Long-distance movement and differential targeting of microRNA399s.

We have previously demonstrated that miR399s control phosphate (Pi) homeostasis by regulating the expression of a ubiquitin-conjugating E2 enzyme (UBC24/PHO2) in Arabidopsis. Changes in miR399-dependent PHO2 gene expression modulate Pi uptake, allocation and remobilization. More recently, we provided evidence that miR399s are able to move in the phloem stream and across grafting junctions from the scions overexpressing miR399 to the wild-type rootstocks. Movement of miR399s serves as a long-distance signal to report and balance the Pi status between shoots and roots. Of note, results from grafting experiments indicate that miR399b is less efficient in cleaving the PHO2 mRNA than is miR399f, despite the similar mobility of the two miR399s. We propose that nucleotide 13 of miR399s, which gives rise to the sequence variation among different miR399 species, could be involved in regulating the abundance of PHO2 mRNA through sequence complementarity to the target sequences of PHO2 mRNA and mimicking target sequence of At4/IPS1 noncoding RNAs.