Transcriptome-wide identification of squalene epoxidase genes from Glycyrrhiza glabra L.: expression analysis and heterologous expression of GgSQE1 suggest important role in terpenoid biosynthesis

Abstract

Squalene epoxidase (SQE) is a crucial regulatory enzyme for the biosynthesis of several important classes of compounds including sterols and triterpenoids. The present paper identified and characterised five SQE genes (GgSQE1 to GgSQE5) from Glycyrrhiza glabra through transcriptome data mining and homology-based cloning, for the first time. The phylogenetic analysis implied their functional divergence. The ORF corresponding to one of the five SQEs, namely, GgSQE1, was cloned and studied for its function in a heterologous system, following transient and stable expressions. The transient expression followed by GgSQE1 encoding protein purification suggested approximately 58.0-kDa protein following the predicted molecular mass of the deduced protein. The gene expression profiling based on qRT-PCR indicated its highest expression (6.4-folds) in the 10-month-old roots. Furthermore, ABA (12.4-folds) and GA₃ (2.47) treatments upregulated the expression of GgSQE1 in the shoots after 10 and 12 hours, respectively, which was also reflected in glycyrrhizin accumulation. The inductive effects of ABA and GA₃ over GgSQE1 expression were also confirmed through functional analysis of GgSQE1 promoters using GUS fusion construct. Stable constitutive expression of GgSQE1 in Nicotiana tabacum modulated the sterol contents. The study could pave the way for understanding the metabolic flux regulation concerning biosynthesis of related sterols and triterpenoids.