In-vitro interaction between chloroplasts and peroxisomes as controlled by inorganic phosphate

Peroxisomes, whole chloroplasts, mitochondria, and broken chloroplasts of spinach (Spinacia oleracea L.), each form 1 band at its typical density, when isolated in sucrose gradients by isopycnic centrifugation in glycylglycine buffer. In potassium-phosphate buffer peroxisomes form a 2nd band at the density of whole chloroplasts. The phosphate effect is half-saturated at a concentration of 10-20 mM. If whole chloroplasts are removed by differential centrifugation before isopycnic centrifugation no second band is formed. Arsenate can be substituted for potassium phosphate while KCl, NaCl, KNO3 and glycolate cannot, showing inorganic phosphate to be the active ion. Evidence is presented showing that during isopycnic centrifugation more slowly sedimenting peroxisomes have to move through faster sedimented bands of whole and broken chloroplasts. In the presence of inorganic phosphate this leads to a specific interaction between whole chloroplasts and peroxisomes visible as a second peroxisomal band at the density of the whole chloroplasts. The relationship of the interaction in vitro to the known association of the two organelles in vivo is considered.