Photosynthesis is one of the most important metabolisms for the sustaining of life on Earth. It is performed by plants, algae, and even bacteria, its purpose is to exploit solar energy for the production of sugars. By using an external source of energy, these organisms are able to convert (reduce) inorganic carbon (CO2 in the air) to organic carbon, which can be used as a source of energy.

This “jump” from the inorganic to the organic world is done also by other organisms, by exploiting different autotrophic metabolisms such as chemosynthesis.

Photoautotrophic organisms are involved in the maintaining of the C cycle in our planet, in particular by sequestrating CO2  from the atmosphere, and trapping it in difficult-to-hydrolyze polymers  (lignin and cellulose), and by producing molecular oxygen as a side product.


Why engineering

The next years will be a challenge in terms of food production: with an expected population of more than 9 billion in 2050, the majority of pressure will weigh on the primary sector, in particular crop production, which is responsible for almost 2/3 of total consumed calories worldwide. Food security is a challenge that humans always needed to face, and one of the main problem in this regard, are food wastes (almost 1/3 of the produced food is discarded each year).

With a so big problem of food managing and preservation, someone can think that the real problem is on wasting, more than increasing productivity. And that would be true, or better, the solution lies in the middle: reducing wastes is possible and is mainly due to education problems than actual technology limits (most wastes comes from houses), but with an increasing need for high-quality animal foods, and reducing land availability, more food needs to be produced per unit of land.

The yield of crops has increased drastically in the last years, in the case of rice and wheat, for example, it almost doubled since 1960. With “yield” I’m referring to the Yield potential (Yp), defined as the mass of harvested material (fruits of grains) collected by a particular genotype in the complete absence of stresses.  The Yp is influenced mainly by:

  1. The efficiency of the plant of intercepting incoming light
  2. The partition of energy and carbon in the harvested part
  3. The conversion of the intercepted light into chemical energy

This increase in productivity, commonly known as Green Revolution, has been possible thanks to improved agronomy and improved crops genotypes, achieved, so far, with traditional breeding and varieties selection. But almost no effort was put into increasing the metabolism responsible for the conversion of the solar energy into usable chemicals by the plant (3rd factor): Photosynthesis.

It seems that in some crops, the improvement of light detection, influenced by physiological and climatic factors, and carbon partitioning in the plant tissue, have reached their biological maximum.  Engineering of the pathway could be possible by “taking inspiration” from the different types of photosynthesis evolved in plants, or in other distantly related species, like bacteria and cyanobacteria.


What is photosynthesis

Photosynthesis is a well-known metabolism, I will reassume just a few aspects that are particularly important for its engineering, and I will leave additional material for the ones interested.

In general, Photosynthesis is split into two different reactions:

During the (i)light reaction, the sunlight energy is captured as reducing potential; a molecule of chlorophyll is excited by light, resulting in an excited high-energetic state. The specific wavelength of light that is absorbed depends on the chlorophyll and other accessory pigments, chlorophyll is found in chloroplasts, specifically in the reaction center of big proteins complexes called photosystems. In this state, energy is transferred in the form of pairs of electrons to a series of other molecules until the final acceptor: NADP+, is then reduced to NADH.

As electrons are passed between each intermediate of the reaction chain, H+ ions are accumulated in the thylakoid lumen, this chemical potential is used to synthesize ATP molecules similarly to what happens during respiration.  To regenerate (give back electrons and H atoms) the starting chlorophyll, water molecules are split and deprived of H atoms in a process called photolysis.

Scheme of the light reaction that occurs in the thylakoid's membrane. The reaction takes place by using two different photosystems to help the transfer of electrons. Source:http://photobiology.info/Schurmann.html

During the (ii)dark reaction, the Calvin cycle is used to fix CO2 from the atmosphere and synthesize triose sugars. The first enzyme of the metabolism is RuBisCO and it catalyzes the carboxylation of Ribulose 1-5P (5C sugar), the so synthesized 6C intermediate is split, releasing two 3C sugars. One of these molecules of 3P glycerate is used as the precursor of sugars, amino acids, or other complex biomolecules, the other one, will continue the cycle and will regenerate the starting ribulose molecule. During the regeneration process, ATP and NADPH produced in the previous reaction are consumed.


Scheme of Calvin Cycle (on the right), compared to Photorespiration (see the last chapter). Source:https://fr.wikipedia.org/wiki/Fichier:Simplified_photorespiration_diagram.jpg


Engineering of Photosynthesis

Our capacity of engineering photosynthesis mainly depends on our knowledge of the reactions and enzymes involved, the availability of computational models for making predictions and performing in silico screenings, and our capability of performing reliable gene editing (CRISPR, Zn fingers, vectors…). Photosynthesis itself is a complex pathway, and we can act at many different levels.

The wavelength absorbed by chlorophyll is in the range of 400-700 nm, slight differences can be found in the 2 major types of chlorophyll: A and B (the latter acts as accessory pigment). With this narrow range of wavelengths, none of the energy of the near-UV or far-infrared spectrum is absorbed, reducing by half the energy that the plant can absorb. Expansion of the spectrum further in the red is limited by the energy of photons at low frequencies, that are not powerful enough to drive the water photolysis. This pushes the limit of wavelengths for oxygenic photosynthetic organisms to 750 nm, like in the case of Acaryochloris marina, a cyanobacterium that uses variants of chlorophyll A to carry out photosynthesis underwater.

Another factor that limits the efficiency of the photosynthetic pathway is the inefficient use of ATP and NADPH, mostly when the incoming radiation saturates all the available pigment, in those conditions, energy is lost by dissipation and fluorescence. Also, the continuous changes of exposure of leaves in the canopy due to the tissue structure, or variable climatic conditions, are another source of inefficiency.

Saturation conditions may also cause chlorophyll molecules in their excited state to transfer their energy to oxygen, leading to the formation of harmful oxygen radicals, that may cause damage, and that require energy to be managed by the cell.

Another important bottleneck of the system is the dark reaction, in particular the activity of the RuBisCO enzyme.


The most abundant protein on earth: RuBisCO

This enzyme is so abundant due to the broad extension of photosynthetic organisms on earth, but also due to its inefficient kinetics. RuBisCo, in fact, is not specific for CO2 but can interact also with O2, leading to a series of wasteful reactions that consume energy and previously fixed carbon (Photorespiration). Due to this aspecific activity, large amounts of it are necessary to support an adequate photosynthetic rate.

So, how we can deal with this bottleneck? Well, nature can show us an example of adaptation. The photosynthesis previously described is common only in some species: C3 plants. In these organisms CO2 is absorbed directly from the atmosphere, however, C4 plants (like maize and sugarcane) have evolved a mechanism to concentrate and translocate CO2 in the vicinity of RuBisCO. This process, which evolved 60 independent times in history as an adaptive measure, has two main steps:

  1. The initial fixation of CO2  happens in the mesophyll (exterior) cells, where phosphoenolpyruvate carboxylase forms the C4 acid oxaloacetate
  2. The C4 acid is then transferred to the cells of the bundle sheath, where it is decarboxylated to release CO2 in the vicinity of Rubisco.
Comparative anatomy of C3 and C4 leaves.

So, a strategy could be the insertions of C4 genes in C3 crops like rice, wheat, and oat, to increase their efficiency. A few results have been achieved in this regard, but the lack of the correct anatomy in C3 plant leaves to sustain this type of CO2 translocation, could be the major problem.

Another solution could be taking inspiration from the mechanisms found in bacteria and cyanobacteria, that use carbonic anhydrases to concentrate CO2 near the RuBisCO site. These enzymes are pretty common and convert CO2 molecules in bicarbonate ions (causing a decrease in pH), that are solubilized and more easily transferred.

This seems simpler than inserting the entire C4 anatomy in C3 crops but requires the insertion of bicarbonate and CO2 pumps in the chloroplast membrane, and correct localization of the anhydrase activity in the plant cell to work. The latter option seems more promising, since, it has been shown that even the addition of only bicarbonate pumps in the chloroplast, successfully increased the efficiency of photosynthesis.


The field of research is open and very productive, interfering and modifying the way in which the light is absorbed by the plant seems to be challenging and maybe more results could be achieved by modifying the dark reactions, and other secondary metabolisms. Modifications of vital metabolisms are not often straightforward and can result in unexpected side, detrimental effects.


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