C4 Wheat? Lend an ear to hear more

One of the Grand Challenges – and, arguably, the Holy Grail – of plant research at present is an attempt to improve the photosynthetic efficiency of plants. Somewhat surprisingly, the fundamental photosynthetic pathway – which is the ultimate piece of biochemistry used by all plants to incorporate (‘fix’) carbon dioxide into organic molecules such as sugars for the plant’s growth and energy needs – so-called C3 photosynthesis – isn’t as efficient as it could be.

One of the reasons for its comparatively low conversion rate of sunlight energy into chemical energy stored within organic molecules is the phenomenon of photorespiration. Whatever else this process may achieve it ‘wastes’ some of the expensively-produced photosynthetic compounds and effectively reduces the overall efficiency of the process. Many plants have managed to overcome the problem of photorespiration by biochemical ‘engineering’ that increases the CO2 concentration within the plant cells so that photorespiration is suppressed. Such botanicals – C4 plants such as maize and sugar-cane – are therefore more efficient – i.e. more productive – than C3 plants. Wouldn’t it be great if C3 plants could be ‘converted’ to C4 plants?

Yes (e.g. this and this). Especially as, compared to C3 plants, C4 plants do better under hotter conditions (which are likely to prevail as climate continues to change globally and get warmer), and are more efficient in their use of water – a resource that is likely to be in shorter supply in future.

Generally, the challenge of converting C3 plants to C4 has seemed extremely daunting requiring the addition of many genes – with accompanying alteration of structure, physiology and biochemical pathways – to be effective. Nevertheless, because of the potential boosts to agricultural production, this has been viewed as extremely desirable, and a major way forward to increase crop productivity and thereby to help alleviate present and future global food shortages.

However, work by Parimalan Rangan et al. suggest that this desired state of affairs might not only already be here, but also that it may have been hiding in plain sight all along. Looking at genes specific to C4 photosynthesis, the team demonstrate existence of a C4 pathway in the developing grain (‘seed’) of wheat (Triticum aestivum presumably, although not explicitly stated in the paper…). This C4 pathway is absent from wheat leaves, which is why that cereal has always been considered to be a true C3 species. Additionally, grain chloroplasts exhibit dimorphism [presence of two distinctly different structural types] that corresponds to chloroplasts of mesophyll- and bundle sheath-cells in leaves of classical C4 plants, such as maize.

Arguably, presence of putative full C4 capacity – albeit in a specialised part of the plant – suggests it may not be that difficult to ‘encourage’ its expression in other parts of the plant, such as the leaves. But, an obvious question we must now ask is, if wheat is already using C4 photosynthesis in its grains – which can contribute up to 42% of total photosynthesis of the ear – how much can we further boost this cereal’s total photosynthesis, if at all?

Examining rice (Oryza sativa – a cereal that famously provides the major calorie intake for approx. one-half of the world’s population – Weijun Shen et al. demonstrate that at least some of the C4-like photosynthetic properties are present in mesophyll cells of the mid-veins of the leaves of this otherwise C3 grass. It would be interesting to know what type of photosynthesis occurs in rice grains.

Taken together these two revelations suggest that C4 photosynthesis may be a latent property within cereals that just awaits ‘reawakening’, which is perhaps not such a daunting challenge as wholescale introduction of the C4 pathway anew…

[Ed. – and it’s worth reminding ourselves that the discovery of the C4 variant of photosynthesis was published 50 years ago by Hal Hatch and Roger Slack, hence its fuller name of the Hatch-Slack Pathway. For a historical – and future – perspective of this brilliant bit of botanical biochemistry, get hold of Robert Furbank’s Darwin Review. For an up-to-date assessment of the evolutionary and taxonomic dimensions of this fabulous phytological photosynthetic phenomenon, we recommend Rowan Sage’s Darwin Review. And for an interesting study into how C4 photosynthesis boosts growth by altering physiology, allocation and size, see Rebecca Atkinson et al.]