During the last two decades the world has experienced an abrupt change in climate. In particular, human activities such as combustion of fossil fuel, industrial processes and deforestation, have led to an increase in atmospheric CO2 concentration by 30% since the mid of the eighteenth century (1). As these emissions continue to increase, projections indicate that the atmospheric CO2 concentration will almost double by the end of this century. The impact of climate change is already being felt around the world in the form of more intense and frequent wildfires, hurricanes, floods, hail, frost, wind, and droughts. A growing body of research shows that these climate extremes and elevated CO2 and other greenhouse gas levels are also threatening plant growth and productivity while exerting considerable direct and indirect effects on the quality and quantity of plant nutrients (2-5). Climate change is expected to not only cause greater uncertainty for food producers but also severely impact crop nutrition in unexpected ways.
Fructans, linear or branched polymers of fructose often with a terminal glucose, are important storage carbohydrates found in approximately 15 % of the flowering plant species. In addition to acting as a reserve carbohydrate in these plants, they play a key role as stress protectants from climate extremes such as frost and drought. Fructans form an important part of our diet as they occur in several common vegetables such as onion, garlic, and leek, fruits such as bananas, dates, and grapefruit, and grains such as wheat and rye that are consumed in large amounts worldwide (6). As we lack the enzymes hydrolyzing fructans to monosaccharides, they reach the colon, almost unchanged, and are fermented by our resident gut bacteria to yield gut-health promoting molecules and foster further growth of beneficial microbes (17). A fructan-rich diet is now widely recognized for providing fermentable food fibers and therefore exerting several health promoting and immunomodulatory effects (7). However, fructans are also emerging as an important trigger for symptoms associated with functional gastrointestinal disorders such as irritable bowel syndrome (IBS). Hence, consuming a reduced fructan containing diet can lead to symptom improvement in IBS patients (18). Understanding how fructan composition of crops is changing under the influence of climate extremes and elevated CO2 levels can not only help define the nutritive value of grains, fruits, and vegetables that are rich in these molecules but also outline safe portion sizes for individuals attempting to restrict fructan consumption. This understanding will also be a key tool in developing strategies to enhance crop tolerance to stressful conditions, particularly under the changing climate prediction.
Although several studies have described the effect of elevated CO2 and temperature on important nutritional parameters such as, protein content, fiber content, carbon/nitrogen ratio and tannin concentration of plants (8-10), the available data on changes in fructan content affected by climate change is limited. An important hurdle in clearly defining the effect of climate change on fructan composition is the species-dependent nature of this interaction. Plant responses to climate change are complicated by significant ‘species X climate’ interactions and show species-group specific responses. Moreover, there is a huge diversity in molecular structure and weight, polymer chain length, and linkage types between fructosyl residues of fructan molecules. This is compounded by the variation in fructan species that occur in different parts of the same plant at different stages of growth and in different seasons (19). Even daily oscillations in fructan content with tolerance to changes in weather and air pollution have been reported (16). Variations in seasonal cycles, local weather, and pollution levels could all be induced by climate change. Hence predicting fructan composition and concentration changes in various plant species, while factoring in the influence of the multifactorial stress imposed by climate change represents a very complex problem.
Despite these hurdles, some studies suggest that fructans and other related storage carbohydrates are expected to increase under elevated CO2 conditions (11, 12). Climate change induced stresses such as drought, have also been shown to alter the fructan composition of fructan accumulating plants to favor water retention, reinforcing their role as stress protectants (13). Tolerance to flooding has also been associated with an increased fructan content (15). A combination of drought exposure under elevated temperatures (+ 3°C) and CO2 concentrations (620 ppm) is also expected to lead to an increase in plant fructan content (14).
Moreover, fructan accumulating plants and non-fructan accumulating plants respond differently to extreme climate conditions and elevated CO2 (14). Compared to the non-fructan accumulating plants, fructan accumulators are able to preserve their tissue quality (protein, macro and micronutrients) better under stress and elevated CO2. This leads us to the speculation whether the robust fructan accumulators - which can better withstand the environmental stresses that would be a consequence of climate change - might dominate our diets in the future. Alternatively, a deeper understanding of the molecular mechanisms of fructan-based stress tolerance might help us design strategies to generate sturdier crop varieties to maintain the current diversity of popular food crops as our planet’s climate changes. Answering such questions will prove to be central to the development of future crop and dietary interventions to ensure that we continue to have access to abundant, safe, and nutritious food in the wake of climate extremes.
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