Author
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Crafts-Brandner, Steven |
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Submitted to: New Phytologist
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 1/1/2005 Publication Date: 3/1/2005 Citation: Crafts-Brandner, S.J. 2005. Fructans and freezing tolerance. New Phytologist 166:708-709. Interpretive Summary: Fructans are a form of carbohydrate that, analogous to starch, serve as a storage reservoir that can be utilized at times when the supply of carbohydrate via photosynthesis is limiting. Fructans are linear or branched polymers of fructose that occur in about 15% of flowering plant species, including many which are cultivated commercially. They are synthesized and stored in the vacuole of plant cells. However, it has long been speculated that their role is wider than just storage. For example, fructan metabolism has been implicated in plant tolerance to drought and frost, in the defense against infection by fructan-producing pathogens. In this brief commentary exciting new findings pertaining to the physiological role(s) of fructan are described. Specifically, two novel enzymes called 6-kestose exohydrolases (6-KEH) were isolated from cold-hardened wheat crown tissue that is adapted to survive at below freezing temperatures. The 6-KEHs are novel among fructan metabolizing enzymes because of their biochemical properties and because they are localized in the apoplastic fluid of plant tissues, far removed from the vacuole. These results indicate that fructan metabolism functions to protect cell membranes exposed to freezing temperature, thus providing the first definitive evidence that fructans have physiological roles in addition to serving as stored carbohydrate. Technical Abstract: Fructans are linear or branched polymers of fructose that occur in about 15% of flowering plant species, including many which are cultivated commercially – they are synthesized from sucrose in the vacuole, where they are stored as reserve nonstructural carbohydrates. However, it has long been speculated that their role is wider than just storage. For example, there is much indirect evidence that fructan metabolism impacts on the tolerance of plants to drought and frost, and aides in the defense against infection by fructan-producing pathogens. In this brief commentary exciting new findings pertaining to the physiological role(s) of fructan are described. Specifically, two novel 6-kestose exohydrolase (6-KEH) isozymes from cold-hardened wheat crown tissue have been cloned and characterized for biochemical properties. The 6-KEHs are novel among fructan metabolizing enzymes because of their high specificity for a single substrate, 6-kestose, and their localization in the apoplastic fluid of fructan-containing sink tissues. These results indicate that fructan degradation in the apoplast functions to stabilize cell membranes exposed to freezing temperature, thus providing the first definitive evidence that fructans have physiological roles in addition to serving as stored carbohydrate. |
