Articles
CHANGES IN GLUTAMINE SYNTHETASE IN HARVESTED ASPARAGUS SPEARS: AN OPPORTUNITY FOR GENETIC MODIFICATION?
In other plant systems it is thought that an ammonia buildup is toxic and causes tissue deterioration.
Glutamine synthetase (GS; EC 6.3.1.2.) has a central role in plant nitrogen metabolism and is a key enzyme in the assimilation of ammonia.
Therefore, we are interested in studying GS in asparagus spear tissues after harvest, as a prelude to the genetic modification of the crop.
Two forms of GS have been identified in asparagus, associated with either the cytosol or the plastids.
The two enzymes have distinct kinetic properties and contain polypeptides of different sizes.
The abundance of the two GS forms changed differently as spears were held in darkness, at 20°C, for 5 days after harvest.
The cytosolic enzyme activity declined after harvest before rising rapidly at 4 and 5 days after harvest.
Ammonia also increased 4 and 5 days after harvest.
Plastid GS enzyme activity declined throughout the 5-day postharvest period in parallel with a decline in the chlorophyll content of the spear tips.
The possibility of overexpressing GS in genetically modified asparagus tissues in order to assimilate the ammonia that is produced after harvest is discussed and rejected.
* Unless otherwise stated, the term ammonia is used to denote ammonia or ammonium ion, without attemping to distinguish between the two forms (Joy 1988).