Metabolic inhibitors induce symplastic movement of solutes from the transport phloem of Arabidopsis roots

The distribution of the phloem-mobile fluorescent probe carboxyfluorescein (CF) within the primary root of Arabidopsis thaliana was imaged using a confocal laser scanning microscope (CLSM) and the tissue and subcellular distribution of the probe was shown to be influenced by treatment with a number of metabolic inhibitors. Sodium azide completely inhibited the phloem transport of CF into the treated region of root. Treatment with both CCCP and probenecid induced the lateral movement of CF from the transport phloem to the adjacent cell layers, and the probe accumulated in the cytoplasm of the pericycle, endodermis, cortex, and epidermis. This lateral transfer of CF was restricted to the pericycle in the presence of plasmolysing concentrations of sorbitol. Ultrastructural investigations demonstrated the presence of a plasmolysing pathway leading from the sieve element-companion cell complex (SE-CC) out into the cortex. The results are consistent with the operation of this symplastic pathway under conditions of metabolic energy reduction and are discussed in relation to the regulation of plasmodesmatal conductance in the transport phloem.