Viruses use light waves to get around
(appeared Apr 2016)

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Plant viruses have been found to divert plantsí machinery to their own use, says S.Ananthanarayanan.

The plantís arsenal for making the best of the environment includes a range of physical phenomena. From quantum efficiency in light gathering for photosynthesis to sophisticated hydraulics in moving the sap in the stem, evolution has helped the plant to adapt and optimize the use of energy. Manipulating the wave properties of light to create all colours of the rainbow is one such adaptation. And another is to use the property of polarization of light to attract insects and birds.

Daniel J. Maxwell, Julian C. Partridge, Nicholas W. Roberts, Neil Boonham, and Gary D. Foster of the School of Biological Sciences, University of Bristol, and also from York and Perth in Australia, report in the journal, PLOS ONE, an analytical detective story of how viruses that affect plants have adapted to get the plants that they infect to modify the plantís own architecture to help the virus spread from host to host.

That animals and insects can sense features of light that humans cannot is now well documented. Light itself is a wave that carries energy. At the level of photosensitive cells of the eyes, light behaves like a particle and transfers a packet or lump-sum of energy to the cells. This is the action of light that humans are able to sense and it serves to make out shapes and colours. But, apart from a straight line path and a frequency or colour, the light wave has another dimension, of the plane of vibration of its electromagnetic composition. Thus, if a beam of light is moving from left to right parallel to this sheet of paper, the electric vibration is either in and out of the plane of the paper or up and down within the plane of the paper, or in any other plane, so long as the plane is perpendicular to the direction of the beam.

Sunlight, which arises from thermal emission of very hot gasses in the sun, consists of waves in all possible planes of vibration. On reflection, however, there is a selection of the plane of vibration and the scattered light from the blue sky and particularly diffused light at sunrise or at dusk is markedly polarized. As humans have evolved to rely on position and colour for navigation and hunting, and benefit by maximum sensitivity, the cells in the human eye respond equally to light waves of all planes of polarization. This, however, is not true of animals, birds or insects, which need to navigate without fixed markers and also to be sensitive to detect prey or food that is not always distinctly visible. Being able to detect the plane of polarization helps know the position of the sun even when it is hidden behind clouds and also to locate specific reflecting surfaces. Examples are water bodies as habitats or suitable egg laying sites.

Plants have evolved to capitalise on a brace of features to attract the right kind of carriers of pollen to help them procreate and flourish. The bright colours of flowers, the scent and the nectar are all means of attracting bees, the main agent, and also butterflies and many other creatures to the plant. While the colours on the surface of petals are found to arise partly from pigments, they also sensitively depend surface geometry, which causes interference of waves and suppression or enhancement of specific colours. In addition, studies have shown that the smoothness of petal and leaf surfaces promote the selection of particular planes of polarization which also influences insects whose eyes are sensitive to such changes. Some of the same researchers in the University of Bristol, in fact, have documented how the bumblebee makes good use of polarized light while foraging.

Entry of the virus

While insect visitors to plants perform a useful function of promoting cross pollination and prosperity, varieties of insects are also the agents that transfer and promote the proliferation of pathogens and diseases that affect plants. An important instance is the transmission of viruses, which is the reason for the loss of valuable food crops, by insects. The current study of the Bristol group was to investigate the manner in which viruses, whose transmission is through insects which are sensitive to polarization of light, may use this sensitivity to their advantage.

The PLOS ONE paper notes that a large number of viruses are transmitted by insects like aphids, whiteflies and thrips. What is more, the paper says, the viruses themselves are not passively moved about by the insects, but they influence the insect and virus host interaction in such a way that the insect finds the host attractive, which favours transmission of the virus.

There are different ways in which the virus affects the plant so that the insects that transmit the virus come and stay, the paper says. One way is by inducing vapours and odours, a common signaling mechanism among insects. Another is to affect the nutritional quality of the host, and there have been studies that have found that insects feed differently on virus infected plants as against healthy ones. But an area that has not received attention is the effect that viruses have on the surface of leaves of infected plants. Modified leaf surfaces would have a different feel and changes in the surface hairs could affect how the insect is able to move. And then, changes in the surface waxes could change the colour and way the leaf looks, and a particular effect, which has not been studied so far, the paper says, is how changes in the surface could affect the polarization of light on reflection from the leaf. There is evidence, the paper says, that insects that transmit plant viruses have the machinery to sense polarization of light, which suggests that effects of the viruses on the polarizing capacity of leaves could be an agency to attract virus-transmitting insects.

Sifting the evidence

The researchers note that the reflectivity of leaves is because of waxes on the surface and changing the composition of the waxes would affect the level of polarization on reflection. The various components of waxes, in turn, arise because of particular molecules, called Very Long Chain Fatty Acids, which are found in the outer layer of leaves. And the formation of these molecules is controlled by a group or genes, called CER genes, in the plant cells. Instances of a virus affecting the waxiness of leaves should then be reflected in the activity of the corresponding genes, the researchers observe. The group hence studied the effect of both insect transmitted viruses as well as viruses transmitted by other means on the reflecting-polarising effect, the waxy surfaces and the expression of the CER gene in common plants

The main effect of infection by insect-transmitted viruses is found to be reduced polarization of light on reflection by leaves, an effect not found on infection by other viruses. As infected leaves are found to be more attractive to virus transmitting insects, it looks like viruses are adapted to benefit by modifying the reflectivity of host leaves. Although the actual polarization sensitivity of aphids and whiteflies has not been verified, the class of insects does have the necessary eye structure, the researchers say. Even if this were not the case, they say, reduced polarization would have the effect of emphasizing pigmentation of the leaf and make it more attractive. There is also the possibility that reduced polarization has the effect discouraging predators of the insects, which would effectively enhance virus transmission, they add.

The study went on to consider the kind of waxes that formed on the leaves, the level of expression of the relevant genes and also hairiness of the leaf surface, which would affect reflectivity. While insect transmitted viruses were found to have different effects on leaves it was also seen that the differences were adapted according to the hosts . But the idea that viruses that depend on insects for their transmission affect the host in ways to attract the insects, in different ways, is borne out. This is not only insight into the complex web of interrelations in the natural world but also direction to find ways of controlling the spread of viruses that affect plants that have economical value.

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