Due to the chemical inertness of the N2 molecule, the availability of metabolically accessible inorganic nitrogen sources was a key limiting factor for plant life on earth. The large scale conversion of N2 into NH3 and NO 3 - and their use as nitrogen fertilizer has been an agricultural standard for less than a century. The highly efficient recycling and detoxification of NH3 released by metabolic processes, as glycine-serine conversion in photorespiration, catabolic metabolism of amino acids and nucleic acids and nitrite reduction, is essential for plants, due to the limiting availability of nitrogen, and the toxicity and volatility of ammonia.