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Knockouts of Either APY1 or APY2 Genes Suppress Root Gravitropism in Arabidopsis.   S. J. Roux, J. Zalawadia, J. Wu, and T. Butterfield. Sect. of Molecular Cell & Developmental Biology. Univ. of Texas, Austin, TX.

   Apyrases (NTPDases) are enzymes that hydrolyze ATP to ADP and ADP to AMP. Many of the best-characterized apyrases in animal cells function primarily as ectoapyrases; i.e., they are localized in the plasma membrane with their active site facing to the outside of the cell. There they lower the concentration of extracellular ATP and ADP (eATP and eADP), agents that can induce signaling pathways that alter cellular functions. eATP and eADP also turn on signaling changes in plant cells, including an increase in the concentration of cytosolic calcium, superoxide and nitric oxide production, and gene expression changes, just as they do in animal cells. Two Arabidopsis ectoapyrases that are 87% identical in protein sequence, AtAPY1 and AtAPY2, co-purify with plasma membranes in cell fractionation assays, and in growing pollen tubes their ability to hydrolyze eATP can be blocked with either polyclonal antibodies that recognize both proteins or chemical inhibitors of apyrase activity. A role for these enzymes in growth control was demonstrated by Wu et al. (Plant Physiology 143: 1638, 2007), who showed that apy1apy2 double knock out mutants are dwarf. Primary roots of 3-d-old mutant seedlings that are suppressed in their expression of either APY1 or APY2 showed significantly reduced gravitropism when curvature measurements were taken 24 h after gravistimulation. The length of the primary roots of either apy1 or apy2 mutants was the same in 3-d-old mutants and wild-type plants, so inhibition of gravitropism was not due to general inhibition of growth. Based on the assumption that root cells release ATP as they grow (see Kim et al., Plant Physiology, 2006), we interpret these results to mean that: (i) a higher level of eATP accumulates in the apy1 and apy2 mutants; (ii) these higher concentrations interfere with normal auxin transport, as reported by Tang et al. (Plant Physiology, 2003), and (iii) reduced auxin transport results in reduced gravitropic curvature. Supported by NASA NAG2-1586 and NSF IBN-0344221 to S.J.R.)