Apple S ‐ RN ase triggers inhibition of tRNA aminoacylation by interacting with a soluble inorganic pyrophosphatase in growing self‐pollen tubes in vitro

Summary Apple exhibits S ‐ RN ase‐based self‐incompatibility ( SI ), in which S ‐ RN ase plays a central role in rejecting self‐pollen. It has been proposed that the arrest of pollen growth in SI of Solanaceae plants is a consequence of the degradation of pollen rRNA by S ‐ RN ase; however, the underlying mechanism in Rosaceae is still unclear. Here, we used S 2 ‐ RN ase as a bait to screen an apple pollen cDNA library and characterized an apple soluble inorganic pyrophosphatase (Md PP a) that physically interacted with S ‐ RN ases. When treated with self S ‐ RN ases, apple pollen tubes showed a marked growth inhibition, as well as a decrease in endogenous soluble pyrophosphatase activity and elevated levels of inorganic pyrophosphate ( PP i). In addition, S ‐ RN ase was found to bind to two variable regions of Md PP a, resulting in a noncompetitive inhibition of its activity. Silencing of Md PP a expression led to a reduction in pollen tube growth. Interestingly, tRNA aminoacylation was inhibited in self S ‐ RN ase‐treated or Md PP a‐silenced pollen tubes, resulting in the accumulation of uncharged tRNA . Furthermore, we provide evidence showing that this disturbance of tRNA aminoacylation is independent of RN ase activity. We propose an alternative mechanism differing from RNA degradation to explain the cytotoxicity of the S ‐ RN ase apple SI process.