A florigen-expressing subpopulation of phloem companion cells expresses other small proteins and reveals a nitrogen-sensitive FT repressor
The precise onset of flowering is crucial to ensure successful plant reproduction. The gene FLOWERING LOCUS T (FT) encodes florigen, a mobile signal that is produced in leaves and initiates flowering in the shoot apical meristem. FT is highly expressed in phloem companion cells that reside in distal leaf regions. Thus far, a detailed molecular characterization of the FT-expressing phloem companion cells has been lacking. Here, we used single-nuclei RNA-seq to investigate gene expression in FT-expressing cells and other phloem companion cells. We discovered that companion cells with high FT expression form a unique cluster and express other genes encoding small proteins, including the anti-florigen BROTHER OF FT (BFT). The promoters of FT and the genes co-expressed with FT were enriched for the consensus binding motif of NITRATE-INDUCIBLE GARP-TYPE TRANSCRIPTIONAL REPRESSOR1 (NIGT1). Overexpression of the paralogs NIGT1.2 and NIGT1.4 in transgenic Arabidopsis lines significantly delayed flowering under nitrogen-rich conditions, consistent with NIGT1s acting as nitrogen-dependent FT repressors. Taken together, our results demonstrate that FT-expressing cells show a distinct expression profile that suggests that these cells produce multiple systemic signals to regulate plant growth and development. Overall design: Promoters driving expression of the NTF reporter were used for FANS and subjected to 10x Genomics single-cell RNA-seq v3.1