High ice nucleation activity located in blueberry stem bark is linked to primary freeze initiation and adaptive freezing behaviour of the bark

• Published in: AoB plants 2014-07, Vol.6, p.plu044-plu044
• Main Authors: Kishimoto, Tadashi, Yamazaki, Hideyuki, Saruwatari, Atsushi, Murakawa, Hiroki, Sekozawa, Yoshihiko, Kuchitsu, Kazuyuki, Price, William S., Ishikawa, Masaya
• Format: Article
• Language: English
• ISSN: 2041-2851; 2041-2851
• DOI: 10.1093/aobpla/plu044

One may have seen wintering rosette leaves totally frozen and wilted in the early morning but recover during the daytime. How can cold hardy plants survive freezing of the tissues, unlike animal tissues? Cold hardy plants seem to have evolved various strategies. One example is extracellular freezing, where icicles primarily form in intercellular spaces whilst the cells are dehydrated, yet the underlying mechanisms remain unclear. In this study, using blueberry stems, we found high ice nucleation activity specifically localized in the cell wall fraction of bark tissues. This activity likely contributes to the primary and spontaneous initiation of freezing in the intercellular spaces of the bark to successfully perform extracellular freezing.AbstractControlled ice nucleation is an important mechanism in cold-hardy plant tissues for avoiding excessive supercooling of the protoplasm, for inducing extracellular freezing and/or for accommodating ice crystals in specific tissues. To understand its nature, it is necessary to characterize the ice nucleation activity (INA), defined as the ability of a tissue to induce heterogeneous ice nucleation. Few studies have addressed the precise localization of INA in wintering plant tissues in respect of its function. For this purpose, we recently revised a test tube INA assay and examined INA in various tissues of over 600 species. Extremely high levels of INA (−1 to −4 °C) in two wintering blueberry cultivars of contrasting freezing tolerance were found. Their INA was much greater than in other cold-hardy species and was found to be evenly distributed along the stems of the current year's growth. Concentrations of active ice nuclei in the stem were estimated from quantitative analyses. Stem INA was localized mainly in the bark while the xylem and pith had much lower INA. Bark INA was located mostly in the cell wall fraction (cell walls and intercellular structural components). Intracellular fractions had much less INA. Some cultivar differences were identified. The results corresponded closely with the intrinsic freezing behaviour (extracellular freezing) of the bark, icicle accumulation in the bark and initial ice nucleation in the stem under dry surface conditions. Stem INA was resistant to various antimicrobial treatments. These properties and specific localization imply that high INA in blueberry stems is of intrinsic origin and contributes to the spontaneous initiation of freezing in extracellular spaces of the bark b