Does long-distance GABA signaling via the phloem really occur?

The ubiquitous nonprotein amino acid known as gamma-aminobutyrate (GABA) has been proposed as a mechanism whereby environmental and developmental cues are translocated in phloem. Information about the composition of translocation fluids from various species that bleed spontaneously reveals that GABA is always present in xylem at minor levels, but sometimes absent in phloem. In species that do not bleed spontaneously, GABA is also absent or present at minor levels in aphid stylet exudate. By contrast, GABA in ethylenediaminetetraacetate-facilitated phloem exudates is relatively abundant compared to other amino acids or the concentration of total amino acids present in xylem. Evidence indicates that xylem-borne GABA is primarily retrieved and metabolized by mature leaves, and tissue pools of GABA reflect in situ biosynthesis. Recovery of significant GABA from phloem might be an artifact of wounding and the use of ethylenediaminetetraacetate. Scarce data from aphid stylectomy experiments indicates that GABA is a wound signal. However, the composition of phloem sap collected by this method is not uniform, shedding doubt on some results in the scientific literature. Elevated GABA levels in the host plant are known to influence the success of biotic interactions; however, these outcomes could be attributed to the impact of wounding at local sites, rather than delivery of GABA via phloem. Further research is required to provide unambiguous support for long-distance gamma-aminobutyrate signaling in phloem.