Companion cells with high florigen production express other small proteins and reveal a nitrogen-sensitive <i>FT</i> repressor.

Abstract

The precise onset of flowering is crucial for successful reproduction. In longer days, the florigen gene <i>FLOWERING LOCUS T</i> (<i>FT</i>) is induced in specific leaf phloem companion cells in <i>Arabidopsis</i>. However, the molecular nature of these cells remains elusive. Here, we utilized bulk nuclei RNA-seq and single nuclei RNA (snRNA)-seq to investigate transcription in <i>FT</i>-expressing cells and other companion cells. Our bulk nuclei RNA-seq demonstrated that <i>FT</i>-expressing cells in cotyledons and true leaves showed differences in <i>FT</i> repressor gene expression. Within true leaves, our snRNA-seq analysis revealed that companion cells with high <i>FT</i> expression form a unique cluster. The cluster expresses other genes encoding small proteins, including the flowering and stem growth inducer FPF1-LIKE PROTEIN 1 (FLP1) and the anti-florigen BROTHER OF FT AND TFL1 (BFT). We also found that the promoters of <i>FT</i> and the genes co-expressed with <i>FT</i> in the cluster were enriched for the binding motif of NITRATE-INDUCIBLE GARP-TYPE TRANSCRIPTIONAL REPRESSOR 1 (NIGT1). Overexpression of <i>NIGT1.2</i> and <i>NIGT1.4</i> repressed <i>FT</i> and delayed flowering under nitrogen-rich conditions, implying the roles of NIGT1s as nitrogen-dependent <i>FT</i> repressors. Taken together, our results indicate that unique <i>FT</i>-expressing phloem cells may produce multiple systemic signals to regulate plant growth and development.