Engineering the Plastids


Synthetic biology isn’t restricted to engineered microbes alone. Though most applications, such as production of secondary metabolites, are better accomplished in microbes, there are diverse applications for which the higher plants are the best-suited candidates.

Plastids are plant organelles that are semi-autonomous of the plant cells that harbour them. They were endosymbionts that over the course of evolution have lost their individuality. Each plastid has several circular DNA copies having over a 100 genes, most of which are involved in photosynthesis. Plastids are increasingly being seen as candidates for genetic transformation. The advantages of plastid transformation over ‘conventional’ transformation are

  • high protein expression levels,
  • absence of epigenetic effects,
  • uni-parental inheritance, which is commercially favoured,
  • easy transgene stacking in operons, and
  • increased biosafety since plastids are maternally inherited, they aren’t transmitted by pollen.

The hurdles to plastid transformation include

  • difficulty in delivering the exogeneous DNA through the double membrane of the plastid, and
  • the enormous copy number – each plastid of each cell must be transformed.

Can we engineer photosynthesis?

Which single event that has the potential to revolutionize global agriculture like never before? It is improvement in the efficiency of photosynthesis. Plants utilize only about 10% of the light that falls on them. If they utilize even 5% more, that would mean 50% more of everything that we obtain from plants.

Which molecule should we look at? It has to be the most abundant protein in the world – RubisCO.

What to look for in the enzyme? Two characteristics are essential – it’s catalytic activity and its specificity to bind CO2 and O2. Improving the latter seems a realistic approach to get a faster RubisCO.

How do we get a faster RubisCO? I’d bet on directed evolution.

Would that be enough? No. Equally precise tools are required for genome editing too, which we have got in CRISPRs.

Why plastids are suitable for synthetic biology?

  • The advantages of plastid transformation, listed above, are well suited to commercial production of pharmaceuticals and nutraceuticals.
  • Over the course of billion years, plastid has lost most of the genes of the original endosymbiont. The plastid is hence representative of a minimal genome.

Read full article here: Bock and Khan; Taming plastids for a green future; Trends in Biotechnology

View my presentation on the topic here.

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