Disruption of glycosylphosphatidylinositol-anchored lipid transfer protein 15 affects seed coat permeability in Arabidopsis.

The hydrophobic biopolymer suberin, which is deposited in the root endodermis and seed coats, functions as an extracellular barrier against uncontrolled water, gas, and ion loss. Suberin monomers synthesized in the endoplasmic reticulum (ER) are exported through the plasma membrane to the apoplast. However, limited information is available about the molecular mechanisms underlying suberin monomer export and assembly. In this study, we investigated the in planta role of LTPG15 encoding a glycosylphosphatidylinositol (GPI)-anchored lipid transfer protein. LTPG15 was predominantly expressed in the root endodermis and seed coat. Fluorescent signals from LTPG15:eYFP were detected in the plasma membrane in tobacco epidermis. Disruption of LTPG15 caused a significant decrease in the levels of fatty acids (C20-C24), primary alcohols (C20 and C22), ω-hydroxy fatty acids (C22 and C24), and α,ω-alkanediols (C20 and C22), but an increase in the amounts of primary alcohols and hydroxy fatty acids with C16 and C18 in seed coats. The mutant phenotype was restored to that of the wild type (WT) by the expression of LTPG15 driven by its own promoter. Seed coats of ltpg15 had an increase in permeability to tetrazolium salts compared with WT seed coats. ltpg15 seeds were more sensitive than WT seeds to inhibition of germination and seedling establishment by salt and osmotic stress treatments. Taken together, our results indicate that LTPG15 is involved in suberin monomer export in seed coats, and this highlights the role of Type G non-specific lipid transfer proteins (LTPGs) in very-long-chain fatty acids and their derivatives' export for suberin polyester formation.

Identification of marneral synthase, which is critical for growth and development in Arabidopsis.

Plants produce structurally diverse triterpenoids, which are important for their life and survival. Most triterpenoids and sterols share a common biosynthetic intermediate, 2,3-oxidosqualene (OS), which is cyclized by 2,3-oxidosqualene cyclase (OSC). To investigate the role of an OSC, marneral synthase 1 (MRN1), in planta, we characterized a Arabidopsis mrn1 knock-out mutant displaying round-shaped leaves, late flowering, and delayed embryogenesis. Reduced growth of mrn1 was caused by inhibition of cell expansion and elongation. Marnerol, a reduced form of marneral, was detected in Arabidopsis overexpressing MRN1, but not in the wild type or mrn1. Alterations in the levels of sterols and triterpenols and defects in membrane integrity and permeability were observed in the mrn1. In addition, GUS expression, under the control of the MRN1 gene promoter, was specifically detected in shoot and root apical meristems, which are responsible for primary growth, and the mRNA expression of Arabidopsis clade II OSCs was preferentially observed in roots and siliques containing developing seeds. The eGFP:MRN1 was localized to the endoplasmic reticulum in tobacco protoplasts. Taken together, this report provides evidence that the unusual triterpenoid pathway via marneral synthase is important for the growth and development of Arabidopsis.