The enzyme rubisco is found in all plants and other photosynthesizing organisms. It plays a key role in fixing carbon from the air and has helped shape life on Earth.
Credit: Getty Images
Readers may recall how I wrote recently about the enzyme RuBisCo, and how, if they understood its significance, it would be a major embarrassment for devotees of creationism's putative intelligent [sic] designer. To give it it's full chemical name, ribulose-1,5-bisphosphate carboxylase/oxygenase is probably the least efficient enzyme in all biology and yet is absolutely essential for the energy almost all living things ultimately depend on.
It is responsible for 'fixing' the inorganic carbon in atmospheric carbon dioxide (CO2) as the first stage in building the carbon chain that is the backbone of the organic sugar, glucose, which is the end product of photosynthesis, along with free Oxygen (O2) as a by-product.
Indeed, so inefficient is RuBisCo that it is the reason there is so much green in the world, wherever there are growing plants. RuBisCo manages about 4 reactions a second against the hundreds or thousands of reactions that most other enzymes are capable of, and, because it frequently mistakes a molecule of O2 for one of CO2, it frequently wrecks the process and causes a loss of carbon. It can also go wrong and produce xylulose-1,5-bisphosphate which actually poisons it!
Because of this inefficiency, there is great pressure on scientists to come up with an alternative, or to develop an improved version, in order to increase food production, and, to that end, multiple universities and research institutes have established the RIPE Project, led by the University of Illinois, USA in partnership with The Australian National University, Chinese Academy of Sciences, Commonwealth Scientific and Industrial Research Organisation, Lancaster University, UK, Louisiana State University, USA, University of California, Berkeley, USA, University of Cambridge, UK, University of Essex, UK, and U.S. Department of Agriculture, Agricultural Research Service.
But one of the programs run by RIPE has just reported a failure. The researchers at the Division of Plant Sciences, Research School of Biology, The Australian National University, Acton, ACT, Australia, had been hoped that, if they could improve the supply of CO2 to RuBisCo, they could improve its efficiency. However, they discovered that the limitation was not the availability of CO2 itself but the ability of RuBisCo to utilise it that was the bottleneck in the process.
As the RIPE news release explains:
Modifying photosynthesis has increasingly been a research target to improve crop yields to feed a growing global population in the face of climate change and other environmental factors. In a recent study, published in the Journal of Experimental Botany, a team from the Australian National University (ANU) investigated the effects of increasing the amount of carbon dioxide channels in plant membranes, but could not detect any impact on photosynthesis in model tobacco plants.Our research targeted the membranes in leaf cells; we wanted to know if we could make CO2 transfer more efficient by adding extra channels for CO2 diffusion into cell membranes
Dr Tory Clarke
Australian National University
Photosynthesis relies on a supply of carbon dioxide (CO2) to the chloroplasts within leaf cells, where it is fixed into sugars by the enzyme Rubisco. To get to the chloroplast, CO2 must diffuse into the leaf and through the leaf mesophyll cells, crossing barriers such as cell walls and membranes. Increasing CO2 diffusion through mesophyll cells into the chloroplast (termed mesophyll conductance) will improve photosynthesis–boosting yields in crops while also improving water-use efficiency.
Copyright: © 2022 The authors. Published by Published by Oxford University Press on behalf of the Society for Experimental Biology.Open access
AbstractImagine a production line where a critical part of the production process is performed by a machine which is so slow at the task that it slows the whole production line down to its very slow pace, and then frequently fits the wrong component and wrecks the product so the next stages in the process can't be performed, and it all has to be dismantled and started again. In the case of this study, the scientists tried increasing the supply of the components the machine need in case it wasn’t being supplied quickly enough. But that wasn't the problem. The problem is that the machine is so badly designed it just can't go any faster.
In plants with C3 photosynthesis, increasing the diffusion conductance for CO2 from the substomatal cavity to chloroplast stroma (mesophyll conductance) can improve the efficiencies of both CO2 assimilation and photosynthetic water use. In the diffusion pathway from substomatal cavity to chloroplast stroma, the plasmalemma and chloroplast envelope membranes impose a considerable barrier to CO2 diffusion, limiting photosynthetic efficiency. In an attempt to improve membrane permeability to CO2, and increase photosynthesis in tobacco, we generated transgenic lines in Nicotiana tabacum L. cv Petite Havana carrying either the Arabidopsis PIP1;2 (AtPIP1;2) or PIP1;4 (AtPIP1;4) gene driven by the constitutive dual 2x35S CMV promoter. From a collection of independent T0 transgenics, two T2 lines from each gene were characterized, with western blots confirming increased total aquaporin protein abundance in the AtPIP1;2 tobacco lines. Transient expression of AtPIP1;2-mGFP6 and AtPIP1;4-mGFP6 fusions in Nicotiana benthamiana identified that both AtPIP1;2 and AtPIP1;4 localize to the plasmalemma. Despite achieving ectopic production and correct localization, gas exchange measurements combined with carbon isotope discrimination measurements detected no increase in mesophyll conductance or CO2 assimilation rate in the tobacco lines expressing AtPIP. We discuss the complexities associated with trying to enhance gm through modified aquaporin activity.
Clarke, Victoria C; De Rosa, Annamaria; Massey, Baxter; George, Aleu Mani; Evans, John R; von Caemmerer, Susanne; Groszmann, Michael
Mesophyll conductance is unaffected by expression of Arabidopsis PIP1 aquaporins in the plasmalemma of Nicotiana
Journal of Experimental Botany; erac065 DOI: 10.1093/jxb/erac065
Open access
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)
Creationists believe the equivalent of this machine was designed by a supremely intelligent production line engineer who didn't realise what a bad job it was doing so saw no reason to start again with a new design, and now modern factory managers are trying to find a way to make it more efficient, so they can continue to supply the demand for the product.
As the product of a utilitarian, near enough is good enough, evolutionary process that occurred early in the evolution of living organism and in an oxygen-free atmosphere, this is perfectly understandable. It is not understandable in terms of intelligent design by a supremely intelligent, omniscient designer who would have known in advance what problems lots of the by-product in the atmosphere - free oxygen - was going to cause.
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