Phloem Long‐Distance Trafficking of Proteins and RNAs

Shmuel Wolf, The Hebrew University of Jerusalem, Rehovot, Israel

Published online: December 2009

DOI: 10.1002/9780470015902.a0021260

Phloem sap contains a wide range of macromolecules, including ribonucleic acids (RNAs) and proteins. Recent advances in genomic and proteomic technologies have enabled the identification of thousands of these macromolecules. However, technical limitations are still the main obstacle to the complete characterization of macromolecule profiles in the sieve tube.

Grafting experiments have established that numerous macromolecules are capable of moving long distances between tissues, providing the foundation for the hypothesis of an interorgan information superhighway in higher plants involving the phloem. This chapter presents the current knowledge on proteins and messenger RNA (mRNA) molecules present in the sieve tube. The challenge now is to characterize the biological role of trafficking macromolecules, such as their involvement in the signal-transduction pathway(s) elicited by environmental cues and in orchestrating developmental processes at the whole-plant level.

Key concepts

  • Phloem sap contains macromolecules such as proteins and RNAs.
  • Protein profiles in the sieve tube are tissue specific.
  • Phloem proteins act as long-distance signalling molecules.
  • Plant viruses move long distance via the phloem.
  • Endogenous RNA molecules are capable of trafficking long distance via the phloem.
  • mRNA molecules may act as long-distance signalling molecules.

Keywords: companion cell; heterograft; plasmodesmata; sieve element

Figure 1. Models for the two mechanisms of phloem loading of sugars into the sieve tube of higher plants. (a) Symplastic phloem loading. Sucrose is transported symplastically all the way from the mesophyll (MC) to intermediary cells (ICs), which are a special type of companion cells (CC). In the ICs, sucrose is converted to raffinose and stachyose which are then translocated long distances within the sieve tube. (b) Apoplastic phloem loading. Sucrose exits to the apoplast and then is actively loaded by sucrose transporters. Note that there are a small number of plasmodesmata (PD) interconnecting the bundle sheath (BS)/phloem parenchyma (PP) with CCs. It is assumed that these PD provide a route for the trafficking of information molecules. SE; sieve element and SAP, phloem sap collected from cut sieve tube.
Figure 2. A model for the ‘wave’ pattern long-distance movement of plant viruses. CC, companion cell and SE, sieve element. Virus particles are unloaded from the SEs to the CCs for replication, forming an additional ‘wave’ for further movement.
Figure 3. Functional classification of transcripts identified in phloem sap collected from melon stems: 1152 characterized ESTs (based on BLAST description) were classified into 10 categories. The percentage of ESTs assigned to each category is indicated.
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Related Sub-Topics:

Plant Development | Plant Genetics and Molecular Biology | Plant Stress Physiology | Viruses Infecting Plants | RNA Viruses
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Article Title: Phloem Long‐Distance Trafficking of Proteins and RNAs
 
How to Cite close
Wolf, Shmuel(Dec 2009) Phloem Long‐Distance Trafficking of Proteins and RNAs. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0021260]