MicroRNA399 is a long-distance signal for the regulation of plant phosphate homeostasis

Abstract

The presence of microRNA species in plant phloem sap suggests potential signaling roles by long-distance lregulation of gene expression. Proof for such a role for a phloem-mobile microRNA is lacking. Here we show lthat phosphate (Pi) starvation-induced microRNA399 (miR399) is present in the phloem sap of two diverse lplant species, rapeseed and pumpkin, and levels are strongly and specifically increased in phloem sap during Pi ldeprivation. By performing micro-grafting experiments using Arabidopsis, we further show that chimeric lplants constitutively over-expressing miR399 in the shoot accumulate mature miR399 species to very high llevels in their wild-type roots, while corresponding primary transcripts are virtually absent in roots, ldemonstrating shoot-to-root transport. The chimeric plants exhibit (i) down-regulation of the miR399 target ltranscript (PHO2), which encodes a critical component for maintenance of Pi homeostasis, in the wild-type lroot, and (ii) Pi accumulation in the shoot, which is the phenotype of pho2 mutants, miR399 over-expressers or lchimeric plants with a genetic knock-out of PHO2 in the root. Hence the transported miR399 molecules retain lbiological activity. This is a demonstration of systemic control of a biological process, i.e. maintenance of plant lPi homeostasis, by a phloem-mobile microRNA.