New steps in mucilage biosynthesis revealed by analysis of the transcriptome of the UDP-rhamnose/UDP-galactose transporter 2 mutant

Upon imbibition, epidermal cells of Arabidopsis thaliana seeds release a mucilage formed mostly by pectic polysaccharides. The Arabidopsis mucilage is composed mainly of unbranched rhamnogalacturonan-I (RG-I), with low amounts of cellulose, homogalacturonan, and traces of xylan, xyloglucan, galactoglucomannan, and galactan. The pectin-rich composition of the mucilage and their simple extractability makes this structure a good candidate to study the biosynthesis of pectic polysaccharides and their modification. Here, we characterize the mucilage phenotype of a mutant in the UDP-rhamnose/galactose transporter 2 (URGT2), which exhibits a reduction in RG-I and also shows pleiotropic changes, suggesting the existence of compensation mechanisms triggered by the lack of URGT2. To gain an insight into the possible compensation mechanisms activated in the mutant, we performed a transcriptome analysis of developing seeds using RNA sequencing (RNA-seq). The results showed a significant misregulation of 3149 genes, 37 of them (out of the 75 genes described to date) encoding genes proposed to be involved in mucilage biosynthesis and/or its modification. The changes observed in urgt2 included the up-regulation of UAFT2, a UDP-arabinofuranose transporter, and UUAT3, a paralog of the UDP-uronic acid transporter UUAT1, suggesting that they play a role in mucilage biosynthesis. Mutants in both genes showed changes in mucilage composition and structure, confirming their participation in mucilage biosynthesis. Our results suggest that plants lacking a UDP-rhamnose/galactose transporter undergo important changes in gene expression, probably to compensate modifications in the plant cell wall due to the lack of a gene involved in its biosynthesis.