A readily assayed HW efflux from sycamore (Acer pseudoplatanus), rye (Lolium perenne), and bean (Phaseolus vulgaris cultivars Red Kidney and Small White) suspensioncultured cells has been detected and partially characterized. The H+ efflux has been shown to require a source of energy, to be significantly stimulated by Na+ and Mg2+ but not by K+ and Ca2+, and (16, personal communication) have found that, following a change from pH 7 to pH 3, the lag before the increase in the growth rate of Avena coleoptiles is no greater than 1 min. The H+-stimulated increased growth rate of the coleoptiles can be maintained for at least 2 hr. The rate of growth of epidermal-stripped coleoptiles is stimulated maximally at pH 5.5 and below. Rayle and Cleland have also found that the pH response closely resembles the auxin response in a number of ways, including the stimulation of the same maximal growth rate and the necessity for the continued presence of the inducing agent for continued rapid growth. The similarity in the growth response to low pH and auxin suggests the possibility that these responses may share a common mechanism, such as the lowering of the pH within the cell wall. Indoleacetic acid may achieve this by activating a H+ pump in the cell membrane (2, 5).There is considerable evidence available which supports the idea that an auxin-regulated hydrogen ion pump may be pres-'This work was supported in part by Grant GB-36569 from the Regulatory Biology Program of the National Science Foundation and by Atomic Energy Commission Contract AT(I 1-1)-1426. ent in plant cell membranes. Kitasato (9) has demonstrated a hydrogen ion pump in the plasma membrane of the green algae, Nitella; and Jaffe (8) has presented evidence consistent with the possibility of a hydrogen ion pump in the plasma membranes of the root tip cells of mung beans (Phaseolus aureus). Hager et al. (5) have published evidence that suggests an auxin-regulated hydrogen ion pump may be present in the cell membranes of oat coleoptiles. Their evidence is based on the observation that CCCP,2 known to make membranes permeable to protons (4), rapidly inhibits auxin-induced elongation of coleoptile segments. Marre et al. (10,11) have demonstrated that in pea internode segments, the stimulation of cell enlargement by the auxins IAA, 2,4-D, and naphthaleneacetic acid, and by fusicoccin, a plant toxin secreted by Fusicoccum amygdali, is accompanied by a significant decrease in the pH of the incubation medium, and that this decrease in pH parallels auxin-induced cell enlargement. They have also shown that inhibitors of respiration, of oxidative phosphorylation, of RNA synthesis, and of protein synthesis inhibit both cell enlargement and the decrease of pH in the incubation medium. Cleland and Rayle (personal communications) have evidence that IAA and 2, 4-D induce an increase in H+ concentration in solutions bathing Avena coleoptile sections from which the cuticle and epidermis have been removed. This increase in H+ concentration is of sufficient magnitude...
