Birch, S. | bioRxiv Science

bioRxiv2025

An ancient anthozoan protein reveals an alternative evolutionary path of antiviral signaling

How antiviral immunity first arose in animals is a central question in evolutionary biology. Using the sea anemone Nematostella vectensis, we identify CARDIB, a previously uncharacterized gene located next to RLRb, a cnidarian homolog of the vertebrate RIG-I-like receptor family. This conserved genomic linkage across Anthozoa reveals an ancient coupling between immune sensing and regulation. Despite sequence similarity to vertebrate MAVS, CARDIB performs an opposing function: it represses immune genes under basal conditions yet is essential for activation upon viral challenge. CARDIB binds RLRb via a single CARD domain, forming a repressive complex. Loss of either gene abolishes antiviral transcription, disrupts apoptosis, and elevates viral load under lab conditions. Both genes as well as the RLRb paralog, RLRa, are essential for antiviral defense under native conditions. Phylogeny places the cnidarian CARDs distinctly from the vertebrate RLR-MAVS families, revealing an ancient antiviral system regulating antiviral response through CARD-based signaling.

evolutionary biology↗

bioRxiv2025

PHO2 suppresses arbuscular mycorrhizal symbiosis in high phosphate conditions

Arbuscular mycorrhizal (AM) symbiosis is an ancient relationship formed between most plants and Glomeromycotina fungi, typically in response to phosphate (Pi) limitation in soils. By hosting these fungi in their roots, plants extend their access to essential mineral nutrients and water beyond the rhizosphere, while providing the fungus with carbon in return. This mutualistic symbiosis presents a promising tool for enhancing sustainability in agriculture, as it not only supports plant nutrition but also immunity and wider soil health. However, achieving high crop yields currently relies on supplementing plants with excess Pi, which suppresses AM symbiosis. We found that this suppression is mediated by a key negative regulator of the Pi starvation response (PSR) in rice (Oryza sativa), Phosphate overaccumulator 2 (PHO2). PHO2 encodes an E2 ubiquitin-conjugating enzyme which targets various proteins involved in the PSR in Pi-sufficient conditions for protein degradation. Here we report that pho2 mutants of rice and Nicotiana benthamiana retained high AM fungal colonisation even in high Pi conditions. Our transcriptomic analysis of uninoculated rice roots revealed that pho2 mutants are less sensitive to Pi treatment and retain susceptibility to AM symbiosis by maintaining expression of a core set of AM-related genes gating the early stage AM fungal entry, such as genes involved in strigolactone biosynthesis, LysM-containing plant receptors for fungal molecules, and components of the common symbiosis signalling pathway (CSSP). Furthermore, isotope tracing using 33P and phosphate transporter (PHT1) gene expression patterns collectively suggest enhanced direct and symbiotic Pi overaccumulation in pho2 mutant leaves. Together, our data reveal a new role for PHO2, as a negative regulator of AM colonisation and symbiotic Pi accumulation in shoots.

plant biology↗