Phloem Structure and Function

Abstract

AbstractThe phloem collects photoassimilates in green leaves, distributes them in the plant and supplies the heterotrophic plant organs (e.g. fruits, buds and roots). Phloem structure is specialized for loading, long‐distance transport and unloading of assimilates. The conducting cells, called sieve elements, are highly modified to create a low‐resistance pathway composed of contiguous living cells, whose long‐term viability is maintained through an intimate association with companion (or Strasburger) cells. The difference in turgor pressure that is generated by osmotically active assimilates within this living conduit is the physical force that drives long‐distance transport through sieve elements. Plant species have evolved a variety of strategies to generate and maintain turgor pressure between source tissues, where assimilates are synthesized or released, and sink tissues, where assimilates are utilized or stored. Signalling molecules accessing the phloem are swept along with assimilates and trigger important growth processes such as flowering.Key conceptsLong‐distance transport of assimilates occurs from source to sink, that is, from the green leaves, which produce a surplus of assimilates to the plant organs which consume assimilates.The conducting cells are the sieve elements in the phloem which offer a low‐resistance pathway, characterized by a reduced cytoplasm and sieve pores in the connecting cell walls.Reduction of the sieve‐element cytoplasm is accompanied by a dependency of sieve elements on neighbouring cells, called companion cells in angiosperms, and Strasburger cells in gymnosperms, that hold the sieve elements alive.The mechanism of phloem transport is based on a high sugar concentration in the sieve element‐companion cell complexes of source leaves, which osmotically builds up a high turgor pressure.Accumulation of sugars in the source phloem is called phloem loading and depends either on sucrose transporters in the plasma membrane of sieve‐element companion cell complexes, or enzyme activity in the companion cells of the source phloem.Phloem transport is utilized for signal transport, including rapid electropotential waves, mobile noncodingribonucleic acids(RNAs) and mobile transcription factors that are responsible for wound responses and developmental switches in the target tissue.