Catalyzes the stereospecific condensation of D-arabinose 5-phosphate and phosphoenolpyruvate to form 3-deoxy-D-manno-octulosonate 8-phosphate (KDO-8-phosphate) and inorganic phosphate. Involved in the biosynthesis of 3-deoxy-D-manno-octulosonate (KDO) which is an indispensable component of rhamnogalacturonan II (RG-II), a structurally complex pectic polysaccharide of the primary cell wall. RG-II is essential for the cell wall integrity of rapidly growing tissues and pollen tube growth and elongation. D-arabinose 5-phosphate + H2O + phosphoenolpyruvate = 3-deoxy-alpha-D-manno-2-octulosonate-8-phosphate + phosphate A number of isoforms are produced. According to EST sequences. Expressed in roots, apical meristem, emerging leaves, hydathodes of young leaves, styles of mature flowers and funicules of mature siliques. Belongs to the KdsA family.