Surviving dehydration: shutdown–restart dynamics in <i>Physcomitrium patens</i> avoids the metabolic collapse seen in <i>Arabidopsis</i>

Crop productivity and food security faces an ever-increasing challenge from drought events world-wide and to establish more drought tolerant crops requires deeper insights into plant dehydration tolerance. We compared the inducible vegetative dehydration tolerance (IVDT) of the moss Physcomitrium patens and the vegetative dehydration sensitivity (VDS) of the dicot Arabidopsis thaliana, using a combination of structural, physiological and transcriptomic analyses. Key components in the IVDT response of P. patens, ELIPs and bZIP transcription factors, were functionally investigated using both transient and stable transformation. Physcomitrium patens exhibited survival after c. 98% water loss, with reversible cellular changes, and a 'shutdown-restart' physiological and transcriptomic program. By contrast, A. thaliana perished below 25% relative water content and suffered irreversible cellular damage. Physcomitrium patens's accumulated protective gene transcripts (e.g. ELIPs, SODs, and bZIPs) during dehydration, whereas Arabidopsis prioritized stress avoidance over protection. Functional validation indicted PpELIPs stabilized photosynthetic pigments in transgenic plants, while PpbZIP transcription factors enhanced water retention via abscisic acid-independent pathways. The comparison of divergent response mechanisms, IVDT and VDS, to dehydration revealed components that protect photosynthesis and alter plant water relations to delay wilting and maintain productivity during water limiting conditions thus offering bryophyte-based strategies for crop improvement for drought tolerance.