Annals of Botany 2008 101(1):NP; doi:10.1093/aob/mcm315
© The Author 2008. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org
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
The secret of good coffee – keep it under wraps
I am sitting at
my computer taking sips from a cup of delicious dark-roast black
coffee and it seems appropriate that I am writing about green
coffee (i.e. dried, unroasted beans), the subject of a paper
by
Selmar et al. (Braunschweig, Germany, pp. 31–38). The
first stage in coffee manufacture is to extract the beans from
the flesh of the coffee ‘cherry’; this is done by
different methods that result in different flavours in the final
product. In wet treatments, the first stage is a mechanical
de-pulping followed by immersion in water (‘fermentation’)
to remove the remaining pulp. The beans are then dried and the
endocarp (‘parchment’) is removed. In semi-dry processing,
the fermentation step is omitted; residual pulp and endocarp
are removed mechanically after drying. In dry processing, whole
cherries are dried and the pulp and endocarp are removed mechanically.
In the authors' research, green coffee was treated by these
three methods under strictly controlled conditions. For the
wet treatment, only half the beans had the endocarp removed.
The green coffee beans were stored at 22 °C and 63 % relative
humidity for up to 2 years. Viability and germination tests
gave very clear results: beans with endocarp retained viability,
in some instances for up to 2 years; bare beans lost viability
rapidly over the first 4 months. The reason for this was unclear
since it seems unlikely that the dried endocarp (parchment)
could affect significantly the metabolism of the bean. Distinct
biochemical changes such as slow loss of sugars and significant
loss of glutamine occurred in all beans and again these changes
gave no clue about changes in viability. Sensory tests showed
that storage of green coffee caused a distinct loss of aroma
within 6 months. Intriguingly, this loss of aroma was slightly
less in the beans with retained endocarp; maintenance of viability
is thus correlated with an improved retention of quality.
Sink signals source in shaded sugar
Following a small
decline in 2000–01, world sugar production has risen steadily
to reach 163 million tonnes in 2006–07. About 70 % of
this comes from sugar cane (
Saccharum spp.), providing a major
source of revenue for many less-developed countries. It is thus
important to understand more about the factors that influence
sucrose synthesis and deposition, as discussed by a South African–Australian
group,
McCormick et al. (pp. 89–102). They disrupted normal
source–sink relationships by reducing the source to just
one leaf; all other leaves being shaded. This led to decreased
hexose concentrations in the unshaded leaf, while sucrose concentrations
were little affected; the reverse was true for culm (the sink
tissue). There was a marked increase in photosynthetic efficiency
of the unshaded leaf, especially evident in the assimilation
rate and the electron transport rate. Then, in a very careful
and thorough study, the authors used a reverse northern macroarray
to look at gene expression in the unshaded leaf. Of the genes
studied, 27 showed changes in expression greater than two-fold;
the majority of these were up-regulated. The cohort of up-regulated
genes showed a remarkable correlation with the changes in leaf
metabolism; for example, expression of PEP carboxylase, representing
the C
4 phase of photosynthesis, increased by between 2·2-
and 4·3-fold in 14 days, while that of the large subunit
of Rubisco increased 3-fold over the same period. Such changes
were typical of the range of genes involved in photosynthesis.
Of the down-regulated genes, the authors note particularly hexokinase
(HXK) and fructokinase (FK). Down-regulation of HXK was strongly
correlated with the decline in leaf hexose concentration, consistent
with a role for HXK as a sensor in hexose signalling. Overall,
the results provide clear evidence that an increased carbon
demand at the sink (because of decreased supply from the source)
leads to signalling, possibly via a hexose-based pathway, thus
up-regulating C-fixation via increased photosynthesis in the
source leaf.
Rubisco renewed as ravages of age are reversed
Rubisco is by far
the most abundant protein in green leaves. It can comprise 50
% or more of total leaf protein and contain up to 35 % of leaf
N. Its synthesis is affected by both light and N supply, and
its expression is regulated at both transcriptional and translational
levels. In senescing leaves it is an important source of N and
C for recycling as synthesis declines and existing protein is
broken down. However, it is apparent from the work of
Imai et al., Sendai, Japan (pp. 135–144) that the senescence-related loss can be reversed. From their
very thorough study we focus on the dynamics of Rubisco synthesis
and turnover in leaves of
Oryza sativa. Senescence was studied
in the 24 days following full leaf expansion. Plants were supplied
with 1·0 or 4·0 m
M N [supplied as (NH
4)
2SO
4]
at intervals. As expected, Rubisco synthesis declined during
this time. Transcriptional regulation was evident in the decline
of mRNAs encoding both the large and small subunits; there was
also regulation at the translational level in that the translational
efficiencies of both mRNAs were decreased. This involved both
the cytoplasmic and chloroplastic translation systems. However,
all these features could be reversed, at least temporarily,
by increased N-flux, especially at 4 m
M. Thus, as shown
by other authors for other aspects of leaf physiology, the senescence
programme can be turned around. In the present work, this reversal
was especially marked in the most extreme treatment when all
tillers and all leaves except for the eighth were removed from
plants fed with N at 4 m
M. Even at a late stage of senescence,
16 days after full expansion, this treatment led to an increased
Rubisco mRNA population (especially of the nuclear-encoded small
subunit mRNA) and increased translational efficiency of both
mRNAs. This resulted in an increase in Rubisco protein in leaves
that had previously been well advanced in senescence.
Small is beautiful when plants are on the pull
Richard Fleischer's
film
Fantastic Voyage tells of a surgical team who were miniaturized
in order to travel in a tiny submarine through a patient's bloodstream.
The purpose of this voyage was to locate and disperse a blood
clot in the brain. Although it is impossible to shrink humans,
sending therapeutic agents to specific places in the body has
become possible through nanotechnology. Tiny particles on a
nanometre scale may be directed, for example, to deliver cytotoxic
drugs to cancer cells. But what about plants? In the late 1980s,
methods were developed for shooting DNA-coated gold or tungsten
particles into plant cells, methods now known as biolistics.
Questions arise as to whether, in addition to genetic modification
via biolistics, nanotechnology has any other applications, including
applications with whole plants. The work of a Spanish research
team,
González-Melendi et al. (pp. 187–195), suggests
that the answer may soon be ‘yes’. The authors suspended
carbon-coated iron nanoparticles in surfactant solution that
was thoroughly mixed into a commercially available succinated
gel to give a ‘biocompatible magnetic fluid’. This
was injected into the petioles of whole
Cucurbita pepo plants,
facilitating entry into and transport through the vascular system.
Furthermore, the transport could be directed by placing magnets
adjacent to parts of the plant. Analysis of plant organs and
tissues by microscopy, confocal microscopy and EM showed that
the particles became concentrated in relation to the magnetic
field. Thus particles accumulated, both inside and outside cells,
in regions of the roots and of petioles adjacent to a magnet.
The way is therefore opened for the delivery of particular cargoes
coated onto the nanoparticles to specific parts of the plant.
The use of magnets to generate particle distribution in individual
plants is not readily applicable to large crop stands, but certainly
could be used with individual plants such as olive and other
fruit trees, as suggested by the authors.
CiteULike 
Connotea 
Del.icio.us What's this?
Related articles in Ann Bot:
- Changes in the Synthesis of Rubisco in Rice Leaves in Relation to Senescence and N Influx
- Kazuhiro Imai, Yuji Suzuki, Tadahiko Mae, and Amane Makino
Ann Bot 2008 101: 135-144.
[Abstract]
[Full Text]
- Nanoparticles as Smart Treatment-delivery Systems in Plants: Assessment of Different Techniques of Microscopy for their Visualization in Plant Tissues
- P. González-Melendi, R. Fernández-Pacheco, M. J. Coronado, E. Corredor, P. S. Testillano, M. C. Risueño, C. Marquina, M. R. Ibarra, D. Rubiales, and A. Pérez-de-Luque
Ann Bot 2008 101: 187-195.
[Abstract]
[Full Text]
- The Storage of Green Coffee (Coffea arabica): Decrease of Viability and Changes of Potential Aroma Precursors
- Dirk Selmar, Gerhard Bytof, and Sven-Erik Knopp
Ann Bot 2008 101: 31-38.
[Abstract]
[Full Text]
- Changes in Photosynthetic Rates and Gene Expression of Leaves during a Source–Sink Perturbation in Sugarcane
- A. J. McCormick, M. D. Cramer, and D. A. Watt
Ann Bot 2008 101: 89-102.
[Abstract]
[Full Text]
