Functional Interchangeability of Nucleotide Sugar Transporters URGT1 and URGT2 Reveals That and Cell Wall Chemotypes Depend on Their Spatio-Temporal Expression.

Nucleotide sugar transporters (NSTs) are Golgi-localized proteins that play a role in polysaccharide biosynthesis by transporting substrates (nucleotide sugars) from the cytosol into the Golgi apparatus. In Arabidopsis, there is an NST subfamily of six members, called URGTs, which transport UDP-rhamnose and UDP-galactose . URGTs are very similar in protein sequences, and among them, URGT1 and URGT2 are highly conserved in protein sequence and also showed very similar kinetic parameters toward UDP-rhamnose and UDP-galactose . Despite the similarity in sequence and function, mutants in led to a specific reduction in galactose in rosette leaves. In contrast, mutants in showed a decrease in rhamnose content in soluble mucilage from seeds. Given these specific and quite different chemotypes, we wonder whether the differences in gene expression could explain the observed differences between the mutants. Toward that end, we analyzed whether URGT2 could rescue the phenotype and by performing a promoter swapping experiment. We analyzed whether the expression of the coding sequence, controlled by the promoter, could rescue the rosette phenotype. A similar strategy was used to determine whether URGT1 could rescue the mucilage phenotype. Expression analysis of the swapped genes, using qRT-PCR, was similar to the native and genes in wild-type plants. To monitor the protein expression of the swapped genes, both URGTs were tagged with green fluorescent protein (GFP). Confocal microscopy analyses of the swapped lines containing URGT2-GFP showed fluorescence in motile dot-like structures in rosette leaves. Swapped lines containing URGT1-GFP showed fluorescence in dot-like structures in the seed coat. Finally, the expression of in mutants rescued galactose reduction in rosette leaves. In the same manner, the expression of in mutants recovered the content of rhamnose in soluble mucilage. Hence, our results showed that their expression in different organs modulates the role of URGT1 and URGT2. Likely, this is due to their presence in different cellular contexts, where other proteins, acting in partnership, may drive their functions toward different pathways.