Changes in chromatin structure associated with germination of maize and their relation with desiccation tolerance

Morphological and physicochemical measurements of chromatin condensation were made on germinating maize (Zea mays L.) radicles to determine whether the loss of genetic activities that occurs during the loss of desiccation tolerance is linked to irreversible changes in chromatin condensation. Chromatin samples were compared at different stages of germination (0, 24 and 72 h after imbibition), before (control) and after 24 h of desiccation. Morphological changes in chromatin structure and condensation were characterized by a qualitative and quantitative electron microscope study of chromatin which was allowed to spread in 0.2 mol m^?3 EDTA and then laid on coated microscope grids. The experiments showed similar levels of chromatin condensation in quiescent embryos and 24-h-old radicles (desiccation-tolerant material). After 72 h of imbibition, when radicle emergence and desiccation intolerance had ceased, the chromatin underwent a major decondensation towards various lower order folded structures. Regardless of the desiccation tolerance stage, an in vivo drying treatment of 24- and 72-h-old radicles before chromatin extraction did not induce significant changes in the extent of condensation compared to their respective controls. Similar conclusions were drawn from measurements of several spectroscopy properties (absorbance ratios: A₂₆₀/A₂₄₀, A₂₆₀/A4₀₀; thermal denaturation, and linear electric dichroism) of chromatin fragments that were obtained after nuclease digestion and then dissolved in 0-2 mol m^?3 EDTA. In quiescent and 24-h-old material, chromatin fragments were poorly soluble but highly stable during thermal denaturation. Chromatin fragments were 3-5-fold more soluble and less thermally stable in 72-h-old material than in 24-h-old material. In vivo desiccation had no significant effects on these properties compared to the respective controls. Collectively these data suggest that desiccation did not induce irreversible changes in the condensation properties of chromatin. The likelihood that the decondensation process occurring during germination is linked to the loss of desiccation tolerance is discussed.