Callose Deposition in Plasmodesmata and Viroid Invasion of the Shoot Apical Meristem

A commentary on Invasion of shoot apical meristems by Chrysanthemum stunt viroid differs among Argyranthemum cultivars Given that they do not encode proteins, viroid RNAs are extremely dependent on host factors for completing their infection cycle. This cycle consists of: (i) access to the organelle wherein they are replicated, the nucleus in the family Pospiviroidae (type member potato spindle tuber viroid, PSTVd) and plastids in the Avsunviroidae (type member avocado sunblotch viroid, ASBVd), (ii) replication, (iii) exit and cell-to-cell movement, and (iv) long-distance movement and downloading to infect distal plant parts. In addition, viroids need to circumvent the defense response mounted by their hosts (see for a review Flores et al., 2015). With respect to movement, when injected into symplasmically connected mesophyll cells of tomato and tobacco, PSTVd moves quickly from cell to cell, strongly suggesting the involvement of plasmodesmata (PD; Ding et al., 1997). The same viroid traffics long distance in the phloem of Nicotiana benthamiana, with this trafficking being probably sustained by replication in the phloem and by plant developmental and cellular factors (Zhu et al., 2001). The latter study also showed that PSTVd was absent from the shoot apical meristem (SAM), a finding with practical implications. Infection by viroids is frequently accompanied by symptoms that include stunting and leaf, flower and fruit deformation and discoloration in plants of economic interest. Therefore, the use of viroid-free plants is crucial for their sustainable production. Several procedures have been implemented to generate viroid-free plants, prominent among which is shoot-tip culture or micrografting. The rationale for this procedure is most likely multiple and includes RNA silencing, as revealed by the involvement of RNA-dependent RNA polymerase 6 in delaying accumulation and precluding meristem invasion of PSTVd in N. benthamiana (Di Serio et al., 2010). Besides this, the relative vascular isolation of the SAM from the surrounding tissue may also hamper viroid invasion, with the fraction of regenerated viroid-free plants being dependent on the size of the shoot tip as well as on the specific viroid, host, and even cultivar. For instance, although PSTVd and the closely related tomato apical stunt viroid (TASVd) cannot invade the tomato SAM (Qi and Ding, 2003; Matsushita et al., 2011), the size of the viroid-free region adjacent to the SAM differs between the two viroids, being smaller in the case of TASVd. Zhang et al. (2015) now report a study on another close relative of PSTVd, chrysanthemum …