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.

Abstract

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 network^{2, 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.

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