A step forward to building an algal pyrenoid in higher plants

Absence of carbonic anhydrase in chloroplasts affects C3 plant development but not photosynthesis

The enzyme carbonic anhydrase (CA), which catalyzes the interconversion of bicarbonate with carbon dioxide (CO2) and water, has been hypothesized to play a role in C3 photosynthesis. We identified two tobacco stromal CAs, β-CA1 and β-CA5, and produced CRISPR/Cas9 mutants affecting their encoding genes. While single knockout lines Δβ-ca1 and Δβ-ca5 had no striking phenotypic differences compared to wild type (WT) plants, Δβ-ca1ca5 leaves developed abnormally and exhibited large necrotic lesions even when supplied with sucrose. Leaf development of Δβ-ca1ca5 plants normalized at 9,000 ppm CO2 Leaves of Δβ-ca1ca5 mutants and WT that had matured in high CO2 had identical CO2 fixation rates and photosystem II efficiency. Fatty acids, which are formed through reactions with bicarbonate substrates, exhibited abnormal profiles in the chloroplast CA-less mutant. Emerging Δβ-ca1ca5 leaves produce reactive oxygen species in chloroplasts, perhaps due to lower nonphotochemical quenching efficiency compared to WT. Δβ-ca1ca5 seedling germination and development is negatively affected at ambient CO2 Transgenes expressing full-length β-CA1 and β-CA5 proteins complemented the Δβ-ca1ca5 mutation but inactivated (ΔZn-βCA1) and cytoplasm-localized (Δ62-βCA1) forms of β-CA1 did not reverse the growth phenotype. Nevertheless, expression of the inactivated ΔZn-βCA1 protein was able to restore the hypersensitive response to tobacco mosaic virus, while Δβ-ca1 and Δβ-ca1ca5 plants failed to show a hypersensitive response. We conclude that stromal CA plays a role in plant development, likely through providing bicarbonate for biosynthetic reactions, but stromal CA is not needed for maximal rates of photosynthesis in the C3 plant tobacco.

Optimizing photorespiration for improved crop productivity

In C3 plants, photorespiration is an energy-expensive process, including the oxygenation of ribulose-1,5-bisphosphate (RuBP) by ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) and the ensuing multi-organellar photorespiratory pathway required to recycle the toxic byproducts and recapture a portion of the fixed carbon. Photorespiration significantly impacts crop productivity through reducing yields in C3 crops by as much as 50% under severe conditions. Thus, reducing the flux through, or improving the efficiency of photorespiration has the potential of large improvements in C3 crop productivity. Here, we review an array of approaches intended to engineer photorespiration in a range of plant systems with the goal of increasing crop productivity. Approaches include optimizing flux through the native photorespiratory pathway, installing non-native alternative photorespiratory pathways, and lowering or even eliminating Rubisco-catalyzed oxygenation of RuBP to reduce substrate entrance into the photorespiratory cycle. Some proposed designs have been successful at the proof of concept level. A plant systems-engineering approach, based on new opportunities available from synthetic biology to implement in silico designs, holds promise for further progress toward delivering more productive crops to farmer's fields.