Gene expression profiling: keys for investigating phloem functions

Phloem is the major route for transport of carbohydrates, amino acids, and other nutrients from source to sink tissues. Hormones, mRNAs, small RNAs and proteins also are transported by the phloem, and potentially play pivotal roles in communication between organs to coordinate plant development and physiology. A comprehensive understanding of the mechanisms involved in phloem transport and signalling is still lacking. Recent transcript profiling in several plant species has provided new insights to phloem-specialized functions. Here, we review conclusions regarding the unique functions of the phloem and discuss putative roles for mRNAs and small RNA species in long-distance signalling.