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Nature 444, 490-493 (23 November 2006) | doi:10.1038/nature05311; Received 28 July 2006; Accepted 4 October 2006; Published online 5 November 2006

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Self-incompatibility in Papaver targets soluble inorganic pyrophosphatases in pollen

Barend H. J. de Graaf1,2, Jason J. Rudd1,2,3, Michael J. Wheeler1,2,3, Ruth M. Perry1, Elizabeth M. Bell1, Kim Osman1, F. Christopher H. Franklin1 & Vernonica E. Franklin-Tong1

  1. School of Biosciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
  2. These authors contributed equally to this work.
  3. Present addresses: Rothamsted Research, Harpenden, Hertfordshire AL5 2JQ, UK (J.J.R.); Institute of Cancer Studies, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK (M.J.W.).

Correspondence to: Vernonica E. Franklin-Tong1 Correspondence and requests for materials should be addressed to V.E.F.-T. (Email: v.e.franklin-tong@bham.ac.uk). Sequences have been deposited in the EMBL Nucleotide Sequence Database (http://www.ebi.ac.uk/embl/) under accession codes AM162550 and AM162551.

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In higher plants, sexual reproduction involves interactions between pollen and pistil. A key mechanism to prevent inbreeding is self-incompatibility through rejection of incompatible ('self') pollen1. In Papaver rhoeas, S proteins encoded by the stigma interact with incompatible pollen, triggering a Ca2+-dependent signalling network2, 3, 4, 5 resulting in pollen tube inhibition and programmed cell death6. The cytosolic phosphoprotein p26.1, which has been identified in incompatible pollen, shows rapid, self-incompatibility-induced Ca2+-dependent hyperphosphorylation in vivo3. Here we show that p26.1 comprises two proteins, Pr-p26.1a and Pr-p26.1b, which are soluble inorganic pyrophosphatases (sPPases). These proteins have classic Mg2+-dependent sPPase activity, which is inhibited by Ca2+, and unexpectedly can be phosphorylated in vitro. We show that phosphorylation inhibits sPPase activity, establishing a previously unknown mechanism for regulating eukaryotic sPPases. Reduced sPPase activity is predicted to result in the inhibition of many biosynthetic pathways, suggesting that there may be additional mechanisms of self-incompatibility-mediated pollen tube inhibition. We provide evidence that sPPases are required for growth and that self-incompatibility results in an increase in inorganic pyrophosphate, implying a functional role for Pr-p26.1.