Abstract text: Beta-1,4-galactans are abundant side chains on rhamnogalacturonan I (RGI). GALS enzymes of the GT92 family catalyze elongation of these side chains as well as their termination with arabinopyranose residues. However, Arabidopsis mutants where all three GALS genes are inactive still exhibit short beta-1,4-galactan stubs on the rhamnose residues of RGI, indicating that additional enzymes are required for biosynthesis. We have identified a group of galactosyltransferases, named Galactan Extender (GALE) enzymes, that specifically add the second galactose to RGI. gale mutants retain single galactose residues on RGI but lack beta-1,4-linked galactose. GALE enzymes expressed in HEK293 cells catalyze the addition of a single galactose to acceptors bearing one galactose on an RGI oligosaccharide, but cannot add the first galactose, nor the third or fourth residues. Overexpression of GALE or GALS in Arabidopsis yields a modest increase in beta-1,4-galactans. gale mutations are epistatic to gals, and overexpression of GALS in a gale background has limited effect. Co-overexpression of GALE and GALS in the same plant produces strong synergistic effects, resulting in a 2.6-fold increase in pectic galactan content. These findings define a two-step pathway for RGI galactan extension and identify targets to manipulate pectic galactan content in plants.