Agriculture Reference
In-Depth Information
Inulin-type fructans, derived from chicory, are widely used as prebiotics in func-
tional food leading to improved health and well-being (Roberfroid 2007 ). Recent
insights suggest that they could counteract oxidative stress in the human body as
well (Stoyanova et al. 2011 ).
4   Sugars as Osmoprotectants
One of the mechanisms that plants use to combat the detrimental effects of en-
vironmental stress is to synthesise different kinds of protective compounds such
as compatible solutes and antioxidants. Soluble carbohydrates (e.g., trehalose, su-
crose, raffinose and fructans) along with certain amino acids (e.g., proline), quater-
nary ammonium compounds (e.g., glycinebetaine), and polyols (e.g., mannitol) are
thought to be compatible solutes. They are synthesized in response to osmotic stress
by definition and can occur at high intracellular concentrations without interfering
with normal cellular metabolism (Shen et al. 1997 ; Mundree et al. 2002 ; Parva-
nova et al. 2004 ). They act as osmoprotectants and some of them (e.g. fructans) are
also storage carbohydrates (Kawakami et al. 2008 ). Compatible solutes facilitate
osmotic adjustments during water stress and in addition may serve as protective
agents by stabilizing proteins and membranes (Hincha et al. 2002 ). It should be
noted that most compatible solutes show excellent capabilities to scavenge ROS in
vitro, suggesting that they have similar roles in plants (Van den Ende and Valluru
2009 ; see also below).
4.1   General Mode of Action under Stress
The primary cause of injury during freezing is the destabilization of cellular mem-
branes (Uemera and Steponkus 1999 ). Also during desiccation, the cell needs to
keep all its membranes (plasma membrane, tonoplast membrane, organellar mem-
branes) and proteins in proper functional state. In general, soluble sugar levels
contribute to the increased cryostability of cellular membranes (Ma et al. 2009 ),
keeping membranes in their proper state which is a prerequisite for survival under
unfavorable conditions. Resurrection plants develop an array of mechanisms to sur-
vive complete dehydration, sugar accumulation being one of them (Djilianov et al.
2011 ). Sugars can replace water under drought stress. As such, they keep mem-
brane surfaces “hydrated” and prevent membrane fusion by maintaining the space
between phospholipid molecules (Mundree et al. 2002 ; Valluru and Van den Ende
2008 ). This can be explained in terms of 'sugar vitrification'. It includes the forma-
tion of a solid, amorphous glass that prevents membrane fusion. Hydrogen bonds
are not present in such glassy states. The magnitude of sugar vitrification depends
on the temperature at which the glass devitrifies (T g ), which itself depends on the
molecular weight of the sugar and on the water content (Levine and Slade 1991 ).
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