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

The Interplay between Enucleated Sieve Elements and Companion Cells

In order to adapt to sessile life and terrestrial environments, vascular plants have developed highly sophisticated cells to transport photosynthetic products and developmental signals. Of these, two distinct cell types (i.e., the sieve element (SE) and companion cell) are arranged in precise positions, thus ensuring effective transport. During SE differentiation, most of the cellular components are heavily modified or even eliminated. This peculiar differentiation implies the selective disintegration of the nucleus (i.e., enucleation) and the loss of cellular translational capacity. However, some cellular components necessary for transport (e.g., plasmalemma) are retained and specific phloem proteins (P-proteins) appear. Likewise, MYB (i.e., APL) and NAC (i.e., NAC45 and NAC86) transcription factors (TFs) and OCTOPUS proteins play a notable role in SE differentiation. The maturing SEs become heavily dependent on neighboring non-conducting companion cells, to which they are connected by plasmodesmata through which only 20-70 kDa compounds seem to be able to pass. The study of sieve tube proteins still has many gaps. However, the development of a protocol to isolate proteins that are free from any contaminating proteins has constituted an important advance. This review considers the very detailed current state of knowledge of both bound and soluble sap proteins, as well as the role played by the companion cells in their presence. Phloem proteins travel long distances by combining two modes: non-selective transport via bulk flow and selective regulated movement. One of the goals of this study is to discover how the protein content of the sieve tube is controlled. The majority of questions and approaches about the heterogeneity of phloem sap will be clarified once the morphology and physiology of the plasmodesmata have been investigated in depth. Finally, the retention of specific proteins inside an SE is an aspect that should not be forgotten.

Role of Plant Virus Movement Proteins in Suppression of Host RNAi Defense

One of the systems of plant defense against viral infection is RNA silencing, or RNA interference (RNAi), in which small RNAs derived from viral genomic RNAs and/or mRNAs serve as guides to target an Argonaute nuclease (AGO) to virus-specific RNAs. Complementary base pairing between the small interfering RNA incorporated into the AGO-based protein complex and viral RNA results in the target cleavage or translational repression. As a counter-defensive strategy, viruses have evolved to acquire viral silencing suppressors (VSRs) to inhibit the host plant RNAi pathway. Plant virus VSR proteins use multiple mechanisms to inhibit silencing. VSRs are often multifunctional proteins that perform additional functions in the virus infection cycle, particularly, cell-to-cell movement, genome encapsidation, or replication. This paper summarizes the available data on the proteins with dual VSR/movement protein activity used by plant viruses of nine orders to override the protective silencing response and reviews the different molecular mechanisms employed by these proteins to suppress RNAi.