Proteins accumulate in the apoplast of winter rye leaves during cold acclimation

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Abstract
During cold acclimation, winter rye (Secale cerealeL.) plants develop the ability to tolerate freezing temperatures by forming ice in intercellular spaces and xylem vessels. In this study, proteins were extracted from the apoplast of rye leaves to determine their role in controlling extracellular ice formation. Several polypeptides in the 15 to 32 kDa range accumulated in the leaf apoplast during cold acclimation at 5°C and decreased during deacclimation at 20°C. A second group of polypeptides (63, 65 and 68 kDa) appeared only when the leaves were maximally frost tolerant. Ice nucleation activity, as well as the previously reported antifreeze activity, was higher in apoplastic extracts from cold‐acclimated than from nonacclimated rye leaves. These results indicate that apoplastic proteins exert a direct influence on the growth of ice. In addition, freezing injury was greater in extracted cold‐acclimated leaves than in unextracted cold‐acclimated leaves, which suggests that the proteins present in the apoplast are an important component of the mechanism by which winter rye leaves tolerate ice formation