SD‐RLK28 positively regulates pollen hydration on stigmas as a PCP‐Bβ receptor in Arabidopsis thaliana

Pollen hydration on dry stigmas is strictly regulated by pollen–stigma interactions in Brassicaceae. Although several related molecular events have been described, the molecular mechanism underlying pollen hydration remains elusive. Multiple B‐class pollen coat proteins (PCP‐Bs) are involved in pollen hydration. Here, by analyzing the interactions of two PCP‐Bs with three Arabidopsis thaliana stigmas strongly expressing S‐domain receptor kinase (SD‐RLK), we determined that SD‐RLK28 directly interacts with PCP‐Bβ. We investigated pollen hydration, pollen germination, pollen tube growth, and stigma receptivity in the sd‐rlk28 and pcp‐bβ mutants. PCP‐Bβ acts in the pollen to regulate pollen hydration on stigmas. Loss of SD‐RLK28 had no effect on pollen viability, and sd‐rlk28 plants had normal life cycles without obvious defects. However, pollen hydration on sd‐rlk28 stigmas was impaired and pollen tube growth in sd‐rlk28 pistils was delayed. The defect in pollen hydration on sd‐rlk28 stigmas was independent of changes in reactive oxygen species levels in stigmas. These results indicate that SD‐RLK28 functions in the stigma as a PCP‐Bβ receptor to positively regulate pollen hydration on dry stigmas. In Arabidopsis thaliana, the pollen coat protein PCP‐Bβ acts in the pollen to regulate pollen hydration and the S‐domain receptor‐like kinase SD‐RLK28 functions as a PCP‐Bβ receptor in the stigma that positively regulates pollen hydration independent of reactive oxygen species levels in stigmas.