Transcriptomic Analysis of L. japonicus Symbiosis Reveals New Candidate Genes for Local and Systemic Regulation of Nodule Function

Abstract

Several aspects of the legume–rhizobia symbiosis are far from being completely understood, such as the transport of compounds through the symbiosome membrane and the molecular actors (receptors, transcription factors and hormones) involved in the systemic regulation of nodulation. In this work, the transcriptomes of L. japonicus plants growing under symbiotic or non-symbiotic conditions were studied in roots and shoots, in order to look for new genes involved in nodule function and regulation both at the local and systemic levels. Several of the genes differentially expressed in roots were well-known nodulins; however, other genes with unknown function were also discovered that showed univocal nodule-specific expression profiles. Transporters of the Nitrate Transporter1/Peptide Transporter Family family, putative oligopeptide transporters, as well as other uncharacterized transporters were upregulated in nodulated roots. Five transcription factors, as well as receptors/kinases and an f-box domain containing protein, all of unknown function, were also more upregulated in nodulated roots. In the shoots of nodulated plants, genes involved in jasmonic acid and indole-3-acetic acid metabolism were differentially expressed. Moreover, three genes encoding for different glutaredoxins, proteins that were recently involved in the systemic signaling of the Arabidopsis nitrogen status, were highly downregulated in the leaves of nodulated plants. Protein–protein interaction network analysis identified nitrate reductase as a central hub in nitrogen metabolism, and a putative protein of the NADH-ubiquinone complex was highly connected to several SWEET transporters. Clustering analysis of the differentially expressed genes also suggested a possible role for a previously uncharacterized ethylene-responsive transcription factor and for LBD38 homologs in L. japonicus nodule function. The new genes identified in this study represent a promising target for the understating and manipulation of symbiotic nitrogen fixation, with the aim of improving crop legumes’ productivity.