DILIPHO · Diffusional limitations to C4 photosynthesis

Diffusional limitations to C4 photosynthesis

When Photosynthesis evolved in Bacteria some 3.5 billion years ago it was arguably the most important innovation in Earth's history. Photosynthetic organisms harvest light and use its energy for chemically reducing CO2 molecules to form the backbones of the plant body, reserves and reproductive organs. Through geological eras the atmosphere was progressively depleted of CO2 through burial of coal, oil and carbonate rock sediments. Around 30 million years ago, some plants evolved a biochemical pump, called C4 photosynthesis, that concentrates CO2 from the atmosphere in a partially sealed compartment inside the leaf. This Carbon Concentrating Mechanism (CCM) is an effective ‘turbocharger’ of the assimilatory machinery and confers higher productivity potential. C4 plants are of primary importance and leading grain (maize, sorghum), sugar (sugarcane), and biofuel (miscanthus) producers. Advanced breeding of C4 plants is currently impinged on negatively by lack of fundamental knowledge of C4 physiology. Importantly, it is unknown why C4 plants despite consuming less water and occupying drier habitats, are inherently more susceptible to water stress than C3 plants. For C4 photosynthesis to operate, a substantial flow of metabolites is continuously exchanged between two partially isolated compartments in the leaf parenchyma arranged concentrically around veins. In the external layer, called mesophyll, CO2 is temporarily fixed into small organic acids composed of four atoms of carbon (hence the name C4). These C4 acids diffuse to the internal layer, called bundle sheath, through microscopic apertures called plasmodesmata, which connect adjacent cytoplasms in a water-based continuum. In the bundle sheath the C4 intermediates are decarboxylated, while the resulting C3 acids diffuse back to the mesophyll, where they are recycled. This gigantic flux of small metabolites needs to be continuously exchanged between the external and the internal layer through channels called plasmodesmata . DILIPHO hypothesizes that hydraulic pressure within cells (called turgor) is required to keep the plasmodesmata section wide open. When water becomes less available (measured by water potential) turgor decreases and this would cause plasmodesmata to shrink. The consequent reduction in plasmodesmata cross section would reduce permeability to diffusion between mesophyll and bundle sheath, slowing down the exchange of metabolites, thus jamming the C4 machinery. DILIPHO consists of three phases, two of which have now been completed. In the first conceptual phase (WP1) the fellow learned concepts of advanced Mathematics and Biophysics, prepared the following experiment and developed a mechanistic model to study metabolite transport at leaf level, DiliMOD. In the second phase (WP2) a dedicated and novel experiment was performed whereby a transient decrease in turgor was induced while photosynthesis and key physiological quantities were assessed in real time. In the third phase (WP3) the acquired data were analysed and interpreted using the novel model to test the hypothesis.

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