Sink Plasmodesmata as Gateways for Phloem Unloading. Myosin VIII and Calreticulin as Molecular Determinants of Sink Strength?

Phloem-mediated movement of photoassimilates is one of the most critical processes in plants. Photosynthetically active leaves (source) produce an excess of photoassimilates that are exported, via sieve elements of the phloem, into photosynthetically inactive tissues (sink). For instance, growing root apices are heterotrophic sink organs that are dependent on the continuous supply of photoassimilates from the above-ground source organs. Thus, root apices represent an ideal object to study mechanisms of the phloem unloading of photoassimilates and of the sink strength. The simplest definition of sink strength considers the competitive ability of heterotrophic organs to import, process, and store photoassimilates (Herbers and Sonnenwald, 1998). It is unfortunate that both factors and molecules that determine the sink strength remain controversial. Nevertheless, general agreement exists that plasmodesmata, which interconnect most cells of higher plants into a symplasmic continuum, substantially contribute to phloem unloading into sink tissues. This has been shown for root apices and for several other sink tissues such as potato (Solanum tuberosum) tubers and Agrobacterium tumefaciens sp. tumors (Fisher and Oparka, 1996; Pradel et al., 1999; Oparka and Santa Cruz, 2000). Plasmodesmata also participate in Suc export from leaves (Stitt, 1996), highlighting their prime importance for source-sink interactions.