Protein Phosphatase 2A B'α and B'β promote pollen wall construction partially through BZR1-activated CEP1 in Arabidopsis.

Abstract

A well-constructed pollen wall is essential for pollen fertility, which relies on the contribution of tapetum. Our results demonstrate an essential role of the tapetum-expressed protein phosphatase 2A (PP2A) B'α and B'β in pollen wall formation. The b'aβ double mutant pollen grains harbored sticky remnants and tectum breakages, resulting in failed release. B'α and B'β function partially through dephosphorylating and activating BZR1. The bzr1 bes1 double and higher-order mutants of this family displayed similar defects in pollen wall, while bzr1-1D, having an active mBZR1 exhibited fertile pollen grains in a B'α and B'β dependent manner. Correspondingly, the level of phospho-BZR1 is increased and dephospho-BZR1 is decreased in b'aβ and bzr1-1D/b'aβ at anther stages 8-9 as compared with Col-0 and bzr1-1D, respectively. A cysteine protease gene CEP1 was identified as a BZR1 target, whose transcriptional activation necessitates BRREs in the promoter region and BZR1 DNA binding domain. The mRNA level of CEP1 at stages 8-9 was extremely low in bzr1 and bzr1 bes1, while higher in Col-0 and bzr1-1D depending on B'α and B'β. Furthermore, cep1 mutants displayed similar defects in pollen wall. In brief, this study uncovered a PP2A-BZR1-CEP1 regulatory module, providing a new insight into pollen maturation mechanism.