Activity in space
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) Karlsruhe Institute of Technology (KIT), Botanical Institute
, Karlsruhe,
Germany
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How the scalar world of molecules is translated into
developmental complexity that is ordered in time and space belongs to
the most challenging topics of biology. The advances in
fluorescence microscopy, along with the availability of
nondestructive fluorescent markers, have revealed that this order
is more dynamic than thought before. As a consequence,
developmental concepts such as polarity, axis, or pattern can
no longer be seen as static objects but have transmutated into
descriptions of activities. Actually, this insight, although not
adopted by many, is not new. Already more than five decades
ago, in his famous textbook, Sinnott (1960) defined polarity as
specific orientation of activity in space. What we see as
biological order is therefore just a snapshot of ordered activity.
The questions, how the dynamic order of the protoplasma
coexists and interacts with the stability of Mendelian
inheritance (e.g. Nakazawa 1960) and how this order is generated
and sustained by directional movement of molecules in
response to chemical and physical gradients (e.g. Harold 1997),
have been central also in this journal, giving rise to classical
publications that are still worth reading today.
Three contributions in the current issue address how the
temporal order of activity contribute to functional organisation
at different levels of complexity: cells, organelles, and organs.
All eukaryotic cells contain acidic organelles that are often
involved in degradation or digestion processes. In yeast cells,
this acidic compartment is represented by the vacuole. The
acidity is actively maintained by the activity of a proton
ATPase. The function of this acidification is thought to
provide a chemical environment favouring hydrolytic reactions.
In their comprehensive study published in the current issue,
Matsumoto et al. (2013) show that, in addition, acidification is
relevant for the proper allocation of proteins into the vacuole.
They block the vacuolar proton ATPase by the specific
inhibitor concanamycin A and then examine the localisation of
more than 70 GFP-tagged luminal and vacuolar membrane
proteins by quantitative image analysis. About a quarter of
these proteins lose their vacuolar targeting after treatment with
the inhibitors. These include hydrolases, transporters, but also
subunits of the vacuolar proton ATPase itself. This points to a
scenario where initial acidity of the vacuole not only acts as
attractor for the proteins destined to act in this acidic
environment but also, in parallel, boosts the chemical reaction
responsible for this acidity, which is a nice illustration of
selfamplifying self-organisation at the subcellular level.
The dependence of protein function on subcellular
distribution is also highly relevant for application, as carefully
demonstrated by Pasare et al. (2013) in the current issue. Goal
of their work was to improve the efficiency of metabolic
engineering to generate potatoes that can be used as functional
food because they contain elevated levels of nutritionally
important carotenoids. The strategy to overexpress key
enzymes of carotenoid biosynthesis, such as phytoene synthase
and -carotene hydroxylase, had not yielded consistent
success during previous work. This led the authors to ask whether
differences in suborganellar localisation of these proteins in
amyloplasts versus chromoplasts might be relevant. They
used fusions of the two key enzymes with red fluorescent
protein and investigated their localisation after stable
transformation either in Nicotiana benthamiana (leaves) or in potato
(tubers). They observed that the localisation of the two key
enzymes differed depending on the type of plastid. In leaves,
the -carotene hydroxylase was found in the stroma, whereas
in tuber amyloplasts, it was sequestered in small vesicles. In
contrast, the phytoene synthase localised to focal dots at the
thylakoid in leaves, whereas it was stromal in the amyloplasts.
This finding not only highlights the importance of subcellular
distribution for the success of transgenic strategies but also
demonstrates very clearly that it can be misleading to infer
subcellular localisation from convenient experimental models
(such as transiently transfected leaves of N . benthamiana ) to
the real situation in different organs (such as tubers). It further
demonstrates that functionality of compartmentalised
metabolism can be controlled even on the level of suborganellar
organisation.
The third contribution by Qi et al. (2013) in the current
issue investigated the molecular events underlying the
characteristic flower pattern of the broad-leafed grape hyacinth
(Muscari latifolium ). The inflorescence of this flower is pale
blue in the upper and purple in the lower domain. To
understand the reason for this peculiar gradient, the authors
measured vacuolar pH with a microelectron as well as content of
metal ions by inductively (...truncated)