Tweaking photosynthesis for a better crop yield

Energy from sunlight is used in photosynthesis, but it also has the potential to harm the tissues it falls on.   | Photo Credit: T_A_Hafeez

We depend on plants for much of our food, clothing and shelter. Plants depend on the material in the soil and earth below, water, air and sunshine for their growth. Sunlight is thus an essential raw material for the growth of plants. Using sunlight, water and carbon dioxide present in air, plants synthesise what is needed for much of their own metabolic needs (and thus our needs). This process, known as photosynthesis, is carried out largely by the leaves (and similar appendages) in plants. Humans and many animals are dependent on the efficiency with which plants photosynthesise, grow and multiply.

As human population increases, we would need more of crops in order to cater to the global demands for food. It thus becomes important to study ways in which plant productivity can be increased. One way of approaching this is to find ways in which photosynthesis can be improved.

An international team of researchers, led by Dr Stephen P. Long of the University of Illinois and Dr Krishna K. Niyogi of the University of California, Berkeley, USA has focused on this problem. Their paper, titled “Improving photosynthesis and crop productivity by accelerating recovery from photo-protection,” has appeared in the 18 November 2016 issue of the journal Science.

Energy from sunlight is captured by the green pigment called chlorophyll in the leaves in order to conduct these chemical reactions. But this energy can also damage the leaves (recall how sunbathers in beaches can get sunburnt). Plants protect themselves from such light-induced damage by releasing heat (but we use sun-tan lotions or dark glasses for protection). Now, such “quenching” of excess solar energy must be quick. If it takes too long (often as long as half an hour) to “relax” and resume the cycle, it may be thought of as a “waste of time.” If only we can hasten this process (termed non-photochemical quenching, abbreviated as NPQ) of recovery safely, argues this research team, we may be able to improve crop productivity.

Quick fix

The team studied how plants “fix” or adjust their photosynthetic cycle as their leaves experience light and shade- as in a natural environment. In full sunlight, NPQ is activated so as not to harm the chlorophyll too much. But as clouds shade the sunlight, in such a low-light situation, NPQ is reduced. Hastening the NPQ process, argues the team, could increase the efficiency of the photosynthesis cycle by anywhere between 8 per cent and 30 per cent. This, in turn, could be a promising strategy for improving crop yield.

Such a switching of NPQ levels is governed in plants by the action of three proteins. One protein abbreviated as ZEP speeds up the NPQ rate. A second one, termed VDE, balances ZEP activity, acting as a moderator, while a third one called PSBS adjusts the NPQ level. If one can play with the levels of the three proteins, one can in turn adjust the efficiency of photosynthesis and hence the crop yield.

To this end, the group inserted the genes for VDE, PSBS and ZEP and obtained a genetically engineered plant of tobacco as the proof-of-principle plant. Why tobacco? When this question was posed by the science writer Hannah Martin Lawrenz, Dr Long is reported to have responded: “Because it is easy to transform [genetically], and it is a crop, so it produces the layers of leaves we needed. [Also] because the process is the same in rice, soybeans, wheat, and cowpea, we have strong confidence that this should work for food crops. But the next step is making the same changes in those crops, which are much more difficult to transform”.

Plants with all the 3 genes VDE, PSBS and ZEP were named VPZ plants and their performance compared with normal, unmodified (scientists call such controls “wild type” or WT) tobacco plants. The VPZ plants displayed faster relaxation of NPQ and they were seen to recover and engage in photosynthesis sooner. The researchers next made the light falling on the plants fluctuate, so as to mimic light and shade (or day and night), and compared the performance of VPZ with that of WT. Here again, the genetically engineered VPZ plants performed better in low light than WT.

And then, in order check the productivity in the field as crops, they planted both types of tobacco plants in the university field. Plants from VPZ had higher leaf area (better for photosynthesis), increased leaf, stem and root weights and anywhere between 14 per cent and 20 per cent more dry weight per plant than the unmodified plants. These findings, the researchers write, provide proof of concept for a route to obtain a sustainable increase in productivity for food crops and a much-needed yield jump.

Yes, the plants are genetically modified. But note, the genes are already plant-based and not foreign to the plant kingdom. So, there should be far less, if any at all, objection or opposition from anti-GM activists.

D Balasubramanian

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Printable version | Oct 22, 2020 4:15:35 AM |

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