Convergent Acquisition of Glucomannan β-galactosyltransferases in Asterids and Rosids

{beta}-Galactoglucomannan ({beta}-GGM) is a primary cell wall polysaccharide in rosids and asterids. The {beta}-GGM polymer has a backbone of repeating glucose and mannose, usually with mono- or di-galactosyl sidechains on the mannosyl residues. CELLULOSE SYNTHASE-LIKE 2 (CSLA2), MANNAN -GALACTOSYLTRANSFERASE (MAGT), and MANNAN {beta}-GALACTOSYLTRANSFERASE (MBGT) are required for {beta}-GGM synthesis in Arabidopsis thaliana. The single MBGT identified so far, AtMBGT1, lies in glycosyltransferase family 47A subclade VII, and was identified in Arabidopsis. However, despite the presence of {beta}-GGM, an orthologous gene is absent in tomato (Solanum lycopersicum), a model asterid. In this study, we screened candidate MBGT genes from the tomato genome, functionally tested the activities of encoded proteins, and identified the tomato MBGT (SlMBGT1) in GT47A-III. Interestingly therefore, AtMBGT1 and SlMBGT1 are located in different GT47A subclades. Further, phylogenetic and glucomannan structural analysis from different species raised the possibility that various asterids possess conserved MBGTs in GT47A-III, indicating that MBGT activity has been acquired convergently among asterids and rosids. Although functional convergence was observed, the acquired amino acid substitutions among the two MBGT groups were not shared, suggesting different evolutionary pathways to achieve the same biochemical outcome. The present study highlights the promiscuous emergence of donor and acceptor preference in GT47A enzymes, and suggests an adaptive advantage for eudicots to acquire {beta}-GGM {beta}-galactosylation.