Topic 10.6

Some Substances Enter the Phloem by Diffusion

Susan Dunford, University of Cincinnati

Many substances, such as organic acids and plant hormones, are found in the phloem sap at lower concentrations than carbohydrates. These substances are probably not actively loaded into the sieve element–companion cell complex but enter the sieve elements via other pathways and mechanisms. They may be taken up directly by diffusion across the phospholipid bilayer of the plasma membrane of the sieve element–companion cell complex or by a passive transporter in the plasma membrane of those cells, or they may diffuse into the sieve elements via the symplast.

Once in the sieve elements, these substances are swept along in the translocation stream by bulk flow, the motive force being generated by the active loading of only certain sugars or amino acids. Many substances not normally found in plants, such as herbicides and fungicides, can be transported in the phloem because of their ability to diffuse through membranes at an intermediate rate. In other words, they diffuse through membranes rapidly enough to allow considerable accumulation in the sieve elements, but slowly enough that they are not lost from the sieve elements completely before reaching a sink tissue. In addition, compounds that are weakly acidic tend to be "trapped" within the sieve elements, becoming negatively charged in the basic environment (low H⁺ concentration) of the sieve elements and thus less likely to diffuse out of the cells across the hydrophobic membrane (Kleier 1988). Substances that are not transported in the phloem (such as calcium) may not enter the sieve elements at all.

A recent study has characterized the Arabidopsis gene AUX1 (auxin influx carrier) that encodes an auxin permease that appears to functin as an auxin carrier (Swarup et al., 2001, Merchant et al. 2002, see textbook Figure 19.19). AUX1 has been localized in protophloem cells in the root (Swarup et al. 2001). Gene expression studies have shown that AUX1 facilitates IAA loading into the leaf vascular transport system, and IAA unloading in the primary root apex and developing lateral root primordium (Marchant et al. 2002).