The M. truncatula SUNN gene is expressed in vascular tissue, similarly to RDN1, consistent with the role of these nodulation regulation genes in long distance signaling

Encoding a conserved protein of unknown function, the Medicago truncatula RDN1 gene is involved in autoregulation of nodulation through signaling in the root. In contrast, the SUNN kinase in M. truncatula has been shown by grafting of mutant scions to control nodule number in the root by communication of a signal from the shoot to the root. GUS staining patterns resulting from expression of the SUNN promoter fused to uidA showed expression of SUNN in most parts of plant including the root, but confined to the vascular tissue, a pattern that overlaps with that published for RDN1. Real Time qRT-PCR analysis showed levels of both SUNN RNA and RDN1 RNA did not change significantly during early nodulation signaling (0-72 hours after inoculation). The similarity in expression in cell types strongly suggests vascular signaling for nodule number regulation, while the lack of changes over early nodule development suggest post transcriptional mechanisms such as protein association or phosphorylation transmit the signal.