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EXTRACELLULAR ATP INHIBITS ROOT GRAVITROPISM AT CONCENTRATIONS THAT INHIBIT POLAR AUXIN TRANSPORT . S. J. Roux, W.-Q. Tang, and Y. Sun. Section of Molecular Cell & Developmental Biology, Univ. Texas, Austin.

     There is normally a steep ATP gradient cross the plasma membrane, with cytoplasmic concentrations typically reaching several mM and concentrations in the bulk fluid of the extracellular matrix typically below nM. Previous results from this lab showed that plants might use this transmembrane ATP gradient to help power the efflux of various compounds across transporters in the ATP-binding cassette family. Here we present data showing that decreasing the gradient with exogenous ATP can inhibit the gravitropism of Arabidopsis roots,  and that this effect can be correlated with the inhibition of auxin export. Raising the level of extracellular ATP [xATP] to the mM levels found intracellularly can block gravitropism in roots of Arabidopsis thaliana. When plants are grown in a medium supplied with 1 mM ATP, their roots grow horizontally instead of growing straight down. Two mM ATP induces root curling, and 3 mM ATP stimulates lateral root growth. Gravity stimulation experiments show that 5 mM ATP is able to reduce or, in some cases, completely block the angle of root bending. These effects, which cannot be mimicked by equivalent concentrations of ADP or inorganic phosphate, may be due to the disturbance of auxin distribution in roots, for xATP can inhibit the extent of auxin (IAA) distribution in Arabidopsis roots, as judged indirectly by DR5-GUS transgenic plants, and it can increase the response sensitivity of plant roots to exogenously added NAA. Additionally, in corn root segments, 30% less ³H-IAA is polarly transported from the apical end to basal end when 5 mM ATP is supplied in receiver agar block.  Taken together, these results suggest that the inhibitory effects of xATP on IAA distribution may happen at the level of IAA export and that there is a potential role for ATP gradients in auxin export and plant root gravity sensing. 

     (Supported by NASA: NAG2-1347.)