Editorial: Evolution of Signaling in Plant Symbioses

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

Dynamics of miRNA mediated regulation of legume symbiosis

Symbiotic nitrogen fixation in legume nodules is important in soils with low nitrogen availability. The initiation and sustainability of symbiosis require cellular reprogramming that involves the miRNA-mediated inhibition or activation of specific nodulation genes. The high-throughput sequencing of small RNA libraries has identified miRNAs and their targets, which are the major players in the post-transcriptional gene regulation (PTGS) of the different stages of legume-rhizobia symbiosis ranging from bacterial colonization and organogenesis to symbiotic nitrogen fixation. Here, we present an overview of information obtained from the miRNA libraries from nodulating tissues that have been sequenced to date. The functional analysis of miRNAs has revealed roles in phytohormone homeostasis and spatio-temporal regulation, as well as the mobility of miRNAs and their functions in shoot to root signalling that affects diverse functions, including bacterial entry, meristem division and differentiation, nitrogen fixation and senescence. Furthermore, small RNA fragments of rhizobial origin repress complementary plant mRNAs. We also consider the roles of miRNAs in determinate or indeterminate nodules. Taken together, this overview confirms that miRNAs are master regulators of the legume-rhizobia symbiosis.