Rhamnogalacturonan‐II (RG‐II) is structurally the most complex glycan in higher plants, containing 13 different sugars and 21 distinct glycosidic linkages. Two monomeric RG‐II molecules can form an RG‐II‐borate diester dimer through the two apiosyl (Api) residues of side chain A to regulate cross‐linking of pectin in the cell wall. But the relationship of Api biosynthesis and RG‐II dimer is still unclear. In this study we investigated the two homologous UDP‐D‐apiose/UDP‐D‐xylose synthases (AXSs) in
Arabidopsis thaliana that synthesize UDP‐D‐apiose (UDP‐Api). Both
AXSs are ubiquitously expressed, while
AXS2 has higher overall expression than
AXS1 in the tissues analyzed. The homozygous
axs double mutant is lethal, while heterozygous
axs1/+ axs2 and
axs1 axs2/+ mutants display intermediate phenotypes. The
axs1/+ axs2 mutant plants are unable to set seed and die. By contrast, the
axs1 axs2/+ mutant plants exhibit loss of shoot and root apical dominance. UDP‐Api content in
axs1 axs2/+ mutants is decreased by 83%. The cell wall of
axs1 axs2/+ mutant plants is thicker and contains less RG‐II‐borate complex than wild‐type Col‐0 plants. Taken together, these results provide direct evidence of the importance of AXSs for UDP‐Api and RG‐II‐borate complex formation in plant growth and development.
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