabolic pathway, and helps prevent the energy‐consuming biosynthesis of PCho through the methylation pathway.

Roles of type II H+-PPases and PPsPase1/PECP2 in early developmental stages and PPi homeostasis of Arabidopsis thaliana

“…For instance, each PECP protein demonstrates some ability to cleave PPi, even if only marginally. In fact, the pyrophosphatase activity measured in vitro for PECP2 was quite low (<1 nmol Pi min -1 mg protein -1 ; May et al, 2011) in comparison to the one associated with PECP3 (177 nmol Pi min -1 mg protein -1 ; Tannert et al, 2021), although another study only revealed a very marginal pyrophosphatase activity for PECP3 (Hasnain et al, 2016). Interestingly, their in vitro specific activities also suggested that each of these three enzymes metabolizes PEtn more efficiently than PCho in vitro (PECP1: 3,665 (PEtn) versus 475 (PCho) nmol Pi min -1 mg protein -1 (May et al, 2012); PECP2: 0.02 (PEtn) versus<0.001 (PCho) nmol Pi min -1 mg protein -1 (May et al, 2011); PECP3: 1,379 (PEtn) versus 315 (Pcho) nmol Pi min -1 mg protein -1 (Tannert et al, 2021).…”

“…This was already suggested following the characterization of hypocotyl elongation in pecp1/2/3 triple mutants and fugu5 pecp1/2 triple mutants (Hanchi et al, 2018), but we performed a more in-depth characterization of these mutants, along with the demonstration that fugu5 PPi homeostasis defects cannot be complemented by pecp2 overexpression. The initial in vitro characterization of PECP2 (May et al, 2011), completed later on by characterizing PECP1 and PECP3 (May et al, 2011;Hasnain et al, 2016;Tannert et al, 2021), remains fundamental for the study of the PECP family, as it clearly demonstrates that each of these enzymes can dephosphorylate a range of different substrates with different affinities. For instance, each PECP protein demonstrates some ability to cleave PPi, even if only marginally.…”

“…However, it remains to be demonstrated if ppspase1/ pecp2 has any detectable impact on PPase homeostasis in planta. A third and final homolog named ThMPase1/PECP3 (encoded by At4g29530) is expressed in plants at low levels, and does not respond to Pi availability (Hanchi et al, 2018;Tannert et al, 2021). Based on heterologous expression in yeast, this homolog was initially shown to dephosphorylate thiamine monophosphate with a high efficiency (Hasnain et al, 2016).…”

“…Based on heterologous expression in yeast, this homolog was initially shown to dephosphorylate thiamine monophosphate with a high efficiency (Hasnain et al, 2016). PEtn was later revealed to be the favored substrate of this enzyme in planta, instead of thiamine monophosphate (Tannert et al, 2021).…”

B vitamin supply in plants and humans: the importance of vitamer homeostasis

“…Recently, TH2 was shown to specifically catalyze this reaction and is reported to be found in mitochondria and the cytosol (Mimura et al., 2016) (Figure 1b). Plastid TMP phosphatases might also exist, but they have yet to be validated in vivo (Hasnain et al., 2016; Tannert et al., 2021). Pyrophosphorylation is catalyzed by two functionally redundant thiamine pyrophosphokinases (TPKs) that are localized to the cytosol in Arabidopsis (Ajjawi et al., 2007) (Figure 1b).…”