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

Abstract

SUMMARYPlants 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.

Regulation of metabolic gene clusters in Arabidopsis thaliana

“…a,b). These pathways have both been implicated in development and in ecological interactions (Field & Osbourn, 2008; Field et al ., 2011; Go et al ., 2012). The genes within each of these two clusters are co-regulated and are expressed exclusively in roots (Supporting Information Fig.…”

Inducible Expression of Arabidopsis Response Regulator 22 (ARR22), a Type-C ARR, in Transgenic Arabidopsis Enhances Drought and Freezing Tolerance

“… [74] , [75] . MRN1 is involved in the biosynthetic pathway of triterpenoids and plays a critical role in growth and development in Arabidopsis [76] . Loss-of-function in Arabidopsis MRN1 caused various phenotypic changes in plant growth and development and also significantly increased electrolyte leakage compared with that of wild-type [76] .…”

Metabolic and functional diversity of saponins, biosynthetic intermediates and semi-synthetic derivatives

“…Interestingly, these effects could not be attributed to the altered expression of genes involved in cell expansion, while it is unknown whether marneral and its derivatives serve as membrane constituents. At the transcriptional level, these compounds have been proposed to have a negative role in flower development, since the delayed flowering phenotype in the mrn1 knockout mutant could be attributed to increased expression of the flowering locus C ( FLC ) gene that encodes a repressor of flowering in A. thaliana (Go et al ., 2012 ).…”

“…However, very little is known about the temporal and spatial embedding of these molecules in the membranes or the role that they may play during normal plant growth. Triterpenes may substitute sterols at certain developmental stages as has been suggested for marneral (Go et al ., 2012 ), or they may have functions in various physiological processes such as nodule formation and cuticle composition. It has also been suggested that isoprenoids, unidentified as yet but other than sterols, are responsible for the activity of plant microRNAs (miRNAs) that guide ARGONAUTE (AGO) protein complexes to regulate expression of complementary RNAs.…”