Desiccation tolerance in Physcomitrella patens: Rate of dehydration and the involvement of endogenous abscisic acid (ABA)

The moss Physcomitrella patens, a model system for basal land plants, tolerates several abiotic stresses, including dehydration. We previously reported that Physcomitrella patens survives equilibrium dehydration to -13MPa in a closed system at 91% RH. Tolerance of desiccation to water potentials below -100MPa was only achieved by pretreatment with exogenous abscisic acid (ABA). We report here that gametophores, but not protonemata, can survive desiccation below -100MPa after a gradual drying regime in an open system, without exogenous ABA. In contrast, faster equilibrium drying at 90% RH for 3-5days did not induce desiccation tolerance in either tissue. Endogenous ABA accumulated in protonemata and gametophores under both drying regimes, so did not correlate directly with desiccation tolerance. Gametophores of a Ppabi3a/b/c triple knock out transgenic line also survived the gradual dehydration regime, despite impaired ABA signaling. Our results suggest that the initial drying rate, and not the amount of endogenous ABA, may be critical in the acquisition of desiccation tolerance. Results from this work will provide insight into ongoing studies to uncover the role of ABA in the dehydration response and the underlying mechanisms of desiccation tolerance in this bryophyte. The moss Physcomitrella patens, as a model system for basal land plants, has an important role in efforts aimed at understanding the evolutionary dynamics of genes that constitute cellular signaling pathways involved in abiotic stress responses. We report that both gametophores and protonemata are sensitive to slow dehydration under equilibrium drying regimes but desiccation tolerance can be induced in gametophores during gradual drying in an open system. Endogenous abscisic acid (ABA) accumulation did not correlate directly with desiccation tolerance, and impairment of ABA signaling pathways did not render the gametophores sensitive to desiccation. It appears that the initial drying rate, and not endogenous ABA amounts, is critical for the acquisition of desiccation tolerance.