Phloem Transport: Cellular Pathways and Molecular Trafficking

Abstract

The phloem transports nutrients, defensive compounds, and informational signals throughout vascular plants. Sampling the complex components of mobile phloem sap is difficult because of the damage incurred when the pressurized sieve tubes are breached. In this review we discuss sampling methods, the artifacts that can be introduced by different sampling procedures, the intricate pathways by which materials enter and exit the phloem, and the major types of compounds transported. Loading and unloading patterns are largely determined by the conductivity and number of plasmodesmata and the position-dependent function of solute-specific, plasma membrane transport proteins. Recent evidence indicates that mobile proteins and RNA are part of the plant's long-distance communication signaling system. Evidence also exists for the directed transport and sorting of macromolecules as they pass through plasmodesmata. A future challenge is to dissect the molecular and cellular aspects of long-distance macromolecular trafficking in the signal transduction pathways of the whole plant.

Involvement of Abscisic Acid in the Coordinated Regulation of a Stress-Inducible Hexose Transporter (VvHT5) and a Cell Wall Invertase in Grapevine in Response to Biotrophic Fungal Infection

“…Our data now suggest that, apart from its role in regulating defense responses, ABA may also have an important role in regulating the carbohydrate supply to tissues under biotic stress. In grapevine leaves, this appears to be mediated through the coordinated increase in VvcwINV and VvHT5 transcription, leading to increased cleavage of Suc derived from the phloem sieve elements by apoplastic VvcwINV and increased transport of hexoses from the apoplast into surrounding companion cell/phloem parenchyma (Turgeon and Wolf, 2009) via VvHT5.…”

Long-distance movement of ArabidopsisFLOWERING LOCUS TRNA participates in systemic floral regulation

“…This RNA based system, unique to plants, acts as a remote-control network involved in gene silencing, pathogen defense, leaf development, tuber formation, phosphate homeostasis and many other physiological processes. [1][2][3] The non-cell autonomous RNAs are trafficked through the phloem translocation stream, probably by forming an RNA-protein complex to allow the stable translocation of RNA molecules. 4 The complexity of phloem sap RNA has been extensively characterized.…”

“…Currently, several thousand mRNAs and small RNAs have been identified from phloem exudates, which raises the intriguing hypothesis that many physiological processes may be regulated by the noncell autonomous RNA molecules. [1][2][3] Floral initiation is orchestrated by various environmental and developmental factors. Classic physiological experiments established that the perception of day length variation relies…”

Intraplant communication in maize contributes to defense against insects

“…1,2 When plants are being challenged by different stresses, including abiotic and biotic stresses, the plants systemically translocate defense signal(s) to induce defense responses. 3,4 Biotic stresses caused by herbivore attack induce plant defenses both locally and systemically.…”