Early events leading to water deficit responses in the liverwort Marchantia polymorpha

To understand the genetic and evolutionary basis of the responses to water deficit in plants, we turned to the basal liverwort Marchantia polymorpha, representing one of the most ancient plant lineages that first colonized land. Thalli exposed to water deficit showed a decreased growth rate and morphological modifications. In addition, water deficit led to relocation of cell organelles, mostly due to changes in volume of the central vacuole, which correlated with adjustments of osmotic potential. Abscisic acid (ABA) is a phytohormone that mediate responses to environmental stress in vascular plants. Consistently, we observed an increase in endogenous ABA levels upon water deficit in Marchantia, which correlated with the increased expression of genes encoding for LATE-EMBRYOGENESIS-ABUNDANT proteins, which are thought to protect cells from damage caused by water deficit. Transcriptome-level changes upon ABA and water deficit treatments revealed that several genes modify their expression under these conditions. Consistent with our cellular level observations, differentially expressed GO categories related to ion binding and transport may explain the changes in vacuole volume. Induction of stress-related genes, including several LEA genes, also points towards a possible conservation of ABA-dependent signaling pathways in M. polymorpha and defines a framework to understand the underlying molecular events.