Physiological characteristics and miRNA sequencing of two root zones with contrasting ammonium assimilation patterns in Populus.

Abstract

The net ammonium fluxes differ among the different root zones of Populus, but the physiological and microRNA regulatory mechanisms are unclear. To elucidate the physiological and miRNA regulatory mechanisms, we investigated the two root zones displaying significant differences in net NH4+ effluxes of P. × canescens. Populus plantlets were cultivated with 500μM NH4Cl for 10days. Six plants were randomly selected to determine the net NH4+ fluxes using a noninvasive microtest technique. High-throughput sequencing were used to determine the dynamic expression profile of miRNA among the different root zones of Populus. Net NH4+ efflux in zone I (from 0 to 40mm from the root apex) was - 19.64pmolcm-2s-1 and in zone II (from 40 to 80mm) it was - 43.96pmolcm-2s-1. The expression of eleven miRNAs was significantly upregulated, whereas fifteen miRNAs were downregulated. Moreover, eighty-eight target genes of the significantly differentially expressed miRNAs were identified in root zone II compared with zone I. Particularly, ptc-miR171a/b/e and their target, SCL6, were found to be important for the difference in net NH4+ effluxes in the two root zones. Moreover, the expression of the target of ptc-miR169d, NFYA3 was upregulated in root zone II compared with root zone I, contributing to increased NH4+ efflux and decreased NH4+ assimilation in root zone II. These results indicate that miRNAs regulate the expression levels of their target genes and thus play key roles in net NH4+ fluxes and NH4+ assimilation in different poplar root zones.