Talks Back: Temporal Patterns of Differential Gene Expression During Establishment of Symbiosis.

Endosymbiotic associations between hornworts and nitrogen-fixing cyanobacteria form when the plant is limited for combined nitrogen (N). We generated RNA-seq data to examine temporal gene expression patterns during the culturing of N-starved in the absence and the presence of symbiotic cyanobacterium . In symbiont-free .  gametophytes, N starvation caused downregulation of chlorophyll content and chlorophyll fluorescence characteristics as well as transcription of photosynthesis-related genes. This downregulation was reversed in . cocultured with . , corresponding to the provision by the symbiont of N-derived NH, which commenced within 5 days of coculture and reached a maximum by 14 days. We also observed transient increases in transcription of ammonium and nitrate transporters in a . -dependent manner as well as that of a SWEET transporter that was initially independent of N-derived NH. The temporal patterns of differential gene expression indicated that . transmits signals that impact gene expression to . both prior to and after its provision of fixed N. This study is the first illustrating the temporal patterns of gene expression during establishment of an endosymbiotic nitrogen-fixing association in this monophyletic evolutionary lineage of land plants. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.