Getting plants to hurry up
(appeared in Aug 2019)

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Tweaking a core process that plants use can raise agricultural output, says S.Ananthanarayanan.

Crop yields have risen dramatically over the last few decades. Over the last century, thanks to expanding cropland and fertilisers, increased agricultural output gave the lie to Malthus' forecast of shortage and disaster. And then, with developments in the use of hybrids and farming techniques, and the use of pesticides, food has stayed on the world's table despite huge rise in population and per capita consumption.

Nevertheless, it is uncertain whether the world's resources would measure up to food demands in the coming decades. There are limits to the land that is available and we need to curtail the pollution caused by the production and use of fertiliser and pesticides. In the context, a group of researchers in the journal, Communication Biology, says that accelerating the process of photosynthesis, through which plants convert the s

emmerer, from the Australian National University, at Acton, Canberra, and the University of Essex, UK, write that their work on how photosynthesis comes about has led them to a way of increasing the output of the process in an important category of plants. The mechanism by which the energy of sunlight, which breaks down the water molecule, is made use of, they write, depends on and is limited by a specific protein. Stepping up the production of this protein would hence lead to faster photosynthesis, even while keeping all other functions of the plant unchanged.

They write that a way to increase the pace of photosynthesis in a larger class of plants, like rice, wheat, barley, even lawn grass, which follow a certain path in photosynthesis, has been known for some time. Doing the same with another class of plants, which include important food crops like maize, sorghum, sugarcane and millets, and sources of bioenergy, would hence be of economic value, the paper says.

Photosynthesis takes place in plants by the action of certain proteins, which are most abundant in the leaves, and the pigment, chlorophyll. The proteins, with the help of the pigment, absorb the energy in sunlight and use the energy to separate hydrogen and oxygen from water. While the oxygen is released, which makes photosynthesis of great value to life itself, the energy in the system gets hydrogen to combine with carbon, usually in carbon dioxide, to form carbohydrates, the food material.

The light absorbing proteins in plants are found in the cell membrane, or in folded structures, which have high surface area, called thylakoids, and these are collected in organs called chloroplasts. The net result of light striking light gathering centres is that water, or H2O, is split into 2H+ and O- , charged components that retain the charges that keep the water molecule together. Pairs of O- get together as O2, or molecular oxygen, but two negative charges move free, with the energy that keeps them away from the 2H+.

Over a series of 'passing the parcel' exchanges, the energetic negative charge and the 2H+ are transported to where CO2 is available, and the combination of CO2 with 2H+, to form carbohydrates, becomes possible. We caČn see that in the normal course, the negative charge could just have neutralised 2H+ to H2 and the only effect would be splitting water into its components and their recombination as water, with some dissipation of heat. It is the sequence followed, of trans

Rate limiting step

The Communications Biology paper delves into the process of transporting negative charge and identifies four main protein complexes that mediate the movement of charge, called electron transfer, in the rice, wheat, etc. group of plants. And the second of these four complexes, 'Cytochrome b6f', the paper says, is a' rate limiting' step in the chain.

The paper then looks into the structure of Cytochrome b6f and says it consists of two forms of Cytochrome, a protein called Rieske FeS, which contains iron and sulphur, and some others. And, "there is an increasing amount of evidence that the amount of Rieske FeS protein......regulates the abundance of Cytochrome b6f ", the paper says. Model plants of this group, which had been genetically primed to produce more Rieske FeS, have shown increase in Cytochrome b6f as well CO2 assimilation, the paper says.

This group of plants with smaller grain, like rice, wheat, have one way of turning CO2 to carbohydrates, while plants like maize, sorghum, sugarcane have another. The first group uses a molecule with three carbon atoms, on the way to forming carbohydrates, and is referred to as the C3 group. The second group makes use of a 4-carbon atom intermediary and the group is known as the C4 group. C4 plants produce more sugars than C3 plants in bright and warm conditions and may have evolved to tolerate conditions of low CO2

Although there are differences in the two routes followed, the paper notes, both pathways include the Cytochrome b6f component. The authors of the paper hence took up a typical C4 plant, a common species of grass, to see if increasing the level of Rieske FeS led to higher content of Cytochrome b6f and faster photosynthesis. What they find, the paper says, is just that ľ first, that the speed of electron transport is what limits the assimilation of CO2 in C4 plants, especially when there is plenty of light and ample CO2,, and second, that the speed of transport depends on the level of Cytochrome b6f.

The conclusion, that introducing GM varieties of C4 plants with higher Cytochrome b6f abundance would then lead to higher CO2 assimilation and greater yield, is then presented as a possible answer to a future crisis of food insufficiency.

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