Although proteins constitute the biggest nitrogen storage in plants, nucleic acids, especially RNA, also serve as a storage for nitrogen, phosphate and carbon. In line with its function as a nutrient store, RNA can be degraded by plants and the contained nutrients released.
The complete elucidation of the purine nucleotide degradation pathway is a major focus within our group. The purine bases guanine and adenine each contain five nitrogen atoms which are released as ammonia in the course of nucleotide degradation. Ammonia is assimilated by the plant and serves as a building block for new biomolecules.
In the model plant Arabidopsis thaliana, we identified and biochemically characterized three enzymes which catalyze the last three steps of purine degradation. These enzymes degrade the purine degradation intermediate allantoate completely to ammonia and glyoxylate without a urea intermediate. The enzymes constituting this part of the pathway are allantoate amidohydrolase (AAH), ureidoglycine aminohydrolase (UGlyAH) and ureidoglycolate amidohydrolase (UAH). Since little was known about the enzymes of the allantoate degradation pathway in other organisms we extended our investigation to the bacterial model organism Escherichia coli where a pathway similar to the one in plants was identified. Another degradation pathway, involving the ureidoglycine transaminase (UGlyTA), was inferred in other microorganisms using a comparative genome approach.
In soybeans a symbiotic interaction with bacteria from the genus Rhizobium allows the plant to fix nitrogen. In root nodules symbiotic bacteria convert atmospheric nitrogen to ammonia which is used by the plant for purine biosynthesis and subsequent degradation to allantoin and allantoate (ureides). These metabolites serve to transport the fixed nitrogen from the nodules to the shoot where they must be hydrolyzed to access the nitrogen. Our work has demonstrated that ureides in soybean are hydrolyzed by similar enzymes as found in Arabidopsis.
Recently we described a plant specific enzyme catalyzing the deamination of guanosine to xanthosine resulting in the release of ammonia. In Arabidopsis, guanosine deaminase (GSDA) is a key enzyme for the production of xanthosine – an important intermediate of the purine nucleotide degradation pathway. In this respect plants differ from other organisms, where guanylates are degraded by a deamination of guanine to xanthine, whereas plants deaminate guanosine to xanthosine.
A genetic blockage in the purine degradation pathway at the level of uric acid caused by a mutation in the uricase enzyme leads to strong reduction in germination rate and the inability to develop the cotyledons. In contrast to other loss-of-function mutants in the purine degradation pathway uricase mutant plants eventually die. A detailed analysis showed that uric acid accumulation in the mutant interferes with the stability of peroxisomes during the development of the embryo. Peroxisomes are essential for the development of cotyledons and are e.g. instrumental for the degradation of storage lipids. The temporal absence of functional peroxisomes in the mutant therefore explains the observed phenotype. The loss-of-function of xanthine dehydrogenase – an enzyme catalyzing the metabolic reaction upstream of the uricase enzyme – in the uricase mutant background results in accumulation of xanthine instead of uric acid which reverts the phenotype. Wheras uric acid has a strong effect on the stability of the peroxisomes in the cotyledons, this is not the case for xanthine.
Pyrimidine nucleotides also release nitrogen, phosphate and carbon upon degradation. The pyrimidine base uracil contains two nitrogen atoms in the ring and cytosine has an additional nitrogen atom at the ring.