Orchestration of phosphate homeostasis by the ITPK1-type inositol phosphate kinase in the liverwort Marchantia polymorpha.

Land plants have evolved sophisticated sensing mechanisms and signalling pathways to adapt to phosphate-limited environments. While molecular players contributing to these adaptations in flowering plants have been described, how non-vascular bryophytes regulate phosphate (Pi) homeostasis remained largely unknown. In this study, we present findings that both male and female plants of the liverwort Marchantia polymorpha respond to altered phosphate availability through substantial developmental changes. We show that the second messenger inositol pyrophosphates (PP-InsPs) respond more quickly to changes in cellular Pi status than the lower inositol phosphates, highlighting a functional relationship between PP-InsP and Pi homeostasis in M. polymorpha. To further corroborate the possible involvement of PP-InsP in Pi homeostasis, we characterized M. polymorpha INOSITOL (1,3,4) TRIPHOSPHATE 5/6 KINASE1 (MpITPK1) that phosphorylates InsP6 to generate InsP7 both in vitro and in vivo. Consistent with the role of PP-InsPs in Pi homeostasis, M. polymorpha lines with enhanced MpITPK1 expression leading to the accumulation of 5-InsP7 and an InsP8 isomer exhibit altered expression of phosphate starvation induced (PSI) genes and display attenuated responses to low phosphate. The characterization of MpPHO1-deficient plants with dramatically increased levels of 1,5-InsP8 further supports the role of PP-InsP in Pi homeostasis in this liverwort species. Notably, our study unveiled that MpITPK1 rescues the deregulated Pi homeostasis in Arabidopsis (Arabidopsis thaliana) ITPK1-deficient plants, suggesting that liverwort and eudicots share a functional ITPK1 homolog. In summary, our study provides insights into the regulation of Pi homeostasis by ITPK1-derived PP-InsPs in M. polymorpha.