Systemic mRNA transport depends on m5C methylation, nuclear mRNA export factors and developmental phase changes

Abstract

In grafted plants mRNAs can be transported from shoot to root, however, how their mobility is regulated remains poorly understood. Recent work has shown that m5C methylation plays an essential role in systemic mRNA mobility in vegetative plants, but its role at later developmental stages remains unclear. To address this, we examined the mobility of three mobile mRNAs, TCTP1, HSC70.1 and GRP7 in mutants deficient in m5C mRNA methylation. By comparing vegetative and flowering stages, we found that in contrast to GRP7, the reduced mobility of TCTP1 and HSC70.1 observed in the vegetative growth phase is restored upon flowering, indicating that m5C-dependent regulation of RNA transport is developmentally controlled. We further identified two RNA-binding proteins, the nuclear mRNA export factors ALY2 and ALY4, as phloem mobile. ALY2 and ALY4 deficient plants show a similar developmental effect on mRNA mobility as m5C mRNA methylation mutants, suggesting that they contribute to the regulation of systemic mRNA transport. Notably, combined perturbation of m5C methylation and ALY2 or ALY4 results in reduced mRNA mobility even in flowering plants. Together, our findings reveal an unexpected developmental layer of control over mRNA transport and identify the nuclear mRNA export factors ALY2 and ALY4 as potential regulators of long-distance RNA movement. SIGNIFICANCE STATEMENTmRNA m5C methylation is essential for shoot-to-root mRNA transport during plant growth; however, whether and how mRNA mobility is regulated, and whether it changes during development remains unclear. We show that TCTP1 and HSC70.1 mRNA transport is reduced in vegetatively growing m5C-deficient mutants but remains unaffected upon flowering, and identify ALY2 and ALY4 proteins, two nuclear export factors, as additional regulators of long-distance mRNA transport, providing new layer of mRNA transport.