Ammonia Assimilation and Metabolite Transport in Isolated Chloroplasts. II. Malate Stimulates Ammonia Assimilation in Chloroplasts Isolated From Leaves of Dicotyledonous but Not Monocotyledonous Species

Abstract

Malate stimulated NH₃ assimilation, as determined by a (2-oxoglutarate, NH₃)-dependent O₂ evolution system, by up to 3-fold in chloroplasts isolated from leaves of dicot but not monocot species. This difference was apparently correlated with the endogenous metabolite pools present in these chloroplast preparations. During NH₃ assimilation the glutamate and glutamine pools were large in spinach (dicot) but small in oat chloroplasts. The reverse was the case for the 2-oxoglutarate (2-OG) pool. The addition of malate substantially increased the glutamate, glutamine and 2-OG pools in spinach chloroplasts but had little effect in oat chloroplasts. This suggests that the supply of 2-OG was apparently limiting NH₃ assimilation in spinach chloroplasts. Malate increased this supply and, consequently, stimulated NH₃ assimilation. On the other hand, NH₃ assimilation in oat chloroplasts seemed to be limited by the supply of glutamate and glutamine which could not be overcome by the addition of malate.

Chloroplasts were also isolated from oat seedlings watered with high nutrient solution. The rates of NH₃ assimilation in these organelles exceeded those obtained in spinach chloroplasts. But the addition of malate had little effect on (2-OG, NH₃)-dependent O₂ evolution in these oat chloroplasts. Since malate did not inhibit this activity it is conceivable that it still might play a role, albeit a 'passive' role, in serving as a counter-ion for 2-OG uptake via the 2-OG translocator and glutamate export via the Dct translocator during NH₃ assimilation.