Heterologous expression of nodulation signaling pathway genes enhances grain yield in rice.

Rhizobial symbiosis, a crucial source of nitrogen for legume hosts, is thought to have evolved from mycorrhizal symbiosis. Both symbioses share a common symbiotic signaling pathway (CSSP) in plants. One hypothesis is that the lack of nodulation-specific genes in the genome of mycorrhizal symbiotic plants limits their ability to establish rhizobial interactions. Here, we introduced nine key genes in nodulation pathway, including NFR1, NFR5, SYMRK, CCaMK, CYCLOPS, NSP1, NSP2, LHK1, and NIN) from Lotus japonicus, into rice (Oryza sativa ssp. japonica cv. Zhonghua 11) to create Nodulation Signaling Pathway Overexpression (NSPO) rice. Analysis of gene expression showed that NFR5 and CCaMK were robustly transcribed in transgenic rice roots determined by qPCR. SYMRK, CYCLOPS, NSP2, and NFR1 showed relatively low transcript abundance, while transcripts of NIN, NSP1, and LHK1 were not detected. NSPO rice did not exhibit enhanced rhizobial colonization at the roots but increased formation of 2,4-D-induced nodule-like structures at the ratoon roots compared to the wild type. Remarkably, field trials demonstrated higher grain yield in NSPO rice, despite a slight reduction in seed-setting rate. Additionally, the expression of immune-related transcription factor genes was downregulated in NSPO rice. These findings suggest that heterologous expression of nodulation-related genes can promote rhizobial interaction and improve agronomic traits, such as yield in non-leguminous crops.