Adenosine triphosphate (ATP) is both an energy source when intracellular and a signaling molecule when extracellular. In plants, ATP is normally much more concentrated inside the cells than in the extracellular space. Tang et al. determined that ATP inhibited gravitropism of the roots of Arabidopsis seedlings and at higher concentrations promoted root curling. Exogenous addition of ATP also stimulated auxin signaling as measured by increased expression of an auxin-sensitive reporter gene in root tips. Consistent with ATP enhancing auxin signaling, lower concentrations of exogenous auxins were required to produce growth inhibition in the presence of ATP than without added ATP. ATP decreased basipetal auxin transport and increased auxin retention in roots. The authors discuss whether the effect of ATP is due to alteration of the steepness of the intracellular and extracellular ATP gradient or whether ATP may be acting as a signaling molecule stimulating gravitropism. W. Tang, S. R. Brady, Y. Sun, G. K. Muday, S. J. Roux, Extracellular ATP inhibits root gravitropism at concentrations that inhibit polar auxin transport. Plant Physiol. 131 , 147-154 (2003). [Abstract] [Full Text]