Agriculture Reference
In-Depth Information
name implies, joining additional fructose units via the 2 position in their ring
structure to the 6 position in the glucose ring in a fructo-oligosaccharide, the
enzyme fructan:fructan 6
G
-fructosyl transferase (6 G-FFT) can also catalyse
the joining of the 2 position of a fructose unit to the 1 position of a fructose unit
in a fructo-oligosaccharide, acting thereby as a fructan:fructan 1-fructosyl
transferase (1F- FFT). The ratio of the 6G- FFT activity to the 1-FFT activity of
the purified enzyme is 2.3:1 (Fujishima
et al.
, 2005). Therefore with just two
enzymes, SST and the FFT, the range of fructo-oligosaccharides found in onion
are synthesized, as shown in Fig. 8.5 (Ritsema
et al.
, 2004; Fujishima
et al.
,
2005). Hydrolytic enzymes that reverse these processes and degrade fructans
by removing and releasing the terminal fructose unit have also been extracted
from onions (Benkeblia
et al.
, 2004).
The sequence of amino acids making up the active site of the FFT enzyme
has been determined (Ritsema
et al.
, 2004; Fujishima
et al.
, 2005). This is
leading towards knowledge of the three-dimensional structure of the active site
of the enzyme, which will clarify how it binds and catalyses in fructo-
oligosaccharide synthesis (Ritsema
et al.
, 2004).
The gene for onion 6 G-FFT has been transferred to make transgenic
chicory plants (Vijn and Smeekens, 1999) and a transgenic line of tobacco cells
(Ritsema
et al.
, 2003), both of which synthesized the fructo-oligosaccharides
found in onion. Thus using alliums, and also other fructan-accumulating
species, the technology is developing to enable the synthesis of defined fructans
in transgenic plants. It will be possible to synthesize, for example, onion-type
fructo-oligosaccharides in sugarbeet, a plant that lacks fructose-hydrolysing
enzymes, and therefore does not degrade any accumulated fructan. This could
be a useful technology for producing particular fructans required for industrial
purposes (Vijn and Smeekens, 1999).
The physiological function of fructans in plants is not fully understood.
They and the FFT enzymes occur in the cell vacuole, and fructans are
synthesized there. Clearly, they can act as reserve carbohydrates and also, by
varying the DP, the osmotic effect of a given number of fructose units can be
varied. For example, in a study using a range of onion cultivars, cell osmotic
potential declined from -0.97 MPa to -1.41 Mpa as bulb percentage dry
matter increased from 6.3 to 22.7%, much less rapidly than if the increased dry
matter had been in the form of free fructose or sucrose (Sinclair
et al.
, 1995b).
Fructan-accumulating species are abundant in temperate climate zones
with seasonal drought or frost, and almost absent in tropical regions. Fructan
production and mobilization are less sensitive to low temperature than starch
metabolism. The mobilization of fructan reserves for rapid expansion growth
when temperatures rise in the spring, as in onion or garlic bulb sprouting, was
an important factor favouring the evolution of fructan accumulation (Vijn
and Smeekens, 1999). There is also evidence that fructan accumulation
bestows resistance to drought and cold stress (Vijn and Smeekens, 1999).
There are numerous reports that the fructan level in onion bulbs declines
