Protein accumulation and rumen stability of wheat γ-gliadin fusion proteins in tobacco and alfalfa

Molecular, biochemical, and proteomic analyses of transplastomic tobacco plants expressing an endoglucanase support chloroplast-based molecular farming for industrial scale production of enzymes

The successful production of recombinant enzymes by tobacco transplastomic plants must maintain compatibility of the heterologous enzyme with chloroplast metabolism and its long-time enzyme stability. Based on previous reports, it has been taken for granted that following biolistic-transformation, homoplasticity could be obtained from the initially heteroplastic state following successive rounds of selection in the presence of the selection agent. However, several studies indicated that this procedure does not always ensure the complete elimination of unmodified wild-type plastomes. The present study demonstrates that CelK1 transplastomic plants, which were photosyntetically as active as untransformed ones, remain heteroplastomic even after repeated selection steps and that this state does not impair the relatively high-level production of the recombinant enzyme. In fact, even in the heteroplastomic state, the recombinant protein represented about 6% of the total soluble proteins (TSP). Moreover, our data also show that, while the recombinant endoglucanase undergoes phosphorylation, this post-translation modification does not have any significant impact on the enzymatic activity. Biomass storage might be required whenever the enzyme extraction process could not be performed immediately following the harvest of tobacco mature plants. In this respect, we have observed that enzyme activity in the detached leaves stored at 4 °C is maintained up to 20 weeks without significant loss of activity. These findings may have major implications in the future of chloroplast genetic engineering-based molecular farming to produce industrial enzymes in transplastomic plants.

The effects of PPO activity on the proteome of ingested red clover and implications for improving the nutrition of grazing cattle

Increasing the rumen-stable protein content of feed would lead to improved nitrogen utilisation in cattle, and less nitrogenous waste. Red clover (Trifolium pratense L.) is a high protein ruminant feed containing high polyphenol oxidase (PPO) activity. PPO mediated protein-quinone binding has been linked to protecting plant proteins from proteolysis. To explore the mechanism underlying the effect of PPO on protein protection in fresh forage feeds, proteomic components of feed down-boli produced from wild-type red clover and a low PPO mutant, at point of ingestion and after 4h in vitro incubation with rumen inoculum were analysed. Significant differences in proteomic profiles between wild-type and mutant red clover were determined after 4h incubation, with over 50% less spots in mutant than wild-type proteomes, indicating decreased proteolysis in the latter. Protein identifications revealed preferentially retained proteins localised within the chloroplast, suggesting that PPO mediated protection in the wild-type operates due to the proximity of target proteins to the enzyme and substrates, either diffusing into this compartment from the vacuole or are present in the chloroplast. This increased understanding of protein targets of PPO indicates that wider exploitation of the trait could contribute to increased protein use efficiency in grazing cattle.

BIOLOGICAL SIGNIFICANCE

One of the main challenges for sustainable livestock farming is improving capture of dietary nitrogen by ruminants. Typically up to 70% of ingested protein-N is excreted representing a loss of productivity potential and a serious environmental problem in terms of nitrogenous pollution of lands and water. Identification of key characteristics of rumen-protected protein will deliver target traits for selection in forage breeding programmes. The chloroplastic enzyme PPO catalyzes the oxidation of phenols to quinones, which react with protein. Little is currently known about the intracellular protein targets of the products of PPO activity or the mechanism underlying protein complexing, including whether there is any specificity to the reaction. Here we have determined significant differences in the proteomes of freshly ingested down boli corresponding to the presence or absence of active PPO. These results show that in the presence of PPO the forage protein is less amenable to proteolysis and provide the novel information that the protected proteins are putatively chloroplastically located. These data also contribute to a growing evidence base that a chloroplastic PPO substrate exists in red clover in addition to the currently known vacuolar substrates.