Agriculture Reference
In-Depth Information
sucrose is a major reserve compound in some plants (e.g., sugar beet, sugarcane),
it should be noted that starch is the most prominent reserve carbohydrate in most
plants. It is a glucose polymer consisting of linear amylose (α(alfa)1,4 linkages)
and branched amylopectin chains (α(alfa)1,4 and α(alfa)1,6 linkages). In contrast to
sucrose which only occurs in plants and some algae, trehalose (Glcα(alfa)1,1Glc) is
found in all domains of the tree of life. It accumulates for instance in fungi and in
insects (Elbein et al.
2003
; Muller et al.
1995
). Upon drying, only a few so-called
resurrection plants are known to accumulate trehalose to a great extent. Most higher
plants contain different trehalases that prevent trehalose accumulation (Muller et al.
1995
), but they keep on synthesizing low amounts of trehalose 6-phosphate (T6P
via Trehalose 6-Phosphate Synthase) and trehalose (via Trehalose 6-Phosphate
Phosphatase), the former most probably involved in sugar signaling (Zhang et al.
2009
; see also below).
3.2 Raffinose Family of Oligosaccharides
Raffinose family of oligosaccharides (RFOs) are α(alfa)(1,6) galactosyl extensions
of sucrose, such as raffinose (DP3), stachyose (DP4) and verbascose (DP5). Raf-
finose is nearly ubiquitous in plants (Vanhaecke et al.
2008
). Lychnose and its de-
rivatives can be considered as alternative RFOs, derived from raffinose (Vanhaecke
et al.
2010
). The metabolism of classic RFOs is well known. First, galactinol syn-
thase (GolS) synthesizes galactinol from UDPGal and myoinositol. Next, raffinose
synthase (RafS) transfers the galactose residue to sucrose to form raffinose. Stachy-
ose synthase (StaS) fulfils a similar reaction with raffinose as acceptor substrate.
With the synthesis of higher DP (> DP4), RFOs occur independent of galactinol.
Galactan:galactan galactosyl transferases (GGTs) are used for this purpose (Taper-
noux-Luthi et al.
2004
). Raffinose and stachyose are synthesized in the cytoplasm
(Schneider and Keller
2009
). Recently, it has been reported that RFO gene expres-
sion and enzymatic activities and RFO accumulation are closely associated with
responses to environmental stress (Nishizawa et al.
2008
; Peters and Keller
2009
).
RFO biosynthesis in
Arabidopsis
seems to be mainly regulated at the level of tran-
scription (Espinoza et al.
2010
). Stachyose typically accumulates in
Arabidopsis
seeds (Taji et al.
2002
). RFOs play a main role in many other seeds too (Blochl
et al.
2008
).
3.3 Fructans
Fructans are water-soluble sucrose-derived fructose polymers accumulating in
about 15 % of the angiosperm flora (Hendry
1993
), but they also occur in a wide
range of bacteria and in some fungi. Some economically-important plant groups
are known to accumulate fructans, such as
Poales
,
Liliales
and
Asterales
(Hendry
