(Questions)n on phloem biology. 2. Mass flow, molecular hopping, distribution patterns and macromolecular signalling

This review speculates on correlations between mass flow in sieve tubes and the distribution of photoassimilates and macromolecular signals. Since micro- (low-molecular compounds) and macromolecules are withdrawn from, and released into, the sieve-tube sap at various rates, distribution patterns of these compounds do not strictly obey mass-flow predictions. Due to serial release and retrieval transport steps executed by sieve tube plasma membranes, micromolecules are proposed to “hop” between sieve element/companion cell complexes and phloem parenchyma cells under source-limiting conditions (apoplasmic hopping). Under sink-limiting conditions, micromolecules escape from sieve tubes via pore-plasmodesma units and are temporarily stored. It is speculated that macromolecules “hop” between sieve elements and companion cells using plasmodesmal trafficking mechanisms (symplasmic hopping). We explore how differential tagging may influence distribution patterns of macromolecules and how their bidirectional movement could arise. Effects of exudation techniques on the macromolecular composition of sieve-tube sap are discussed.

► Phloem sap contains a multitude of signalling macromolecules.
► The mode of collection strongly affects macromolecular composition of phloem sap.
► Macromolecules“hop” along the phloem at sieve-element/companion-cell interfaces.
► Membrane-permeant molecules “hop” at companion cell/phloem-parenchyma interfaces.