AJ Cann
Chrysolaena obovata, an aster of the Brazilian Cerrado, presents seasonal growth, marked by senescence of aerial organs in winter and subsequent regrowth at the end of this season. The underground reserve organs, the rhizophores, accumulate inulin-type fructans and confer tolerance to drought and low temperature. Fructans and fructan-metabolizing enzymes show a characteristic spatial and temporal distribution in the rhizophores during the developmental cycle. Previous studies have shown correlations between abscisic acid (ABA) or indole acetic acid (IAA), fructans, dormancy and tolerance to drought and cold, but the signalling mechanism for the beginning of dormancy and sprouting in this species is still unknown. A new paper in Annals of Botany examines the fructan metabolism in this species in response to environmental changes.
Plants were sampled from the field across phenological phases including dormancy, sprouting and vegetative growth. Endogenous concentrations of ABA and IAA were determined and measurements made of fructan content and composition, and enzyme activities. The relative expression of corresponding genes during dormancy and sprouting were also determined. Plants showed a high fructan 1-exohydrolase activity and expression during sprouting in proximal segments of the rhizophores, indicating mobilization of fructan reserves, when ABA concentrations were relatively low and precipitation and temperature were at their minimum values. Higher IAA concentrations were consistent with the role of this regulator in promoting cell elongation and plant growth. With high rates of precipitation and high temperatures in summer, the fructan-synthesizing enzyme sucrose:sucrose 1-fructosyltransferase showed higher activity and expression in distal segments of the rhizophores, which decreased over the course of the vegetative stage when ABA concentrations were higher, possibly signalling the entry into dormancy.
These results show that fructan metabolism correlates well with endogenous hormone concentrations and environmental changes, suggesting that the co-ordinated action of carbohydrate metabolism and hormone synthesis enables C. obovata to survive unfavourable field conditions. Endogenous hormone concentrations seem to be related to regulation of fructan metabolism and to the transition between phenophases, signalling for energy storage, reserve mobilization and accumulation of oligosaccharides as osmolytes.
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