Gene Expression in Nitrogen-Fixing Symbiotic Nodule Cells in Medicago truncatula and Other Nodulating Plants.

Peter Mergaert,Eva Kondorosi,Attila Kereszt

doi:10.1105/tpc.19.00494

Abstract

Root nodules formed by plants of the nitrogen-fixing clade (NFC) are symbiotic organs that function in the maintenance and metabolic integration of large populations of nitrogen-fixing bacteria. These organs feature unique characteristics and processes, including their tissue organization, the presence of specific infection structures called infection threads, endocytotic uptake of bacteria, symbiotic cells carrying thousands of intracellular bacteria without signs of immune responses, and the integration of symbiont and host metabolism. The early stages of nodulation are governed by a few well-defined functions, which together constitute the common symbiosis-signaling pathway (CSSP). The CSSP activates a set of transcription factors (TFs) that orchestrate nodule organogenesis and infection. The later stages of nodule development require the activation of hundreds to thousands of genes, mostly expressed in symbiotic cells. Many of these genes are only active in symbiotic cells, reflecting the unique nature of nodules as plant structures. Although how the nodule-specific transcriptome is activated and connected to early CSSP-signaling is poorly understood, candidate TFs have been identified using transcriptomic approaches, and the importance of epigenetic and chromatin-based regulation has been demonstrated. We discuss how gene regulation analyses have advanced our understanding of nodule organogenesis, the functioning of symbiotic cells, and the evolution of symbiosis in the NFC.

Highlights

In response to nitrogen starvation and the presence of specific compatible nitrogen3 fixing bacteria in the rhizosphere, plants of the nitrogen-fixing clade (NFC) in theRosid clade form symbiotic organs on their roots known as nodules
We review what we have learned from transcriptome approaches aimed at characterizing the specific features of nodules, focusing mostly but not exclusively on the transcriptional reprogramming taking place during the formation of symbiotic nodule cells in the model legume, Medicago truncatula
Transcriptomic analysis of nodule formation has been instrumental in the discovery of 1052 new genes that were subsequently shown by reverse genetics to be important for 1053 nodule formation

Summary

INTRODUCTION

In response to nitrogen starvation and the presence of specific compatible nitrogen fixing bacteria in the rhizosphere, plants of the nitrogen-fixing clade (NFC) in the. These studies confirmed the activation of cell cycle-related processes early in the nodulation signaling pathway (Breakspear et al, 2014; Jardinaud et al, 2016; Larrainzar et al, 2015), which is in agreement with the known role of the cell cycle in the establishment of a nodule primordium (Foucher and Kondorosi, 2000), cell differentiation (Vinardell et al, 2003; Suzaki et al, 2014), and infection (Van Brussel et al, 1992; Yang et al, 1994; Suzaki et al, 2014) Another striking finding is the identification of the gene encoding the rapid alkanization factor MtRALF1 peptide as a Nod factor-induced gene controlling infection thread development (Combier et al, 2008). The biological meaning of this will certainly be worth 1048 deciphering

CONCLUDING REMARKS

Findings

IId IIp IZ III roots nodules