By Angela White
Photosynthesis is a major target area for crop improvement. In July 2014, I caught up with three plant scientists researching photosynthesis to discover their latest findings, which were presented at the Society for Experimental Biology’s annual main meeting in Manchester.
Understanding evolutionary intermediates between two photosynthetic pathways
Marjorie Lundgren, a PhD student at the University of Sheffield, is researching how different photosynthetic mechanisms evolve. She works on the grass Alloteropsis semialata, which is unique in having both C3 and C4 photosynthetic pathways within this single species. Excitingly, Marjorie’s work has discovered populations of this species with intermediate photosynthetic phenotypes (known as C2 plants), helping us to understand how C4 evolved from the C3 pathway.
Marjorie’s research has three main findings. Firstly, she’s confirmed the existence of intermediate photosynthetic states using a range of physiological techniques. Secondly, she’s established that this intermediacy arose in Central Africa. And finally, Marjorie has elucidated clear links between environment, leaf anatomy and physiology. Together, her preliminary work suggests that leaf anatomical traits which are important for the C3 to C4 transition respond to environmental changes. This responsiveness is known as ‘phenotypic plasticity’ and may affect the evolution of photosynthetic types.
“There’s a huge amount of variation within this species,” says Marjorie. “It’s a brilliant system.” Marjorie hopes that her research will inform the multinational C4 rice consortium, which aims to introduce the efficient C4 photosynthetic pathway into rice. She is working to identify important anatomical turning points in the evolutionary process which leads from C3 to C4 photosynthesis.
The next challenge is to use this wild grass species to identify the genetic variation that underpins evolution of the C4 photosynthetic pathway, and see how it affects physiology. This understanding is crucial if we are to successfully engineer C4 traits into C3 plants to improve crop efficiency and yield.
Breeding wheat with high photosynthetic efficiency and yield
Dr Elizabete Carmo-Silva, at Rothamsted Research, is working on an exciting project to identify wheat varieties with high photosynthetic efficiency and high yield. This work is undertaken by the photosynthesis research group, led by Professor Martin Parry. The group studies the characteristics of wheat cultivars in the field to inform traditional breeding and exploit the use of genetic modification to increase the efficiency of the photosynthetic enzyme Rubisco.
Rubisco is made up of subunits encoded by a combination of genes, some of which are found in the nucleus and some in the chloroplast. We still don’t know how to genetically modify the wheat chloroplast but that isn’t the only difficulty; public acceptance of GM is still a long way off in the UK. However, Elizabete hopes that in 15 to 20 years we might see the first Rubisco-modified plants in the field.
“We need to marry the technology with our knowledge of the Rubisco we want to use,” explains Elizabete, who is characterising dozens of wheat varieties to identify potential targets for crop improvement. The group has found some varieties which combine high photosynthetic rate with low levels of Rubisco. This enzyme constitutes approximately half the total leaf protein, so lower Rubisco levels mean lower nitrogen (and thus lower fertiliser) requirements. “It would be great to improve nitrogen use efficiency as well as yield,” says Elizabete, who recognises the importance of resource-use efficiency for improved farming practice. It’s also important to look for cultivars which produce a good crop under variable weather conditions, a trait known as resilient yield.
Understanding stomatal dynamics and the impact on photosynthesis and water use efficiency
With climate change rearing an ever-uglier head, the threat of yield loss due to drought is an increasing issue for UK farmers. Dr Tracy Lawson of the University of Essex is researching the impacts of different light conditions on stomata, the tiny pores which allow carbon dioxide to enter leaves. When carbon dioxide enters the leaf, water is lost – so efficient opening and closing of these pores can reduce the amount of water used by the crop.
Tracy’s laboratory, including Tracy’s PhD student Lorna McAusland, is investigating the rates of stomatal opening in response to light (which allows carbon dioxide to reach the photosynthetic organelles). Of the species analysed, grasses such as maize and oats have faster responses than legumes such as broad bean. This is due in part to the different morphology of their guard cells – the pair of aptly-named cells which surrounds each little pore. Although such anatomical differences are already well-established in the scientific literature, Tracy and Lorna have confirmed with this work that different dynamic properties exist as well.
“Stomatal responses are an order of magnitude slower than the response of photosynthesis,” explains Tracy. “Their responses can overshoot.” In field conditions, where light levels are highly variable, it is important for stomata to respond as fast as possible. Stomatal closure helps plants to avoid excessive water loss when conditions are not optimal for photosynthesis, whilst speedy stomatal opening avoids restriction of carbon dioxide diffusion when the environment is optimal for photosynthesis. Stomata open and close when ions are pumped across guard cell membranes, so targeting these processes would allow us to alter stomatal behaviour. “There is definitely potential to manipulate some of the ion channels and proton pumps in guard cells,” says Tracy. We now need to increase our understanding of the signalling pathways regulating coordination between mesophyll photosynthesis and stomatal conductance, which is a focus of the research in Tracy’s laboratory.
Plants and plant scientists face the future
Plants face multiple challenges, such as fluctuating light levels, ever-increasing carbon dioxide levels, changing temperatures and soil water levels. These abiotic factors interact to create a plethora of environmental combinations, not to mention biotic stresses imposed by pests and pathogens! Research on factors contributing to photosynthetic efficiency is not only fascinating, but of critical importance for future food security. Marjorie’s work looking at the evolution of more efficient photosynthesis, Elizabete’s characterisation of high-photosynthesis, high-yielding wheat varieties, and Tracy’s investigation of fast-responding stomata are examples of exciting novel research in this area. Plant scientists are striving to increase crop yield for future generations, and an understanding of how plants evolve, respond to and thrive in their environment is vital.
Recent reviews co-authored by these scientists
Marjorie Lundgren, University of Sheffield:
Dr Elizabete Carmo-Silva, Rothamsted Research:
Dr Tracy Lawson, University of Essex:
Angela White is a PhD student at the University of Sheffield. Her research focuses on understanding the limitations on plant growth rates imposed by sink and source activity, using a holistic approach involving both experimental and modelling work.