Phloem restriction of viroids in three citrus hosts is overcome by grafting with Etrog citron: potential involvement of a translocatable factor

Abstract

Viroid systemic spread involves cell-to-cell movement from initially infected cells via plasmodesmata, long-distance movement within the phloem and again cell-to-cell movement to invade distal tissues including the mesophyll. Citrus exocortis viroid (CEVd), hop stunt viroid, citrus bent leaf viroid, citrus dwarfing viroid, citrus bark cracking viroid and citrus viroid V remained phloem restricted when singly infecting Citrus karna, Citrus aurantium and Poncirus trifoliata, but not Etrog citron, where they were additionally detected in mesophyll protoplasts. However, when CEVd-infected C. karna was side-grafted with Etrog citron -with the resulting plants being composed of a C. karna stock and an Etrog citron branch -the viroid was detected in mesophyll protoplasts of the former, thus indicating that the ability of Etrog citron to support viroid invasion of non-vascular tissues was transferred to the stock. Further results suggest that a translocatable factor from Etrog citron mediates this viroid trafficking.

Phloem Exit as a Possible Control Point in Selective Systemic Transport of RNA

“…More important, CEVd in the rootstock acquires the ability to exit from vascular bundles to the mesophyll (Bani-Hashemian et al, 2015). These data show that a phloem-mobile factor enabling exit to mesophyll can be transported from the scion to the rootstock.…”

Hop stunt viroid: A polyphagous pathogenic RNA that has shed light on viroid–host interactions

“…Remarkably, several phloem proteins have been proven to have RNA‐binding capabilities to facilitate RNA trafficking through the phloem (Ham & Lucas, 2017; Pallas & Gómez, 2013). One of those factors has been proposed to be necessary to overcome the phloem restriction of viroids in three citrus hosts (Bani‐Hashemian et al, 2015).…”

Hitchhikers, highway tolls and roadworks: the interactions of plant viruses with the phloem

“…Segmented multipartite viruses may traffic as a net of inter-segmentally base-paired RNAs stabilised by proteins including CP and host proteins, in order to ensure that recipient cells receive all genome components [16]. Viroids, noncoding pathogenic RNAs, also rely on their complex secondary structures for systemic transport [17], as well as on host proteins, some of which are graft-mobile [18].Thus, intra-or intermolecular RNA stem-loop-type structures are probably available in all RNA viruses that do not move systemically as encapsidated virions.…”