Glass formation in plant anhydrobiotes: survival in the dry state*1

Abstract

Anhydrobiotes can resist complete dehydration and survive the dry state for extended periods of time. During drying, cytoplasmic viscosity increases dramatically and in the dry state, the cytoplasm transforms into a glassy state. Plant anhydrobiotes possess large amounts of soluble non-reducing sugars and their state diagrams resemble those of simple sugar mixtures. However, more detailed in vivo measurements using techniques such as Fourier transform infrared spectroscopy and electron paramagnetic resonance spectroscopy reveal that these intracellular glasses are complex systems with properties quite different from those of simple sugar glasses. Intracellular glasses exhibit a high molecular packing and slow molecular mobility, resembling glasses made of mixtures of proteins and sugars, which potentially interact with additional cytoplasmic components such as salts, organic acids, and amino acids. Above the glass transition temperature, the cytoplasm of biological systems still exhibits a high stability and low molecular mobility, which could serve as an ecological advantage. All desiccation-tolerant organisms form glasses upon drying, but desiccation-sensitive organisms generally lose their viability during drying at water contents at which the glassy state has not yet been formed, suggesting that other factors are necessary for desiccation tolerance. Nevertheless, the formation of intracellular glasses is indispensable to survive the dry state. Storage stability of seeds and pollens is related to the molecular mobility and packing density of the intracellular glass, suggesting that the characteristic properties of intracellular glasses provide stability for long-term survival.