Annals of Botany 2008 102(2):v; doi:10.1093/aob/mcn123
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John Bryant takes a closer look at some of this month's Original Articles
J. A. Bryant, Professor
University of Exeter, UK
E-mail j.a.bryant{at}exeter.ac.uk
Carbs in the coffee cup
If I were asked
to name the compounds that affect coffee flavour, polysaccharides
would probably not feature in my list. However, this omission
would be a mistake because in coffee (
Coffea sp.), polysaccharides
are laid down as nutrient reserves in the grain. Thus, as pointed
out by
Pré et al. at Tours, France (pp. 207–220) 25 % of the dry weight of a mature coffee bean consists of galactomannans,
polysaccharides with a backbone of mannose units and side groups
of single galactose units. They exist in the seed mainly as
thickening material in the endosperm cell walls. They are important
because of their own solubility profiles (which are affected
by the degree to which the mannan backbone is modified by addition
of galactose residues) and because of their effects on the solubility
of other compounds. Because of the importance of these compounds
in the coffee industry, the authors have studied the regulation
of galactomannan synthesis at the level of gene expression.
Two cDNAs encoding mannan synthases (ManS1 and 2) and two encoding
galactomannan galactosyl transferases (GMGT 1 and 2) were identified.
Gene sequence comparisons showed that
ManS1 and
2 are related
to two groups within the
cellulose-synthase-like (
Csl) gene
family,
CslA and
CslC, whereas
GMGT1 and
2 are members of the
glycosyl transferase family 34. During development of the grain,
ManS1 and
GMGTI were expressed at a high level in the developing
endosperm in both
Coffea arabica and
C. canaphora, suggesting
that the ManS1 and GMGT1 enzymes synthesize specifically the
grain galactomannans. There were differences in the timing and
level of expression between species and between varieties within
a species, but within any one variety the developmental patterns
of expression of the two genes were very similar. This suggests
that the two genes are coordinately regulated and thus the enzymes
they encode are able to act together as required in galactomannan
synthesis.
Glucan goes down as rice reaches up
A common response
of plants to submergence is elongation growth. This in turn
implies changes in cell wall biochemistry and, indeed, several
such changes have been demonstrated. These include hydrolysis
of the mixed β-1,3: 1,4 glucan polymers, leading to wall
loosening and thus allowing cell elongation, as discussed by
Kimpara et al. (Saitama and Osaka, Japan; pp. 221–226).
These authors have focused on the reciprocal process, namely
the synthesis of β-1,3: 1,4 glucans. Rice seedlings, grown
in the dark for 60 h, were submerged, with or without aeration
for 12 h. β-1,3: 1,4 glucan synthase decreased to
less than 40 % of control values during this time. Aeration
of submerged plants prevented most of this decline: enzyme activity
was approx. 80 % of control values, indicating that the key
factor was hypoxia rather than submergence itself. The reduction
in synthase activity was at least partly due to down-regulation
of gene expression. The synthase is another member of the cellulose-synthase-like
(Csl) protein family, of which a particular group, the CslFs,
are restricted to the Poaceae. The authors state that the rice
genome contains at least eight
CslF genes; in this study, the
activity of one of these,
CslF6, declined in submerged seedlings
to less than 20 % of control values. Aeration led only to a
small recovery (36 % of control values) so it seems that factors
in addition to hypoxia are involved in regulating gene expression.
Do these changes in gene expression and in enzyme activity result
in a decline in β-1,3: 1,4 glucan synthesis? Certainly
the amount of polymer in seedlings submerged for 12 h was
only 75 % of that in control seedlings. It is difficult to ascertain
the relative roles of decreased synthesis and increased hydrolysis
in this reduction but it seems that the amount of β-1,3:
1,4 glucan is controlled by both synthesis and degradation in
relation to the submergence-induced elongation growth.
Promoting the hop's bitter harvest
A combination of
beer and botany is one that will appeal to many of our readers.
Some will have strong views about what makes a good beer and
will name enthusiastically their favourite brews. It is widely
known that hop (
Humulus lupulus) is used as a source of the
bitter flavours of beer; more specifically it is secondary metabolites
such as bitter acids, produced by the female cone, that are
the relevant compounds. Moreover, as pointed out by
Castro et al. (Hobart and Bellerive, Tasmania; pp. 265–273) there is now an interest in the medicinal properties of some
of these metabolites, providing a further motivation for study.
The authors review evidence showing that metabolite profiles
of different varieties are very constant and thus can be used
in varietal identification. What then controls the differences
between varieties? In this paper, the authors have focused on
the enzyme valerophenone synthase (VPS), which catalyses the
first step in the pathway leading specifically to the bitter
acids. Two main approaches were used. In the first, the
VPS genes from 13 hop varieties were sequenced, revealing nine single-nucleotide
polymorphisms (SNPs). Seven of these were synonymous (did not
result in an amino change in the protein); the two non-synonymous
polymorphisms did not correlate with bitter acid content. The
second approach used real-time quantitative PCR to investigate
VPS expression in one low-acid and three high-acid varieties.
As an aside, we note that they used a novel internal control,
poly-ubiquitin, for this. There was a clear correlation between
the level of
VPS gene expression in the developing cones and
the accumulation of bitter acids: expression was ten-fold lower
in the low-acid variety than in the high-acid varieties. Differences
in expression levels are often mediated by differences in the
interactions between regulatory molecules and gene promoters.
Detailed analysis of
VPS gene promoters is thus an obvious next
step.
Shocking treatment goes against the grain
All our students
know that GA induces
-amylase synthesis in the aleurone layer
of germinating cereal grains and that
-amylase is one of a suite
of enzymes that mobilize the nutrient reserves of the endosperm.
However, it is much less well known that
-amylase may be produced
prematurely in grain development. Sometimes this is associated
with precocious germination of the grain while still on the
parent plant but often it happens without any other visible
sign of germination. There are obvious implications for the
quality of the grain at harvest and thus
Farrell and Kettlewell at Harper Adams University College, Shropshire, UK (pp. 287–293) have conducted a systematic investigation of possible factors
that may cause pre-maturity
-amylase (PMA) activity. Winter
wheat genotypes that differed in their susceptibility to PMA
were grown in the glasshouse; at anthesis they were transferred
to ‘high’ (30/15 °C) or ‘mid’
(20/10 or 22/22) temperature regimes. At 25–30 d after
anthesis (depending on the pre-treatment), plants were exposed
for 8 d to a temperature shock (mid-to-low, 22/22
12/12 °C;
mid-to-high, 20/10
30/20; high-to-mid, 30/15
18/12). Other
plants were partially de-grained in order to increase grain
size in the remaining grains; some of these were also subjected
to a mid-to-low (25/15
13/11) temperature shock. Amylase was
assayed both by enzyme activity and by ELISA determination of
protein levels. Analysis of results by ANOVA showed very clearly
that the key factors in the occurrence of PMA were environment
and genotype. Both high and low temperature heat-shock treatments
(but especially low temperature) induced PMA in susceptible
varieties but hardly at all in resistant varieties. Cold-shock
treatment also led to the formation of larger grains, giving
a correlation between grain size and PMA. Grain size itself
was not a determinant of PMA, as indicated by the data obtained
with the larger grains of partially de-grained plants.
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Related articles in Ann Bot:
- Characterization and Expression Analysis of Genes Directing Galactomannan Synthesis in Coffee
- Martial Pré, Victoria Caillet, Julien Sobilo, and James McCarthy
Ann Bot 2008 102: 207-220.
[Abstract]
[Full Text]
- β-1,3 : 1,4-Glucan Synthase Activity in Rice Seedlings under Water
- Tomoya Kimpara, Tsutomu Aohara, Kouichi Soga, Kazuyuki Wakabayashi, Takayuki Hoson, Yoichi Tsumuraya, and Toshihisa Kotake
Ann Bot 2008 102: 221-226.
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- DNA Sequence and Expression Variation of Hop (Humulus lupulus) Valerophenone Synthase (VPS), a Key Gene in Bitter Acid Biosynthesis
- Consuelo B. Castro, Lucy D. Whittock, Simon P. Whittock, Grey Leggett, and Anthony Koutoulis
Ann Bot 2008 102: 265-273.
[Abstract]
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- The Effect of Temperature Shock and Grain Morphology on Alpha-amylase in Developing Wheat Grain
- A. D. Farrell and P. S. Kettlewell
Ann Bot 2008 102: 287-293.
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