Plant cells elongate irreversibly only when load-bearing bonds in the walls are cleaved. Auxin causes the elongation of stem and coleoptile cells by promoting wall loosening via cleavage of these bonds. This process may be coupled with the intercalation of new cell wall polymers. Because the primary site of auxin action appears to be the plasma membrane or some intracellular site, and wall loosening is extracellular, there must be communication between the protoplast and the wall. Some "wall-loosening factor" must be exported from auxin-impacted cells, which sets into motion the wall loosening events. About 20 years ago, it was suggested that the wall-loosening factor is hydrogen ions. This idea and subsequent supporting data gave rise to the Acid Growth Theory, which states that when exposed to auxin, susceptible cells excrete protons into the wall (apoplast) at an enhanced rate, resulting in a decrease in apoplastic pH. The lowered wall pH then activates wall-loosening processes, the precise nature of which is unknown. Because exogenous acid causes a transient (1-4 h) increase in growth rate, auxin must also mediate events in addition to wall acidification for growth to continue for an extended period of time. These events may include osmoregulation, cell wall synthesis, and maintenance of the capacity of walls to undergo acid-induced wall loosening. At present, we do not know if these phenomena are tightly coupled to wall acidification or if they are the products of multiple independent signal transduction pathways.

The ability of low pH and CO2 to induce rapid cell elongation and wall loosening in the Avena coleoptile has been examined with the use of a continuous growth-recording technique and an Instron extensometer, respectively. In particular, the properties of the response to hydrogen ions have been examined in detail and have been compared with the responses initiated by CO2 and auxin. The optimal pH for growth is about 3.0, and both the maximal growth rate and wall extensibility are similar to that produced by optimal auxin. The timing (initiated in less than 1 minute) and duration (up to 2 hours) of the response to hydrogen ions, as well as certain other aspects of the growth and wall-loosening responses, are described. It is shown that the pH response can be clearly separated from the CO2 response. Possible mechanisms for the initiation of the growth response to low pH are briefly discussed.The ability of hydrogen ions to promote the growth of coleoptile segments has been known for some time; however, this particular phenomenon has not been extensively studied nor has it been described in any detail. In 1934, Bonner (1) reported that the growth of coleoptile sections was 8 times greater at pH 4.1 than at 7.2. He also noted that a low pH induced a rather large increase in the extensibility of the cell wall. Nitsch and Nitsch (9) reported that hydrogen ions had a stimulatory effect on cell enlargement both in the presence and absence of IAA. Evans (5) has also briefly studied the effect of hydrogen ions on elongation and was the first investigator to note the rapidity of the response. Menzel (8) has examined the effect of low pH on the extensibility O0 plasmolyzed Helianthus hypocotyl segments, and has reported that hydrogen ions increase the extensibility of cell walls. Unfortunately, the techniques used in this particular study do not lend themselves to direct comparison with other systems.In this paper we shall describe in some detail the effect of hydrogen ions on the growth and wall extensibility of coleoptile segments. Because the effect described is in many ways similar to the effect of IAA on growth and wall properties, this information may lead to a more general understanding of the cell enlargement process.The effect of C02-saturated solutions on the growth of coleop-1 This study was supported by National Science Foundation, Grant 5385 X to R.E.C., and by a National Science Foundation Postdoctoral Fellowship to D.L.R. tile segments was also investigated and compared with the hydrogen ion effect. The descriptive aspects of the CO2 response have been investigated in some detail by Evans and his co-workers (5, 7). The response is initiated rapidly (lag 0-2 min) and lasts a relatively short time (about 30-60 min). However, since in most experiments CO2 was bubbled into water and the solution then tested (pH 3.8), it was never clearly established whether the effect was due to CO2 or H+. In this paper we shall show that the two responses can be separated and, indeed, appear to be quite different. MATERIA...

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The dose response curve for hydrogen-ion-induced extension growth in Avena coleoptile segments has been reinvestigated. The previously published optimum (pH 3.0) is in error by about two orders of magnitude. The correct optimum is around pH 5.0. This discrepancy is thought to be due to the impermeable nature of the cuticle to hydrogen ions. In the present study the cuticular barrier to H(+) entry was circumvented by using coleoptile segments from which the epidermis with cuticle were physically removed. Using such peeled coleoptile sections, it was also found that auxin can rapidly (20-30 min) initiate H(+) secretion and that the magnitude of auxin-induced secretion is sufficient to initiate considerable cell-extension growth. Furthermore, it is shown that the secretion response is specific for active auxins, and inhibited by agents which inhibit auxin-induced growth (dinitrophenol, abscisic acid, cycloheximide, valinomycin and others). These results make it very likely that H(+) secretion is responsible, at least in part, for the initiation of auxin-induced cell wall loosening and extension growth.