Table 1.

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

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.