The Responses Of Plant Plasma Membrane H~+-ATPase To Phosphorus Deficient Stress

It has been reported that oil rape could release a large amount of organic acids under phosphorus (P) deficiency. It is well established that organic anions and rhizospere acidification can increase P availability in soils. White lupin is model plant which is able to adapt to phosphorus deficiency by producing proteoid roots, which release citric acids. It is convinced that the acidification of proteoid roots is related to the activity of plasma membrane H⁺-ATPase, which is an important mechanismus of white lupin to acclimate to the P deficiency. But, it should be taken seriously whether the increment of plasma membrane H⁺-ATPase activities in roots pronouncing a universal response of plant roots under P deficiency. Until now, the responses of plasma membrane H⁺-ATPase of oil rape roots to P deficiency are not fully elucidated. In this study, oil rapes were cultivated with and without P nutrient and root plasma membrane was isolated by the two phase system method. The plasma membrane H⁺-ATPase activity was investigated. The results are as follows,1. In comparison with the P sufficient plants, in vivo, the H⁺ exudation from the P deficient plant roots was increased and was inhibited by vanadate.2. in vitro, the pH optimum of plasma membrane H⁺-ATPase for the P-sufficient and -deficient plant was pH 6.0. Both hydrolytic activity and V_max of the plasma membrane H⁺-ATPase in roots of P deficient plants were increased, but K_m and the vanadate sensitivity decreased.3. Immunodetection with a polyclonal antibody specific for the central part of plant plasma membrane H⁺-ATPase showed that a higher intensity for the P deficient rape roots, as compared with the P sufficient plant roots.In conclusion, these data revealed that the higher concentration of the plasma membrane H⁺-ATPase in the P deficient oil rape roots was responsible for higher H⁺-ATPase hydrolytic activity, which is one of the physiological adaptations to the phosphate deficiency. Increased plasma membrane H⁺-ATPase activity may promote the H⁺ exudation, which can be used as the company cations for the release of organic anions from the roots. All of these mechanisms can contribute to the remobilization of unavailable phosphate in the soil.The studies related to the adaptation of plant plasma membrane H⁺-ATPase to phosphorus deficiency were all concentrated on dicotyledonous plants and it is known little about that of the monocotyledonous plants. The adaptation of rice root plasma membrane H⁺-ATPase activity to P deficiency was studied. Rice plants {Oryza sativa L. japonica ssp. cv. Wuyujing) were fed with phosphate or without phosphate in hydroponics culture experiment. At the seedling stage the plasma membrane of rice roots was isolated by two phase system. The plasma membrane H⁺-ATPase hydrolytic activity was analyzed for elucidating the response of the plasma membrane H⁺-ATPase of rice root to the P deficiency. The results showed that,1. Plasma membrane H⁺-ATPase hydrolytic activity and ATPase K_m,V_max obtained from P deficient rice root were lower than those from P sufficient rice root. But I₅₀ showed no significant difference between these two treatments.2. The pH optimum of the plasma membrane H⁺-ATPase of P deficient rice root was 6.0, but was about 6.4 for that of P sufficient rice root.3. Western blot showed that the enzyme concentration of plasma membrane H⁺-ATPase from P deficient rice root was similar to. that from P sufficient rice root.The results indicated that the lower activity of H⁺-ATPase from P deficient rice root is not resulted from the decreased number of H⁺-ATPase units per membrane area. The different H⁺-ATPase isoforms might be induced in the P deficient rice root in comparison with the P sufficient rice root. It could be an adaptation mechanism of plasma membrane H⁺-ATPase of rice root to the P deficiency.