The metabolism of asparagine in plants

Asparagine is a nitrogen transport compound found in many plants. It is synthesized in seeds by asparagine synthetase upon germination, when protein reserves are hydrolysed, or in roots during nitrate assimilation and dinitrogen fixation. Asparagine synthetase has not been isolated from other plant organs, although evidence suggests that asparagine is synthesized in leaves. Asparagine is catabolized by two separate pathways, deamidation and transamination, releasing nitrogen for amino acid and protein synthesis. Asparaginase activity in many plant species is dependent on potassium and is active in tissues requiring nitrogen for growth such as developing seeds, roots, and leaves. In growing leaves, asparaginase activity undergoes diurnal variation in activity; modification of asparaginase activity has also been observed in microorganisms. Asparagine aminotransferase activity is present in pods and developing and mature leaves, possibly supplying nitrogen for leaf growth through the photorespiratory pathway, and is not found in other organs. In older leaves nitrogen is not required for growth, and several reports have suggested that transpirationally derived asparagine in older leaves undergoes little metabolism and is re-exported to the apex, or to developing fruits.