THE CURRENT STATUS OF THE ACID-GROWTH HYPOTHESIS

Excised coleoptile segments that were depleted of endogenous auxin after pre-incubation in water respond to the phytotoxin fusicoccin (FC) and the plant hormone auxin (indole-3-acetic acid, IAA) with an enhancement of growth. This promotion of organ elongation is caused by an increase in the extensibility of the thick, growth-limiting outer epidermal wall. The acid-growth hypothesis postulates that both FC and IAA cause wall-loosening and the concomitant induction of growth by rapid acidification of the extension-limiting cell wall. At saturating concentrations FC and IAA cause very similar growth responses. However, the equilibrium-pH in the incubation medium (similar to pH Of the peripheral cell wall) of abraded FC-treated segments is 3.5-4.0, whereas in the presence of IAA a pH of 4.8-5.0 is established. The FC- and IAA-mediated induction of growth can be stimulated by an external buffer of pH 3.5-4.0. Acid buffers of pH 5, however, cause almost no enhancement of growth compared with the water-control. FC-mediated growth can be inhibited by neutral buffers infiltrated into the outer epidermal wall, but a substantial growth response occurs after addition of IAA under identical conditions. A suboptimal concentration of FC was used to mimic the effect of IAA on acid secretion. When a pH of 4.8-5.0 was established by FC in the peripheral wall no promotion of growth occurred. Hence, IAA-induced proton excretion is insufficient to cause cell-wall loosening in the coleoptile. Abraded segments that rapidly elongate in acid buffers (pH 3.5-4.0) respond to IAA with an additional enhancement of growth, but under identical conditions FC is without effect. The growth-promoting effects of acid and IAA are additive, i.e. IAA and acid act via separate mechanisms. These results are consistent with the acid-growth hypothesis of FC action. However, they indicate that IAA induces growth by a mechanism independent of cell-wall acidification. Alternative hypotheses of IAA-mediated cell-wall loosening are discussed.