Evolutionary origins and natural functions of fructans--a climatological, biogeographic and mechanistic appraisal

Genes for fructan synthesis arose in plants on several occasions ranging from the Cambrian (in Dasycladales-like algae), the late Devonian (in Jungermannialian liverworts) through to the Tertiary (six orders of angiosperms, including the evolutionarily advanced Poales and Asterales). The widespread occurrence of fructan processing in eubacteria may have been in response to the rapidly increasing availability of fructan in the angiosperms and the expansion of grass-eating mammals in the mid-Tertiary. The origin and evolution of the majority of fructan-rich angiosperm families, from fossil evidence, occurs in a relatively narrow window from the Oligocene to the mid-Miocene (30-15 Ma ago). These epochs coincide with the development in several regions, in most continents, of seasonal rainfall and more prolonged periods of water shortage. The replacement of tropical and temperate forests by a fructan flora in drylands and semi-arid to arid regions was world-wide and accounts for the rapid expansion of fructan-rich families in the mid-Tertiary and again in drier phases of the early Holocene. The natural distribution of the contemporary fructan flora confirms the association of fructan-rich families with temperate through to sub-tropical regions with seasonal or more sporadic rainfall and not with regions of low temperatures. It is argued that the natural function of fructan is in maintaining growth during periods of limited water availability. The function of fructan as a vacuolar and soluble carbohydrate in osmoregulation and in growth by cell inflation rather than temperature-dependent cell division is given as the most likely explanation for the global success of the 15% of the angiosperm flora which store fructan as a principal reserve carbohydrate.