Overexpression of UDP‐sugar pyrophosphorylase leads to higher sensitivity towards galactose, providing new insights into the mechanisms of galactose toxicity in plants

SUMMARY

Galactose toxicity (Gal‐Tox) is a widespread phenomenon ranging from Escherichia coli to mammals and plants. In plants, the predominant pathway for the conversion of galactose into UDP‐galactose (UDP‐Gal) and UDP‐glucose is catalyzed by the enzymes galactokinase, UDP‐sugar pyrophosphorylase (USP) and UDP‐galactose 4‐epimerase. Galactose is a major component of cell wall polymers, glycolipids and glycoproteins; therefore, it becomes surprising that exogenous addition of galactose leads to drastic root phenotypes including cessation of primary root growth and induction of lateral root formation. Currently, little is known about galactose‐mediated toxicity in plants. In this study, we investigated the role of galactose‐containing metabolites like galactose‐1‐phosphate (Gal‐1P) and UDP‐Gal in Gal‐Tox. Recently published data from mouse models suggest that a reduction of the Gal‐1P level via an mRNA‐based therapy helps to overcome Gal‐Tox. To test this hypothesis in plants, we created Arabidopsis thaliana lines overexpressing USP from Pisum sativum. USP enzyme assays confirmed a threefold higher enzyme activity in the overexpression lines leading to a significant reduction of the Gal‐1P level in roots. Interestingly, the overexpression lines are phenotypically more sensitive to the exogenous addition of galactose (0.5 mmol L ⁻¹ Gal). Nucleotide sugar analysis via high‐performance liquid chromatography‐mass spectrometry revealed highly elevated UDP‐Gal levels in roots of seedlings grown on 1.5 mmol L ⁻¹ galactose versus 1.5 mmol L ⁻¹ sucrose. Analysis of plant cell wall glycans by comprehensive microarray polymer profiling showed a high abundance of antibody binding recognizing arabinogalactanproteins and extensins under Gal‐feeding conditions, indicating that glycoproteins are a major target for elevated UDP‐Gal levels in plants.

Significance Statement

Studies on Gal‐Tox in different model organisms suggest that Gal‐1P plays a central role as toxic agent. Here we generated transgenic lines of Arabidopsis thaliana with significantly reduced Gal‐1P levels under Gal‐feeding conditions by pyrophosphorylase overexpression, which are surprisingly more sensitive towards galactose, indicating that other galactose‐containing metabolites like UDP‐Gal are much more crucial for the development of Gal‐Tox showing different mechanisms of Gal‐Tox in plants.