Agriculture Reference
In-Depth Information
1993
). The fructan syndrome probably arose some 30 ± 15 million years ago dur-
ing a climatologic shift to seasonal drought (Hendry
1993
). Thus, drought might
have been an important evolutionary trigger and fructans may have played a role
in drought resistance (De Roover et al.
2000
). Plants with the ability to synthesize
fructans are important components of ecosystems that experience frequent envi-
ronmental changes (Albrecht et al.
1997
). Typically, the DP of fructans is modified
during such changes (Amiard et al.
2003
).
The fructans have been classified in five different types depending on the type
of linkages between the fructosyl units and their branching: Inulin consisting of
β(beta)(2,1) linkages, levan containing β(beta)(2,6) linkages and graminan hav-
ing both β(beta)(2,1) and β(beta)(2,6) linkages. Neo-inulin and neo-levan type of
fructans as occurring in
Lolium
,
Asparagus
and
Allium
contain an internal glucose
residue (Van den Ende et al.
2002
). Fructans are believed to be synthesized in the
central vacuole. The first step in fructan synthesis is catalyzed by sucrose: sucrose
1-fructosyltransferase (1-SST), synthesizing glucose and 1-kestotriose from two
sucrose molecules. Depending on the species, further elongation occurs through the
action of a specific number of other fructosyltransferases (FTs; 1-FFT, 6G-FFT and
6-SFT) by further adding β(beta)(2,1)- and/or β(beta)(2,6)-linked fructosyl moieties
(Van den Ende et al.
2011
and references therein). Fructan breakdown is accom-
plished by fructan exohydrolases (FEHs). 1-FEH, 6-FEH, 6-KEH (6-kestose exo-
hydrolase), and 6&1-FEH type of enzymes have been described in fructan plants
(Kawakami et al.
2005
and references therein). However, such FEHs also occur in
non-fructan accumulating plants but their functions in such plants remain unclear
(De Coninck et al.
2005
). The regulation of fructan biosynthetic and breakdown
genes is mainly controlled at the transcriptional level (Van Laere and Van den Ende
2002
). Dicot FT genes are specifically induced by sucrose and dicot FEH genes are
induced by cold (Michiels et al.
2004
). However, many FEHs are also inhibited by
sucrose at the posttranslational level (Van den Ende et al.
2001
). Bacterial fructans
(levans, inulins) are much longer than plant fructans and involve the activity of
levansucrases and inulosucrases (Banguela et al.
2011
).
Fructans can act as long-term (inulins, dicots) and as short-term (other fructan
types, grasses) reserve carbohydrates in different organs such as roots, stems, grains
and sometimes in leaves (Pollock and Cairns
1991
; Van Laere and Van den Ende
2002
). Similar to invertases splitting sucrose into glucose and fructose (Koch
2004
;
Roitsch and Gonzalez
2004
), the sucrose splitting capacities of some FTs can be
used to establish sucrose gradients and regulate sink strength (Ji et al.
2010
). It can
be speculated that storing fructans might be advantageous compared to starch. First,
FTs are less inhibited by cold compared to starch synthases (Pollock et al.
1999
).
Second, remobilization of water soluble fructans is likely to occur at a faster rate
compared to the insoluble starch (Van Laere and Van den Ende
2002
).
Furthermore, fructans and FEH regulate osmosis during flower opening in
Cam-
panula rapunculoides
and other species (Vergauwen et al.
2000
). Fructans fuel
rapid regrowth in grasses and act as membrane stabilizers under stress (Valluru and
Van den Ende
2008
; Lothier et al.
2010
).
