Plants use sunlight to produce their own nutrients and energy: photosynthesis. If we want to keep the planet and its growing population running in a sustainable way, we will need plants to produce far more food, energy and applicable biomass than they do now. Photosynthesis, the green engine of life on Earth, needs tuning.
Frequently asked questions:
“Currently, photosynthesis by plants is probably optimal for a wild, natural situation, but may be optimised for a crop cultivation setting,” says Mark Aarts, personal professor at the Laboratory of Genetics of Wageningen University & Research. “It is a delicate and yet rather aggressive process, involving potentially dangerous energy fluxes, which includes many protective mechanisms to keep this process under control. This is probably best for natural conditions in the field. Crops however are often grown under more controlled conditions, whether in the field or in greenhouses. This means that they might be able to do without some of the protective mechanisms, especially if they pose a limit to production," says Aarts.
Under optimal conditions, the differences in the efficiency of photosynthesis are small. However, stress such as excessive cold, heat or drought can bring out substantial differences in efficiency, and this allows for selection. The current research is carried out on the model plant Arabidopsis, or thale cress, because all its genes have been mapped and their DNA sequences are exactly known.
Optimizing photosynthesis is complicated, and thus difficult, to find the genetic component in the variation in photosynthesis. Firstly, to discover the comparatively tiny differences, it is necessary to repeatedly carry out simultaneous measurements in controlled conditions. “This is very challenging to do in the field,” says Mark Aarts, personal professor at the Laboratory of Genetics of Wageningen University & Research. “And even if you would temporarily move the plants into a climate cell you will have trouble identifying heritable differences. Plants must first acclimatise before the variation is detectable. And those small differences are relevant: if we can boost ten different characteristics with this method, the ultimate improvement will be 10 to 20%. Moreover, there is not necessarily a linear relationship between photosynthesis efficiency and production. A higher efficiency can lead to larger yields, but also, for example, to increased production of substances that protect the plants from predators. This means that the result of selection may not be obvious at first sight.”
A second issue is that the specialised measurement equipment did not exist back then. "The measuring technology is crucial. We measure the efficiency of photosystem II (one of the two systems that harvest light and convert it into chemical energy) by giving the plant a light pulse and registering what happens with a camera. This is best done on plants that are left alone and not moved around, to get the most useful measurements.”
In the Netherlands different horticulture crops, such as tomato, cucumber and sweet peppers are mostly grown in heated greenhouses, and the energy used comprises 20% of the total production costs. Decreasing the greenhouse temperature by 2°C would save 16% of the energy consumed for the production of vegetables. Heating the greenhouses is necessary in order to achieve high yields for the varieties that are currently available in the market. At lower temperatures key enzymes involved photosynthesis and sugar metabolism have a reduced activity and that leads to the accumulation of phosphorylated sugar in leaves (source organs). These sugars trigger feedback inhibition of photosynthesis and result in a limited/delayed fruit development. Wild tomato accessions, in particular the ones naturally growing in cold and high altitude regions, are potential sources of enzymes with a higher activity at lower temperatures. Within this Towards Biosolar Project we are targeting enzymes involved in the reduction of sucrose accumulation in leaves in particular sucrose synthases (susy) and invertases (inv) (Figure 1), and characterizing the activities of different alleles from several tomato wild species.
Figure 1: Sucrose transport from source cell to sink cell. In source cell (leave), trio-phosphates generated from photosynthesis are partly converted into sucrose. Sucrose then can be used in the source cell or exported to sink cells through companion cell first, then sieve tube elements, and phloem. The transported sucrose through phloem is subsequently unloaded into sink cells (e.g. roots, fruits, tubers) and is processed further by several enzymes such as invertase and sucrose synthase.
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