Phloem Long‐Distance Trafficking of RNAs and Proteins

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.

Keywords: phloem; long‐distance signalling; PD ; companion cell; sieve element

Figure 1. Schematic representation of the plant vascular system and directional transport of signals induced by diverse stimuli, such as insect feeding, virus and bacterial infection (left), as well as photoperiod (right, represented by a clock). Xylem is represented as red lines, and the phloem as blue lines; likewise, phloem flow is depicted as blue arrows, while transpiration flow as red arrows.
Figure 2. Phloem development. A precursor cell undergoes asymmetric cell division (ACD) giving rise to two distinct cells (CC, companion cell; SE, sieve element). CC resumes symmetric cell division while SE begins to elongate and lose some organelles. CC‐SE plasmodesmata maintain these cell types interconnected with some ribosomes attached, while the sieve plate is formed. To the left are the organelles that degenerate in the mature SEs; ribosomes in this cell type are depicted as different from those in the CC, because their functionality is unknown.
Figure 3. Transport of RNAs (ribonucleic acids) and proteins through the phloem translocation stream. Different types of RNAs move long‐distance; miRNAs (micro ribonucleic acids) and siRNAs (small interfering ribonucleic acids) are involved, among other processes, in response to phosphate deficit, and the latter are capable of inducing DNA (deoxyribonucleic acid) methylation in several loci. Several mRNAs, on the other hand, influence leaf morphology and root architecture, while others could passively diffuse from the CC to the SE. Finally, several proteins could have a role in long‐distance signaling, such as FT (FLOWERING LOCUS T), involved in flower induction and tuberisation, while other proteins have a role in SE maintenance. Blue arrows represent the directionality of phloem flow, while the red arrows indicate the direction of reverse flow in phloem, or xylem flow (see text).
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Further Reading

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Xoconostle‐Cázares, Beatriz, Martínez‐Navarro, Angélica C, and Ruiz‐Medrano, Roberto(Sep 2016) Phloem Long‐Distance Trafficking of RNAs and Proteins. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0021260.pub2]