The xipotl Mutant of Arabidopsis Reveals a Critical Role for Phospholipid Metabolism in Root System Development and Epidermal Cell Integrity

Abstract

Phosphocholine (PCho) is an essential metabolite for plant development because it is the precursor for the biosynthesis of phosphatidylcholine, which is the major lipid component in plant cell membranes. The main step in PCho biosynthesis in Arabidopsis thaliana is the triple, sequential N-methylation of phosphoethanolamine, catalyzed by S-adenosyl-l-methionine:phosphoethanolamine N-methyltransferase (PEAMT). In screenings performed to isolate Arabidopsis mutants with altered root system architecture, a T-DNA mutagenized line showing remarkable alterations in root development was isolated. At the seedling stage, the mutant phenotype is characterized by a short primary root, a high number of lateral roots, and short epidermal cells with aberrant morphology. Genetic and biochemical characterization of this mutant showed that the T-DNA was inserted at the At3g18000 locus (XIPOTL1), which encodes PEAMT (XIPOTL1). Further analyses revealed that inhibition of PCho biosynthesis in xpl1 mutants not only alters several root developmental traits but also induces cell death in root epidermal cells. Epidermal cell death could be reversed by phosphatidic acid treatment. Taken together, our results suggest that molecules produced downstream of the PCho biosynthesis pathway play key roles in root development and act as signals for cell integrity

CHOLINE TRANSPORTER-LIKE1 is required for sieve plate development to mediate long-distance cell-to-cell communication

“…This organic phosphate ester might serve for phosphate and nitrogen storage and is distributed by the vascular system throughout the plant 14,15 . The idea that choline can be taken up from the media and distributed within the plant is further supported by the observation that externally supplied choline can rescue the developmental defects of xipotl, a phosphocholine biosynthesis mutant 16 . Choline might also be transported by the proline transporter BvBet/ProT1, which is expressed in phloem and xylem parenchyma cells of sugar beet 17 .…”

“…2d), indicating that CHER1 may be involved in the regulation and/or maintenance of choline metabolite levels in plants. Furthermore, we analysed the expression of pAPL::GFPer and the distribution of the freely moving GFP driven by the phloem CC-specific pSUC2 promoter during phloem development in the biosynthetic xipotl mutant, which also shows reduced levels of choline and phosphocholine 16,37 . Similar to cher1-1, we observed a reduced movement and unloading defects of pSUC2::GFP ( Supplementary Fig.…”

The Phosphate Fast-Responsive Genes PECP1 and PPsPase1 Affect Phosphocholine and Phosphoethanolamine Content

“…Furthermore, PCho has been shown to affect the expression of XIPOTL1 indirectly, through the action of an upstream regulatory mORF (Alatorre- Cobos et al, 2012). Previous studies have reported altered root architecture and epidermal cell death in a XIPOTL1/PEAMT1 KO mutant (xpl1; Cruz-Ramírez et al, 2004). However, even though PCho content was effectively diminished in this mutant, the described phenotype was more likely caused by an impaired balance of choline, PtdCho, and phosphatidic acid (Cruz-Ramírez et al, 2004;AlatorreCobos et al, 2012).…”

“…Results from our work and others indicate that altered PtdCho and choline contents are responsible for these phenotypes, rather than PCho or PEth, since specifically affecting their concentrations did not influence root architecture. For instance, XIPOTL1 converts PEth to PCho through a triple methylation and is strongly involved in the PtdCho balance in membranes (McNeil et al, 2001;Cruz-Ramírez et al, 2004). Furthermore, PCho has been shown to affect the expression of XIPOTL1 indirectly, through the action of an upstream regulatory mORF (Alatorre- Cobos et al, 2012).…”

“…Previous studies have reported altered root architecture and epidermal cell death in a XIPOTL1/PEAMT1 KO mutant (xpl1; Cruz-Ramírez et al, 2004). However, even though PCho content was effectively diminished in this mutant, the described phenotype was more likely caused by an impaired balance of choline, PtdCho, and phosphatidic acid (Cruz-Ramírez et al, 2004; AlatorreCobos et al, 2012).A connection between choline content and root hair elongation was also suggested following the mutation of PLDz1, an enzyme responsible for the degradation of PtdCho into PA and choline (Qin and Wang, 2002;Ohashi et al, 2003). However, the double KO pldz1 pldz2 contradicted these results since it did not exhibit any root hair alteration (Li et al, 2006).…”