Plasmodesmata and solute exchange in the phloem

Abstract

Photoassimilates and other substances accumulate against concentration gradients in the phloem, a process known as loading. In mature leaves, the sieve element–companion cell complexes (SE–CCCs) of minor veins, where loading occurs, are connected to surrounding cells by plasmodesmata. These pores appear to participate in loading in plants that translocate raffinose and stachyose, but in sucrose- and polyol-translocating species their function is less certain. Indeed, large numbers of plasmodesmata between the SE–CCCs and surrounding cells should cause a dissolution of the concentration gradient unless the size exclusion limit of the pores is small enough to retain accumulated solute species. In leaves of willow, Salix babylonica L., a sucrose-translocating plant with a high degree of symplastic connectivity into the minor vein phloem, the sucrose concentration gradient is absent between mesophyll and phloem, leading to the conclusion that phloem loading does not occur. Once inside the SE–CCC, solute may be able to pass freely between sieve elements and companion cells since they are also symplastically connected. However, due to net flux into the sieve tubes in source leaves, this should cause a continual drain of metabolic intermediates out of companion cells. It is proposed that this transport step is regulated in minor veins to prevent continual loss of needed solute molecules to the translocation stream.