How Far Can a Molecule of Weak Acid Travel in the Apoplast or Xylem?

The plant hormones auxin, abscisic acid (ABA), and the gibberellins (GAs) are all weak acids subject to the ion-trapping mechanism that tends to remove them from the extracellular space and concentrate them in the cytoplasm of plant cells. If a molecule of one of these compounds enters the extracellular space, it can therefore travel only a limited distance before reenter- ing a cell. Influx carriers can only shorten this distance. Here I present a simple but quantitative estimate of this distance, and discuss its relevance for various models of short- and long-range signaling in plants. To review, the weak acids of interest all have one or more carboxyl groups, with dissociation constants be- tween 4 and 5 (Table I). In the weakly acidic apoplast, a fraction of each hormone will be protonated and thus membrane permeable. However, once in the approx- imately neutral cytoplasm, the molecules dissociate andbecomemembrane-impermeableanions.Intheab- sence of transmembrane efflux carriers, the molecules will accumulate in the cytoplasm—the so-called ion- trapping mechanism. Although the principle of ion trapping has been known for decades (Rubery and Sheldrake, 1973), its contribution to the overall hor- mone economy has remained vague. The actual accu- mulation of a weakly acidic plant hormone in any cell is dominated by the activity of influx and efflux car- riers, if present, with additional contributions from biosynthesis and metabolism pathways (Davies, 2004). However, the fact remains that a weak acid, upon en- tering the extracellular space, will tend to be trapped by adjacent cells. How far can we expect a molecule of hormone to travel? Consider the idealized situation shown in Figure 1A. A transmitter cell secretes a pulse of hormone into the apoplast. The hormone then moves through the