Diversity of Pectin Rhamnogalacturonan I Rhamnosyltransferases in Glycosyltransferase Family 106

Rhamnogalacturonan I (RG-I) comprises approximately one quarter of the pectin molecules in land plants, and the backbone of RG-I consists of a repeating sequence of [2)-alpha-L-Rha(1-4)-alpha-D-GalUA(1-] disaccharide. FourArabidopsis thalianagenes encoding RG-I rhamnosyltransferases (AtRRT1 to AtRRT4), which synthesize the disaccharide repeats, have been identified in the glycosyltransferase family (GT106). However, the functional role of RG-I in plant cell walls and the evolutional history of RRTs remains to be clarified. Here, we characterized the sole ortholog of AtRRT1-AtRRT4 in liverwort,Marchantia polymorpha, namely, MpRRT1. MpRRT1 had RRT activity and genetically complemented the AtRRT1-deficient mutant phenotype inA. thaliana. However, the MpRRT1-deficientM. polymorphamutants showed no prominent morphological changes and only an approximate 20% reduction in rhamnose content in the cell wall fraction compared to that in wild-type plants, suggesting the existence of otherRRTgene(s) in theM. polymorphagenome. As expected, we detected RRT activities in other GT106 family proteins such as those encoded by MpRRT3inM. polymorphaandFRB1/AtRRT8inA. thaliana, the deficient mutant of which affects cell adhesion. Our results show thatRRTgenes are more redundant and diverse in GT106 than previously thought.