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

The ubiquitous nonprotein amino acid known as γ-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 γ-aminobutyrate signaling in phloem.

A new method for rapid screening of xenobiotic phloem mobility in plants

This paper originates from a presentation at the International Conference on Assimilate Transport and Partitioning, Newcastle, NSW, August 1999 The deposition of hydrophobic polymers in the xylem of wheat grain floral axes prevents water and solute movement into grains via the xylem (xylem discontinuity). The only pathway for translocation of photosynthate or externally applied xenobiotics into wheat grains is via the phloem. We have developed a new method based on the xylem discontinuity for rapidly screening phloem mobility of xenobiotics. By quantifying xenobiotic concentration in grains and excised plants after the compounds were applied through the cut stems, the phloem mobility can be estimated quantitatively. The phloem mobility obtained with our new grain-based method was correlated to xenobiotic chemical properties such as log Kow, pKa and electrical charge, and is consistent with published literature. Phloem mobility values determined by the grain-based assay were correlated to those from the direct phloem sap (aphid stylet exudate) assay of excised and intact plants, indicating that the grain-based assay is as reliable as the direct assay with aphid stylectomy. The new grain-based method is simple, quick, and can be scaled up for rapid screening of xenobiotic phloem mobility in plants. Similar seed (fruit)-based assay could also be developed with wide ranges of plant species that use the phloem as the only pathway for supplying water and nutrients into their seeds or fruits.