PYD1 (dihydropyrimidine dehydogenase) initiates the degradation of pyrimidine nucleobases and is located in plastids. In this study, a physiological analysis of PYD1 employing T-DNA knockout mutants and overexpressors was carried out. PYD1 knockout mutants were restricted in degradation of exogenously provided uracil and accumulated high uracil levels in plant organs throughout development, especially in dry seeds. Moreover, PYD1 knockout mutants showed delayed germination which was accompanied by low invertase activity and decreased monosaccharide levels. Abscisic acid (ABA) is an important regulator of seed germination, and ABA-responsive genes were deregulated in PYD1 knockout mutants. Together with an observed increased PYD1 expression in wild-type seedlings upon ABA treatment, an interference of PYD1 with ABA signalling can be assumed. Constitutive PYD1 overexpression mutants showed increased growth and higher seed number compared with wild-type and knockout mutant plants. During senescence PYD1 expression increased to allow uracil catabolism. From this it is concluded that early in development and during seed production PYD1 is needed to balance pyrimidine catabolism versus salvage.
The metabolism of nitrogen‐rich nucleosides in Arabidopsis seedlings was investigated at the level of import and subsequent salvage or degradation. Uptake and fate of nucleosides imported by equilibrative nucleoside transporter 3 (ENT3) was analysed and, furthermore, a comprehensive analysis of the effect of exogenously fed nucleosides at the level of metabolic as well as transcriptomic alterations was performed. Expression of nucleoside transporters ENT1 and ENT3, together with nucleoside import, was increased upon nitrogen limitation. Thereby a role for ENT3, which is expressed mainly in the vasculature of roots and leaves, as a major import route for nucleosides was supported. Exogenously fed nucleosides were able to attenuate nitrogen starvation effects such as chlorophyll breakdown, anthocyanin accumulation, RNA breakdown and reduced levels of amino acids. In response to nucleoside supply, up‐regulation of genes involved in nitrogen distribution in plants was observed. In addition, genes involved in nucleoside metabolism were identified as regulated upon nitrogen limitation. In summary, an overall beneficial effect of nucleoside supply to Arabidopsis seedlings, especially under limiting nitrogen conditions, was observed.
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