Perspective: phloem transport of viruses and macromolecules - what goes in must come out.

Phloem transport of endogenous macromolecules and plant viruses remains poorly understood. Selective movement into and out of the phloem is tightly regulated, yet the mechanisms governing this selectivity have not been elucidated. Recent advances in identifying nonviral proteins and nucleic acids with the capacity for phloem transport will hopefully shed new light on the regulation of phloem loading and unloading.