Phloem Long‐Distance Trafficking of RNAs and Proteins

Abstract

Abstract The vascular tissue has been essential for the evolutionary radiation of land plants, and has allowed them to colonise a wide range of habitats. The phloem is involved in the distribution of fixed carbon throughout the whole plant. The concept of this tissue as a central distributor of signals that regulate plant development and responses to environmental cues has emerged recently. Indeed, several phenomena, such as post‐transcriptional gene silencing, flowering and tuber induction, as well as systemic acquired resistance to pathogens rely on signals, originating in source leaves, and transported via the phloem to systemic tissues. Considering that the phloem translocation stream contains a large array of macromolecules. Given that the functional phloem consists of interconnected enucleate cells, the sieve elements (SEs), the majority of proteins and ribonucleic acids (RNAs) found in phloem sap exudates must originate from the adjoining companion cell (CC). While several proteins play a role in the maintenance of the SEs, it is possible that several species of proteins and RNAs function in long‐distance signalling. In this article the current state of the art of this subject is described, as well as perspectives and possible applications for crop improvement. Key Concepts The vascular tissue consists of xylem and phloem. The xylem is composed of vessels that are dead cells upon maturity that transport mineral nutrients from roots to shoots. The phloem is composed of vessels formed from enucleated living cells involved in the transport of fixed carbon from photosynthetic to heterotrophic organs. The phloem is involved in the delivery of signals that control development as well as responses to external stimuli, such as defence response, post‐transcriptional gene silencing and response to phosphate starvation. Viruses and viroids are transported through the phloem during systemic infection. These probably take advantage of a system enabling long‐distance transport of endogenous proteins and RNAs. The phloem translocation stream contains a wide variety of proteins and RNAs, some of which appear to have a signalling function. RNA and protein exchange can occur between plant pathogenic fungi and parasitic plants and their hosts. Knowledge of this RNA and protein phloem transport system can have applications for crop improvement.