The rich flora of southern Africa has provided us with many garden
plants, including members of the iris family that naturalise readily [1]. Among
the most popular of these plants is montbretia, or coppertips (Crocosmia x crocosmiiflora – a cross of C. aurea and C. pottsii [2]), the result of horticultural inter-breeding and now
popular world-wide for its attractive foliage and, especially, its orange-red
flowers.
Montbretia has two methods of reproduction: fertilisation of
the flowers by insects (and, in some closely-related Crocosmia plants, by wind or humming birds) to produce seeds; and vegetative
reproduction, where each plant produces stolons, or runners, that form new
plants when roots and leaves grow from nodes.
Flowering occurs over a period of days, with the first
flowers at the base of the spike and then sequentially towards the tip, the production
of seeds following the same sequence. This habit ensures that some flowers are
likely to be fertilised during optimal conditions for pollination, and the
seeds, similarly, are produced over time to ensure that dispersal is optimal. This
flowering habit is a feature of many plants, but it is important to recognise
that it evolved to the advantage of the plant a very long time before humans
appeared. It was not designed by, and for, gardeners.
Vegetative growth by stolons ensures local colonisation and
many gardeners like to divide dense clumps, as they tend to choke off other
plants (a measure of the success of the strategy). Each individual montbretia
grows from a corm and this develops after the successful germination of seeds
and the growth of the first colonising plant. It is likely that the casual
disposal of corms, rather than the spread of seeds, allowed montbretia (coppertips)
to be such a successful coloniser of natural habitats, frequently forming large
and vivid clumps on embankments and in coastal regions [2]. When established,
its growth habit ensures its spread and it may now be regarded by some as a
"wild flower".
Each corm is a store of starch grains produced synthetically
after photosynthesis and this store allows the growth of leaves and stems at
the beginning of the growing season. It is also used in shorter time scales to
allow the efficient metabolism of the plant. Starch grains that are stored by
plants for months often have their surface eroded [3], resulting from the
action of naturally-occurring enzymes involved in releasing easily-metabolised sugars
from the more complex starch. Among these is α–amylase and this enzyme is disabled in montbretia corms by the
presence of a compound called montbretin. This ensures that starch grains are
retained in good condition during resting phases, montbretin being of less
significance when starch is being metabolised.
The presence of montbretin, the metabolism of the plant, the
succession of flowers and seeds, and the vegetative spread by stolons, are all
extraordinary adaptations that make montbretia such a successful plant and an
effective invader. In addition, montbretia has recently received attention in
the world human health, something which always results in wide publicity. It
has been discovered that montbretin not only serves to protect starch grains
from the action of α–amylase in
Crocosmia corms, but may also inhibit
the action of these enzymes in humans, something that offers the possibility of
new drug treatments against diabetes [4]:
Type 2 diabetes mellitus is a
condition that affects well over 320 million people worldwide and is closely
associated with obesity. Among the oral antidiabetic drugs used for its
treatment are the α-glucosidase
inhibitors, which prevent hyperglycosemia by slowing digestion of starch and
malto-oligosaccharides in the gut. Partial hydrolysis of starch is accomplished
by salivary α-amylase, with
principal cleavage provided by human pancreatic α-amylase (HPA) within the gut, generating linear and branched
malto-oligosaccharides. These in turn are broken down to glucose by α-glucosidases that are anchored in the
epithelium of the small intestine..
..Selective inhibition targeted at
only HPA, the enzyme at the top of the starch digestion pyramid, could be used
to quantitatively modulate blood glucose levels by restricting or even shutting
down starch degradation, thereby minimizing the specificity problems that arise
with currently available α-glucosidase
inhibitors.
This is written in the technical language of a scientific
paper, but Williams, Zhang et al. [4] then describe the selective nature of
montbretin A (MbA) and its possible value in medicine:
Because MbA is such a potent
inhibitor and is easily isolated from the corms of a readily grown plant (Crocosmia sp.), it has potential as a
new agent for controlling blood glucose levels in diabetics and obese patients.
So, the mechanism that evolved in Crocosmia to conserve starch grains is one in which the pharmaceutical
industry is likely to show a great deal of interest, given the numbers of
diabetics and the "epidemic" of obesity in many countries. This
medical application could lead more people to think about the evolution of Crocosmia and its Natural History,
helping us to move away from our dominant anthropocentric view and towards a
sense of wonder in the capabilities of all living organisms.
Well, it
should do.
[1] Mark Van Kleunen, Steven D. Johnson and Markus Fischer
(2007) Predicting naturalization of southern African Iridaceae in other
regions. Journal of Applied Ecology
44: 594-603.
[3] A. T Modi and R. Mare (2016) Alpha amylase activity and sprouting
during short term storage of taro corms. Journal
of Agricultural Science and Technology 18: 1053-1063.
[4] Leslie K. Williams, Xiaohua Zhang + 11 authors (2015)
The amylase inhibitor montbretin A reveals a new glycosidase inhibition motif. Nature Chemical Biology: published
online 27th July 2015 DOI 10.1038/NCHEMBIO.1865.
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