Fructan Metabolism in Plant Growth and Development and Stress Tolerance

Photosynthesis is a fundamental process for life, converting solar energy into chemical energy. This then powers the assimilation of carbon into organic compounds, like carbohydrates, which are used to synthesize other compounds such as organic acids, amino acids and lipids to form the basic components for biomass accumulation. Water-soluble carbohydrates (WSCs) play a central role in the metabolism of plants as carbon and energy sources in cells, and their levels are continuously adjusted as a result of the balance between supply and demand of carbon at the whole plant level. As a consequence, the metabolism of sugars is very dynamic and varies with the stage of development of plants and in response to the environment. Fructans are the largest reserve of carbohydrates in approximately 15% of higher plants. They are synthesized from sucrose in the vacuole by a group of fructosyltransferase (FT) enzymes and catalysed by fructan exohydrolase (FEH) enzymes. They can be found in vegetative organs – stem, leaves and roots – and in the grain, depending on the state of development of the plant and environmental conditions, such as light intensity, temperature and water availability. Along with its role as a carbohydrate reserve, fructans confer tolerance to cold and drought, contribute to the maintenance of osmotic potential, participate in the stabilization of the membranes and play an important role during grain filling. In temperate cereals, such as wheat (Triticum aestivum) and barley (Hordeum vulgare), carbohydrates are stored mainly in the stem as WSCs and are composed predominantly of grass-type fructans, which may represent more than 80% of the WSCs, followed by sucrose and, to a lesser extent, glucose and fructose. In the absence of stress, fructans accumulate in the stem until they reach a maximum content at early grain filling; later they are degraded and partially remobilized to the grain for the synthesis of starch in late stages of grain filling. However, under unfavourable environmental conditions, fructans can be degraded in the early stages of grain filling to effectively compensate for the decrease in photosynthates and to sustain the rate of grain filling. Therefore, stem fructans can play an important role in grain yield under stressed conditions, and they contribute significantly to the final grain weight in cereals. In addition, genotypic differences in the pattern of fructan accumulation and in the expression genes regulating fructan metabolism have been reported for wheat and other cereals.