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 Optimum pH is 6.5-9.5. Optimum temperature is 65 degrees Celsius. A number of isoforms are produced. According to EST sequences. Expressed in shoots. Belongs to the KdsA family.