“In this paper, we showed that PGPs directly mediated cellular auxin transport. This is quite significant, as PGP1 was found to have much narrower substrate specificity than its mammalian homologs.”

Until the publication of this paper, there was no clear documentation of cellular efflux of the plant hormone auxin. Physiological and developmental studies had strongly implicated the PIN efflux carriers and P-glycoproteins (PGPs) in auxin efflux, but the mechanism had not been demonstrated. The PIN proteins had been shown to be required for normal polar development and to align with the transport vector, so it was presumed that PGPs regulated PIN activity and/or membrane stability.

In this paper, we showed that PGPs directly mediated cellular auxin transport. This is quite significant, as PGP1 was found to have much narrower substrate specificity than its mammalian homologs. The paper is also very thorough, as it reports on a collaborative effort between Professor Enrico Martinoia’s group at the University of Zurich and my own.

The paper described new applications and refinements of existing methodologies. PGPs were successfully expressed in yeast and human HeLa cell lines. A HeLa cell is an immortal cell line used in medical research. The HeLa cell line was derived from cervical cancer cells taken from Henrietta Lacks, who died from her cancer in 1951.

A method developed in our lab to track radiolabelled auxin applied in nanoliter quantities was refined to track hormone movement through small tissues, and protoplast transport assays were used to monitor cellular auxin efflux. PGPs were also immunolocalized in plant tissues for the first time.

This and successive papers using its methodology have removed the word "putative" from the designation of the auxin efflux and influx proteins, thus paving the way for a new understanding of how this hormone functions in plant development.

I had no intention of working in the area of auxin transport. A mistake made while working on anion transporters and their role in metal ion homeostasis produced a startling outcome that resulted in the identification of PGPs as components of auxin transport complexes.

Manipulation of plant form is one of the most expensive aspects of horticulture. More recently, we have shown that manipulation of plant architecture via modulation of auxin transport streams can be used to produce crop plants with enhanced mineral nutrition and water-stress tolerance.