Carbonic Anhydrase and Its Influence on Carbon Isotope Discrimination during C4 Photosynthesis. Insights from Antisense RNA in Flaveria bidentis

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Abstract
In C4 plants, carbonic anhydrase (CA) facilitates both the chemical and isotopic equilibration of atmospheric CO2 and bicarbonate (HCO3−) in the mesophyll cytoplasm. The CA-catalyzed reaction is essential for C4 photosynthesis, and the model of carbon isotope discrimination (Δ13C) in C4 plants predicts that changes in CA activity will influence Δ13C. However, experimentally, the influence of CA on Δ13C has not been demonstrated in C4 plants. Here, we compared measurements of Δ13C during C4 photosynthesis in Flaveria bidentis wild-type plants with F. bidentis plants with reduced levels of CA due to the expression of antisense constructs targeted to a putative mesophyll cytosolic CA. Plants with reduced CA activity had greater Δ13C, which was also evident in the leaf dry matter carbon isotope composition (δ13C). Contrary to the isotope measurements, photosynthetic rates were not affected until CA activity was less than 20% of wild type. Measurements of Δ13C, δ13C of leaf dry matter, and rates of net CO2 assimilation were all dramatically altered when CA activity was less than 5% of wild type. CA activity in wild-type F. bidentis is sufficient to maintain net CO2 assimilation; however, reducing leaf CA activity has a relatively large influence on Δ13C, often without changes in net CO2 assimilation. Our data indicate that the extent of CA activity in C4 leaves needs to be taken into account when using Δ13C and/or δ13C to model the response of C4 photosynthesis to changing environmental conditions.