Wednesday, 10 August 2016

Crocosmia, invasion, diabetes and obesity

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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