Macromolecular trafficking in the phloem

Prior to the early 1990s, the functional aspects of long-distance movement in the phloem were viewed primarily in terms of the transport of sugars and other photoassimilates. The soluble proteins in phloem exudates were often considered to be artifacts arising from the cytoplasmic degeneration of the conducting elements or a confounding anomaly of the sugar transport system. Recent work indicates a much more complex system where proteins and other macromolecules play significant roles in phloem function and long-distance signaling throughout the plant.