Evidence That Auxin-Induced Growth of Tobacco Leaf Tissues Does Not Involve Cell Wall Acidification1

Interveinal strips (10 × 1.5 mm) excised from growing tobacco ( Nicotiana tabacum L. cv Xanthi) leaves have an auxin-specific, epinastic growth response that is developmentally regulated and is not the result of ethylene induction (C.P. Keller, E. Van Volkenburgh [1997] Plant Physiol 113: 603–610). We report here that auxin (10 μ m naphthalene acetic acid) treatment of strips does not result in plasma membrane hyperpolarization or detectable proton efflux. This result is in contrast to the expected responses elicited by 1 μ m fusicoccin (FC) treatment, which in other systems mimics auxin growth promotion through stimulation of the plasma membrane H + -ATPase and resultant acid wall loosening; FC produced both hyperpolarization and proton efflux in leaf strips. FC-induced growth was much more inhibited by a strong neutral buffer than was auxin-induced growth. Measurements of the osmotic concentration of strips suggested that osmotic adjustment plays no role in the auxin-induced growth response. Although cell wall loosening of some form appears to be involved, taken together, our results suggest that auxin-induced growth stimulation of tobacco leaf strips results primarily from a mechanism not involving acid growth.