Table 1.

Tools and technologies relevant to engineering photosynthesis, their potential applications, and current limitations

Technology/toolsApplicationsLimitations
Bacterial transformationEngineering photosynthesis in cyanobacteriaNo serious technical limitations in the most important cyanobacterial model species.
Nuclear transformationEngineering of nucleus-encoded components of the photosynthetic apparatus; expression of novel genes and pathways. Development of synthetic chromosomes.Lack effective strategies to avoid transgene silencing; lack effective strategies for high transgene expression levels (often significantly lower than with plastid transformation); lack sufficient characterized promoters, terminators, and chloroplast transport signals; lack mature technologies to stably transform very large DNA segments (>10 genes) into plants; lack efficient tools for site-directed engineering; lack sufficient information on centromere and telomere sequences.
Plastid transformationEngineering of plastid-encoded components of the photosynthetic apparatus; expression of novel genes and pathways of carbon metabolism.Small number of transformable species, which do not include cereals or other major crops; lack of robust and flexible regulatory strategies.
Mitochondrial transformationEngineering of mitochondrially encoded components of the respiratory chain to minimize respiratory losses; expression of novel pathways of carbon metabolism.Not yet possible in any seed plant; currently possible only in the algal species Chlamydomonas reinhardtii.
Multigene engineeringEngineering of protein complexes in the electron transfer chain; engineering of carbon fixation pathways.No serious limitations; can be performed via synthetic operons in cyanobacteria and plastids and/or by combinatorial transformation in the nucleus, but mature technologies to introduce very large fragments (>10 genes) into plants are needed.
Protein designRedesign of the electron transfer chain; Rubisco engineering; redesign of carbon-fixing enzymes.Computer-based structural modeling not yet sufficiently developed; limited success with rational design and rational optimization of proteins; limited understanding of protein dynamics.
Synthetic genomicsRadical redesign of the photosynthetic apparatus via synthetic plastid genomes and/or artificial (mini)chromosomes in the nucleus.Incomplete parts list of the photosynthetic apparatus, its assembly factors and regulators; incomplete knowledge about dynamic (quantitative) changes in response to environmental cues; limitations in synthesis and efficiency of assembly of very large DNA molecules (>1 Mbp) and in stability of minichromosomes.
Design of logic circuits; development of sensors for light intensity, light quality, temperature, and CO2 concentrationSmart canopy concept.Incomplete parts list: insufficient understanding of the structure and organization of the genetic and metabolic networks underlying photosynthesis and its regulation; limited knowledge about pool sizes of metabolites in different cellular compartments and subcompartments; incomplete knowledge of transporters.
Phenotyping in the fieldEvaluation of design concepts under field conditions and further optimization through mutagenesis.Insufficient sensitivity of current imaging methods; lack of sensitive spectrometric methods for large-scale and continuous measurement of the dynamics of photosynthetic parameters, dynamics of radiation quality and quantity within the canopy, and growth of stands of many genotypes/transformants in the field.