Redesigning photosynthesis for food security

In order to secure the increases in yield so vital if we are to feed an increasingly populated planet, every possibility to improve photosynthetic efficiency is needed. A fascinating and exciting new paper, conceived at the “Redesigning Photosynthesis–Identifying Opportunities and Novel Ideas” workshop, addresses these possibilities, and reads like a to-do list from some of the best minds in this field. The prospect of fast advances in the new field of synthetic biology means that much more complex ideas, involving the movement or alteration of many genes, are looking more and more feasible.

by Joanna Wolstenholme, Communications intern

Starting with light capture, the authors walk us through the photosynthetic process, and point out possibilities for improvement. Currently, most of the time, much more light is absorbed by the leaf than can be used, and so conversion efficiency is very low. They suggest that if we can reduce the number of light harvesting complexes (as has been done in algae for similar reasons), conversion efficiencies would rise as the leaf is subject to less photobleaching (damage caused by too much light being absorbed).

When it comes to light energy conversion, there are two photosystems which currently compete with each other for photons, rather than exploiting different areas of the spectrum. The authors suggest that they could be engineered to intercept a wider range of light energy by introducing elements already found in algal and bacterial systems. They suggest potentially altering photosystem I to use chlorophyll b, and photosystem II to use chlorophyll d (rather than the chlorophyll a which they are already using), as well as shuffling other aspects of the system, could lead to efficiency doubling thanks to the formation of two very different photosystems.

Currently CO₂ transport from the intercellular airspace of leaves to the site of carboxylation (the key step in fixing carbon in a form the plant can use) within the mesophyll chloroplast is a major limiting factor in carbon fixation. Two possibilities are proposed for improving this – the introduction of CO₂ channels and transporters, and the engineering of carbon concentrating mechanisms into crop plants. At Cambridge much work is already being done on the latter, with Julian Hibberd’s group pioneering C4 rice, and Mortiz Meyer in the Griffiths group working to understand algal carbon concentrating mechanisms, with a view to eventually engineer these into crops. Carbon concentration is important as it ensures that Rubisco is saturated with carbon, whereby reducing the number of oxygenation reactions which occur, and the cost of photorespiration (which recaptures the products of the oxygenation reaction). However, with the recent advances in synthetic biology, it is no longer so outlandish to think about redesigning the photorespiratory pathway to be less costly, or even more radically, bypass the imperfect Rubisco altogether by importing oxygen-insensitive pathways for the key carbon fixation reaction.

All of these ideas start to come together, and be advanced, with the concept of smart canopies – ‘an assemblage of plants that interact co-operatively (rather than competitively) at the canopy level to maximize the potential for light harvesting and biomass production per unit land area’. This means that traits could alter along the plant’s height, in order to minimise light interception at the top of the crop canopy (to prevent photobleaching), and maximise it at the base of the plant, to ensure all light is used. Gradients are envisaged in a number of traits in order to achieve this – for example from more vertically angled, C4 using upper leaves to more horizontal, C3 lower leaves. Smart canopies seem to encapsulate the general feeling of the paper – that we are now (or at least soon will be) able to think about manipulating plants in every aspect, to truly design the most efficient crops that are possible. This is an exciting time! Of course there will be much work needed to see if this is actually possible – but the potential is certainly there. The next hurdle will, of course, be regulation and tackling public opinion.

By taking the approach of deconstructing photosynthesis and building it up from scratch, in the most efficient way, and then considering biological implications, the authors are able to find many novel solutions. It will be interesting to see how ideas from this paper develop into the future.

See the full paper here: http://www.pnas.org/content/112/28/8529.abstract