Bresciani, L. | Search bioRxiv

bioRxiv2026

Phyllosphere bacterial communities in milkweeds: composition diverge as the season progresses, and links to cardenolides and arthropods depend on host identity

Leaf bacterial communities shape plant defense and interactions with herbivores, yet how host filtering and stochastic processes assemble them remains unclear. We predicted that host identity, chemistry, and arthropods structure them, and that stochastic and deterministic contributions shift seasonally. Across one growing season we sampled leaf bacteria monthly and arthropods weekly on four Asclepias species (Apocynaceae: A. curassavica, A. incarnata, A. syriaca, A. tuberosa) spanning a cardenolide gradient. We sequenced 16S rRNA genes and quantified selection and dispersal contributions to turnover using null models. Host identity shaped bacterial richness and composition, with A. tuberosa hosting the richest communities, and both richness and phylogenetic diversity rose through the season. Early on, homogenizing dispersal made communities more similar among plants; by mid-season no single process dominated turnover, and by late season dispersal limitation prevailed. Homogeneous selection was episodic, not sustained, and arthropod associations were host-specific, negatively so on A. syriaca. Phyllosphere assembly thus shifts from convergence to divergence in one season: young leaves recruit from a shared pool delivered by wind, rain, and arthropods, whereas exchange among ageing plants declines and communities drift apart. Herbivores arriving late meet plant-specific microbial environments, so microbial mediation of herbivory should be host-specific and seasonally contingent.

ecology↗

bioRxiv2026

Human ASPDH is a 2-aminomuconate reductase that produces L-2-aminohex-3-enedioic acid in tryptophan catabolism

Most tryptophan catabolism in animals occurs through the kynurenine pathway, which generates the essential NAD cofactor and multiple bioactive metabolites. Knowledge of this pathway in eukaryotes ends at the unstable intermediate 2-aminomuconate (2-AM). Here, by leveraging evolutionary information from more than 5,000 eukaryotes, we identify two distinct genes acting downstream of 2-AM in fungi and metazoa. The fungal gene is homologous to bacterial 2-AM deaminase, whereas the metazoan gene is homologous to aspartate dehydrogenase (ASPDH), which in prokaryotes catalyses the first reaction of NAD biosynthesis. Biochemical and structural analyses show that human ASPDH has evolved an unprecedented function as an NAD(P)H-dependent 2-AM reductase (AMR) in tryptophan catabolism. The reaction forms L-2-aminohex-3-enedioic acid, an unsaturated -amino acid absent from current biological databases. Isotope-labeling NMR experiments and structural modelling support a mechanism in which hydride transfer is coupled to double-bond rearrangement of the conjugated system. These findings reveal a previously unknown metazoan branch of the kynurenine pathway, expand the repertoire of endogenous amino acids, and illustrate how comparative genomics can uncover hidden reactions in human metabolism.

biochemistry↗