Abstract

BackgroundGlucosinolates are an important class of secondary metabolites characteristic to the order Brassicales. They are known to play a major role in plant defense and from the human perspective, can be anticarcinogenic or antinutritive. GTRs are plasma-membrane localized high affinity glucosinolate transporters, which are important components of the source (leaf) to sink (seed) translocation of intact glucosinolates in members of Brassicaceae family. GTRs are identified as major candidates for Brassica crop improvement, thus dictating a need for their functional characterization. However, currently there are limitations in availability of heterologous assay systems for functional characterization of plant secondary metabolite transporters. To date, the animal-based Xenopus oocyte system is the best established heterologous system for functional characterization of these transporters. Inherent biochemical and physiological attributes unique to the plant membranes necessitate the need for developing plant-based transporters assay systems as well.MethodsIn this study, Agrobacterium mediated transformation was used to develop GTR expressing cotton cell lines (CCL-1) for functional characterization of the Arabidopsis high affinity glucosinolate transporters, AtGTR1 and AtGTR2. Following sub-cellular localization of AtGTRs, we standardized the glucosinolate uptake assays using cell suspension cultures of AtGTR expressing CCL-1 its requirement of pH, salt, and time based glucosinolate uptake. Using the GTR expressing CCL-1, we subsequently performed kinetic analysis of AtGTR1 and AtGTR2 for different glucosinolate substrates, sinigrin, gluconapin and sinalbin.ResultsSeveral clones expressing each of AtGTR1 and AtGTR2 were obtained showing high level of GTR expression and were maintained through regular sub-culturing. Both AtGTR1 and AtGTR2 are predominantly plasma-localized proteins when overexpressed in CCL-1 cells. Uptake assays were standardized, suggesting that glucosinolate uptake of GTR expressing CCL-1 is robust within the physiological pH range 5–6, and at lower concentration of nitrate salts. GTR expressing CCL-1 cells show increasing glucosinolate accumulation in time course experiment. Kinetic studies over a wide glucosinolate concentrations (10–800 µM) revealed that our novel assay system displayed robust GTR-mediated uptake of different glucosinolates and unambiguously helps elucidate the saturable kinetics of GTRs. Our system confirms the high affinity of AtGTRs for both aliphatic and aromatic glucosinolates.ConclusionThe transporter assay system described in this study holds potential for studying sub-functionalization amongst GTR homologs present across Brassicaceae family. The fast growing CCL-1 cells, confer the benefits of an in vitro system for quick assays and is plant based thus enabling optimal expression without sequence modifications. The efficient functioning of the GTR transporters in the heterologous CCL-1 opens the possibility of using this plant cell suspension system for functional characterization of other metabolite transporters.

Highlights

  • Glucosinolates are an important class of secondary metabolites characteristic to the order Brassicales

  • We describe a novel method for functional characterization of two well-known glucosinolate transporters, AtGTR1 and AtGTR2 by heterologously expressing them in a fast-growing cell-suspension line (CCL-1) derived from cotton, Gossypium hirsutum

  • Agrobacterium mediated transformation of the fast growing Cotton cell suspension line-1 (CCL-1) cells was performed with T-DNA constructs comprising of gene specific coding sequences (CDS) of Arabidopsis AtGTR1 and AtGTR2 under the CaMV35S constitutive promoter and Basta resistance gene (BlpR) as the selection marker (Fig. 2a)

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Summary

Introduction

Glucosinolates are an important class of secondary metabolites characteristic to the order Brassicales. Currently there are limitations in availability of heterologous assay systems for functional characterization of plant secondary metabolite transporters. There have been limited attempts towards using this system for study of plant secondary metabolite transporters such as the GTRs. A major hurdle was that detection of substrate uptake by the transporter under investigation required the utilization of the substrate or its metabolite for growth. A major hurdle was that detection of substrate uptake by the transporter under investigation required the utilization of the substrate or its metabolite for growth In this context, attempts at genetic engineering of yeast with sulphatase gene from Helix pomatia or myrosinase gene from Brassica napus, in order to enable cells to use glucosinolate hydrolytic products as a Sulphur source for growth, have as yet been unsuccessful [24]. Detection of substrate uptake was enabled through coupling with mass spectrometry based techniques, providing scope for future utilization of the yeast system for study of plant secondary metabolite transporters [8]

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