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Another way to exploit the potential of CAM would be to engineer its molecular machinery into C₃ plants (Borland et al., 2014). Indeed there is great interest in general in engineering a wider range of CCMs, taken from C₄ plants and from microbes, into commercial crops. Complex metabolic alternatives, such as CAM, typically require a whole suite of co‐adapted changes, and implementing them will therefore be a massive challenge. However, promising results may be obtained from less ambitious approaches. Figure 3.19, for example, illustrates the results when the photosynthetic rate of a standard variety of soybean, Glycine max, was compared with that of a transgenic variety that had been transformed to express the ictB gene taken from cyanobacteria, where it plays a crucial role in carbon uptake and is essential for their survival at anything other than very high CO₂ concentrations. In both varieties, photosynthetic rates increased with the concentrations of CO₂ in plant tissues but levelled off at higher concentrations. However, these rates, and especially the maximum rate, were significantly higher in the transgenic variety (Figure 3.19). Crucially, this difference translated into significant improvements in yield in the transgenic variety. As the authors of the study conclude, work such as this indicates that even single genes can contribute to the enhancement of yield in a major commodity crop ‘… and point to the significant role that biotechnological approaches to increasing photosynthetic efficiency can play in helping to meet increased global demands for food’.

Figure 3.19 Bioengineering of a gene from cyanobacteria into soybean increases its rate of photosynthesis. A comparison of soybean (Glycine max) wild type (blue) and a variety genetically engineered to express the cyanobacterial membrane protein ictB (red) in terms of the effect of a plant’s internal CO₂ ([CO_2i]) concentration on its net rate of photosynthesis (A_net). The fitted lines were non‐rectangular hyperbolas reflecting current understanding of the underlying physiology.

Source: After Hay et al. (2017).

the evolution of C₄ and CAM

The evolution of the C₄ and CAM pathways, and of CCMs generally (that increase the concentration of CO₂ around RuBisCO), has been reviewed by Raven et al. (2008). They describe the very strong evidence that these mechanisms are evolutionarily primitive (the earliest appearing at least 300 million years ago) but also that the C₄ and CAM systems must have arisen repeatedly and independently during the evolution of the plant kingdom – the most recent being the appearance of C₄ in land plants only 20–30 million years ago. This prolonged coexistence of multiple paths to carbon fixation foreshadows a pattern we will see many more times in later chapters – of coexisting species utilising the same resources but in different ways.