Is plastidic glutamine synthetase essential for C 3 plants? A tale of photorespiratory mutants, ammonium tolerance and conifers.

Agriculture faces the considerable challenge of having to adapt to a progressively changing climate (including the increase in CO 2 levels and temperatures); environmental impact must be reduced while at the same time crop yields need to be maintained or increased to ensure food security. Under this scenario, increasing plants' nitrogen (N) use efficiency and minimizing the energy losses associated with photorespiration are two goals of crop breeding that are long sought after. The plastidic glutamine synthetase (GS2) enzyme stands at the crossroads of N assimilation and photorespiration, and is therefore a key candidate for the improvement of crop performance. The GS2 enzyme has long been considered essential for angiosperm survival under photorespiratory conditions. Surprisingly, in Arabidopsis GS2 is not essential for plant survival, and its absence confers tolerance towards ammonium stress, which is in conflict with the idea that NH 4 + accumulation is one of the main causes of ammonium stress. Altogether, it appears that the 'textbook' view of this enzyme must be revisited, especially regarding the degree to which it is essential for plant growth under photorespiratory conditions, and the role of NH 4 + assimilation during ammonium stress. In this article we open the debate on whether more or less GS2 is a desirable trait for plant productivity.