Nature Plants

High accumulation of heterologous proteins expressed from the plastid genome has sometimes been reported to result in compromised plant phenotypes. Comparisons of transplastomic plants to wild-type are typically made in environmentally-controlled chambers with relatively low light; little is known about the performance of such plants in field conditions. Here we report on two plastid-engineered tobacco lines expressing the bacterial cellulase Cel6A.  Field-grown plants producing Cel6A at ~20% of total soluble protein (TSP) exhibit no loss in biomass or Rubisco content and only minor reductions in photosynthesis compared to wild-type. These experiments demonstrate that when grown in the field, tobacco possesses sufficient metabolic flexibility to accommodate high levels of the recombinant protein by increasing total protein synthesis and accumulation and/or by reallocating unneeded endogenous proteins. Based on current tobacco cultivation practices and readily achievable recombinant protein yields, we estimate that particular proteins could be obtained from field-grown transgenic tobacco plants at costs three orders of magnitude less than current cell culture methods.