Plants flex their muscles… [or, Onion feeling the pinch…?]

Could onions peel back the layers of future developments in artificial muscle construction and soft robotics?
Image: Wikimedia Commons.
Image: Wikimedia Commons

Would you ‘Adam-and-Eve’ it? And an item that is this month’s winner in the ‘I’m really not making this up’ category is the revelation that plant material has been used to construct artificial muscles(!).

For important background, ‘muscle is a soft tissue found in most animals. Muscle cells contain protein filaments of actin and myosin that slide past one another, producing a contraction that changes both the length and the shape of the cell. Muscles function to produce force and motion’. In seeking to identify an artificial muscle, Chien-Chun Chen et al. have used the epidermis of onion*.

In the raw state the uniseriate layer of tissue is not suitable, but after freeze-drying (to remove the water), sulphuric acid treatment and sandwiching between sheets of gold, a working ‘muscle’ was created. When given low voltages (up to 50 V) the structure both elongated and bent downwards; at higher voltages (50–1000 V) it simultaneously contracted and bent upwards.

Using two of these so-called ‘actuators’ (a type of motor that converts energy into motion) close together, and each receiving either low or high voltage, they managed to produce a pair of tweezers that was able to pick up a ball of cotton weighing 0.1 mg. Small beginnings (!); and ‘we still don’t fully understand the structure of the cell walls and its associated properties’, said one of the team members, Wen-Pin Shih, ‘we’re just reporting what we have so far to exchange ideas’.

But the device is expected to have potential in the development of ‘soft robotics’ with biomedical applications. Whilst plants don’t actually have muscles (despite allusions thereto by that ‘wood-working duo’ of Nigel Chaffey and Peter Barlow), they do contain those main and quintessential muscle ingredients of actin and myosin, and so have potential for muscle-like phenomena.

And this has been underlined by work of Keishi Okamoto et al. who have shown that straightening of inflorescence stems of arabidopsis is regulated by an actin–myosin XI cytoskeleton in specialized immature fibre cells. The team also postulate that the long actin filaments in elongating fibre cells act as a bending tensile sensor to perceive the organ’s posture and trigger the straightening system. Regulation of organ straightening and plant posture by an actin–myosin system? Plants, muscling-in on traditional animal preserves? Now that’s exciting!

* Although not specified in the article, one assumes the onion in question is Allium cepa. I don’t know – what is it with non-biological journals that they don’t provide scientific names of the organisms of study…?

[But – and you just know that you want to know too – what on Earth inspired them to try onion in the first place??? – Ed.]