Charura, N. M. | bioRxiv Science

bioRxiv2023

Root cap cell corpse clearance limits microbial colonization

Programmed cell death occurring during plant development (dPCD) is a fundamental process integral for plant growth and reproduction. Here, we investigate the connection between developmentally controlled PCD and fungal accommodation in Arabidopsis thaliana roots, focusing on the root cap-specific transcription factor ANAC033/SOMBRERO (SMB) and the senescence-associated nuclease BFN1. Mutations of both dPCD regulators increase colonization by the beneficial fungus Serendipita indica, primarily in the differentiation zone. smb-3 mutants additionally exhibit hypercolonization around the meristematic zone and a delay of S. indica-induced root-growth promotion. This demonstrates that root cap dPCD and rapid post-mortem clearance of cellular corpses represent a physical defense mechanism restricting microbial invasion of the root. Additionally, reporter lines and transcriptional analysis revealed that BFN1 expression is downregulated during S. indica colonization in mature root epidermal cells, suggesting a transcriptional control mechanism that facilitates the accommodation of beneficial microbes in the roots. Key findings in bullet pointsO_LIThe process of programmed cell death in root development (dPCD) influences the extent and outcomes of fungal symbiosis C_LIO_LIFungal colonization of the root tip and differentiation zone is restricted by SMB-mediated clearance of dead cells, which preserves the meristem and regulates symbiosis C_LIO_LIExpression of plant nuclease BFN1, which is associated with senescence, is modulated to facilitate root accommodation of beneficial microbes C_LI

plant biology↗

bioRxiv2022

A fungal endophyte-generated nucleoside signal regulates host cell death and promotes root colonization

The intracellular colonization of plant roots by the beneficial fungal endophyte Serendipita indica follows a biphasic strategy. After an early biotrophic phase, the interaction transitions to a host cell death phase restricted to the epidermal and cortex layers of the root. Host cell death contributes to the successful accommodation of the fungus during the beneficial interaction in Arabidopsis thaliana. How host cell death is initiated and controlled is largely unknown. Here we show that two fungal enzymes, the ecto-5-nucleotidase SiE5NT and the nuclease SiNucA, act synergistically in the plant apoplast at the onset of cell death to produce deoxyadenosine (dAdo), a potent cell death inducer in animal systems. The uptake of extracellular dAdo, but not the structurally related adenosine (Ado), activates a previously undescribed cell death mechanism in A. thaliana. Mutation of the equilibrative nucleoside transporter ENT3 in A. thaliana results in resistance to cell death triggered by extracellular dAdo and reduced fungal-mediated cell death during root colonization. A library screen of A. thaliana T-DNA insertion lines identified a toll/interleukin-1 receptor nucleotide-binding leucine-rich repeat (TIR-NLR) protein as an additional intracellular component in dAdo-triggered cell death. Mutation of this previously uncharacterised TIR-NLR, which we have named ISI (induced by S. indica), affects host cell death, fungal colonization and growth promotion, suggesting a key role in the regulation of root cell death and plant-microbe interaction. Our data show that the combined activity of two fungal apoplastic enzymes leads to the production of a metabolite that, upon uptake, triggers TIR-NLR-modulated plant cell death, providing a link to immunometabolism in plants. Short summaryEfficient intraradical colonization by the beneficial fungal endophyte Serendipita indica requires restricted host cell death. How this symbiotic host cell death is initiated and controlled is largely unknown. Here we show that two fungal enzymes, the ecto-5-nucleotidase SiE5NT and the nuclease SiNucA, act synergistically in the apoplast at the onset of cell death to produce deoxyadenosine (dAdo), a potent cell death inducer in animal systems. Uptake of extracellular dAdo activates a previously undescribed cell death mechanism in plants. Mutation of the A. thaliana equilibrative nucleoside transporter ENT3 leads to resistance to cell death triggered by uptake of extracellular dAdo and to reduced fungal-mediated cell death during colonization. A library screen of A. thaliana T-DNA insertion lines identified a TIR-NLR protein as an additional intracellular component in dAdo-triggered cell death, providing a link to immunometabolism in plants. In a nutshellRegulated host cell death is part of the plant defense strategy against pathogens, but it is also involved in the accommodation of certain beneficial microbes in the roots. We have identified extracellular metabolites and intracellular metabolic signals that contribute to colonization by beneficial root fungal endophytes and uncovered a conserved cell death mechanism likely co-opted for establishing plant-endophyte symbiosis.

plant biology↗