Nostoc Talks Back: Temporal Patterns of Differential Gene Expression During Establishment of Anthoceros-Nostoc Symbiosis

Endosymbiotic associations between hornworts and nitrogenfixing 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 Anthoceros punctatus in the absence and the presence of symbiotic cyanobacterium Nostoc punctiforme. In symbiont-free A. punctatus 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 A. punctatus cocultured with N. punctiforme, corresponding to the provision by the symbiont of N2-derived NH4+, 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 N. punctiforme-dependent manner as well as that of a SWEET transporter that was initially independent of N2-derived NH4+. The temporal patterns of differential gene expression indicated that N. punctiforme transmits signals that impact gene expression to A. punctatus 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.

Automated summary

This study investigated the temporal gene expression patterns during the establishment of an endosymbiotic nitrogen-fixing association between hornworts and nitrogen-fixing cyanobacteria. The results showed that nitrogen starvation resulted in downregulation of chlorophyll content, chlorophyll fluorescence characteristics, and transcription of photosynthesis-related genes in symbiont-free hornwort gametophytes. However, these downregulations were reversed when the hornwort was cocultured with the symbiotic cyanobacterium, which provided NH4+ derived from N2. The study also revealed transient increases in transcription of ammonium and nitrate transporters, as well as a SWEET transporter that was initially independent of N2-derived NH4+. The findings suggest that the symbiotic cyanobacterium transmits signals to impact gene expression in the hornwort both before and after the provision of fixed nitrogen.