Fig. 7: Updated model for ureide biosynthesis and export in bean nodules.

The model integrates our experimental data with the current knowledge but also includes some speculative aspects. In particular the way allantoin (Aln) and allantoate (Alc) are exported from the ER to the xylem vessels in the vascular tissue is unknown. Here, a vesicle transport has been suggested, but transporter-mediated export through the cytosol may occur instead. It is also not clear if Aln and maybe in part also Alc are transported mainly through the symplast or the apoplast from the central infected zone to the vascular tissue. We assume that both pathways play a role, although a proliferation of tubular ER and its close association to peroxisomes in the uninfected cells⁵¹ suggest that the transport mainly occurs symplastically in the ER. Furthermore, it is unknown whether the inner plastid membrane contains an IMP exporter and how uric acid (Ura) enters the peroxisome and Aln leaves the peroxisome and enters the ER. The ER entry is here shown in the vascular tissue but may already occur in the uninfected cells of the central zone, where the tubular ER is strongly proliferated^{26, 28, 51}. Whether xanthosine (Xao), xanthine (Xan) and Ura diffuse through plasmodesmata between infected and uninfected cells or whether transport proteins are involved is unknown as well. Figure created with Affinity Designer by Serif. Metabolites: PRPP phosphoribosyl pyrophosphate, IMP inosine monophosphate, AMP adenosine monophosphate, Ino inosine, Hpx hypoxanthine, XMP xanthosine monophosphate, Xao xanthosine, Xan xanthine, GMP guanosine monophosphate, Guo guanosine, Ura urate, Aln allantoin, Alc allantoate. Enzymes: IMPDH IMP dehydrogenase, GMPS GMP synthetase, XMPP XMP phosphatase, GSDA guanosine deaminase, NSH1 and NSH2 nucleoside hydrolases 1 and 2, XDH xanthine dehydrogenase, UOX urate oxidase, ALN allantoinase, ER endoplasmic reticulum. ‘R’ in the chemical formulas is either a proton for the nucleobases (Hpx, Xan) or ribose for the nucleosides (Ino, Xao, Guo) or ribose-5-phosphate for the nucleotides (IMP, XMP, GMP).