Phosphoproteomic profiling reveals ABA‐responsive phosphosignaling pathways in Physcomitrella patens

Summary Abscisic acid (ABA) and its signaling system are important for land plants to survive in terrestrial conditions. Here, we took a phosphoproteomic approach to elucidate the ABA signaling network in Physcomitrella patens, a model species of basal land plants. Our phosphoproteomic analysis detected 4630 phosphopeptides from wild‐type P. patens and two ABA‐responsive mutants, a disruptant of group‐A type‐2C protein phosphatase (PP2C; ppabi1a/b) and AR7, a defective mutant in ARK, identified as an upstream regulator of SnRK2. Quantitative analysis detected 143 ABA‐responsive phosphopeptides in P. patens. The analysis indicated that SnRK2‐mediated phosphorylation and target motifs were partially conserved in bryophytes. Our data demonstrate that the PpSnRK2B and AREB/ABF‐type transcription factors are phosphorylated in vivo in response to ABA under the control of ARK. On the other hand, our data also revealed the following: (i) the entire ABA‐responsive phosphoproteome in P. patens is quite diverse; (ii) P. patens PP2C affects additional pathways other than the known ABA signaling pathway; and (iii) ARK is mainly involved in ABA signaling. Taken together, we propose that the core ABA signaling pathway is essential in all land plants; however, some ABA‐responsive phosphosignaling uniquely developed in bryophytes during the evolutionary process. Significance Statement Phosphoproteomic profiling in Physcomitrella patens revealed ABA‐dependent phosphosignaling, leading us to compare ABA responses between bryophytes and higher plants at the signaling level.