Carbonic anhydrases (CAs) are zinc metalloenzymes that interconvert CO 2 and HCO 3 2 . In plants, both a-and b-type CAs are present. We hypothesize that cytoplasmic bCAs are required to modulate inorganic carbon forms needed in leaf cells for carbonrequiring reactions such as photosynthesis and amino acid biosynthesis. In this report, we present evidence that bCA2 and bCA4 are the two most abundant cytoplasmic CAs in Arabidopsis (Arabidopsis thaliana) leaves. Previously, bCA4 was reported to be localized to the plasma membrane, but here, we show that two forms of bCA4 are expressed in a tissue-specific manner and that the two proteins encoded by bCA4 localize to two different regions of the cell. Comparing transfer DNA knockout lines with wild-type plants, there was no reduction in the growth rates of the single mutants, bca2 and bca4. However, the growth rate of the double mutant, bca2bca4, was reduced significantly when grown at 200 mL L 21 CO 2 . The reduction in growth of the double mutant was not linked to a reduction in photosynthetic rate. The amino acid content of leaves from the double mutant showed marked reduction in aspartate when compared with the wild type and the single mutants. This suggests the cytoplasmic CAs play an important but not previously appreciated role in amino acid biosynthesis.Carbonic anhydrases (CAs) are zinc metalloenzymes that catalyze the interconversion of CO 2 and HCO 3 2 . Flowering plants possess members of the aCA, bCA, and gCA families. While all three CA families contain zinc, they clearly have evolved independently (Hewett-Emmett and Tashian, 1996). Most aCAs are monomeric, although there are notable exceptions (Whittington et al., 2001;Hilvo et al., 2008;Suzuki et al., 2010Suzuki et al., , 2011Cuesta-Seijo et al., 2011). The aCA active site contains a single zinc molecule coordinated by three His residues and a water molecule (Liljas et al., 1972). bCAs also contain a zinc active site, although the coordinating molecules are two Cys residues, a His, and a water molecule (Bracey et al., 1994). The active unit of the bCA is a dimer where the active site is located at the interface of the two monomers (Kimber and Pai, 2000). In contrast, gCAs are trimers that have their active site zinc ion situated at the interface of two subunits coordinated by His residues from both subunits (Kisker et al., 1996;Iverson et al., 2000).In Arabidopsis (Arabidopsis thaliana), there are three gCA proteins and two g-like proteins that interact to form an extra structure of complex I of the mitochondrial electron transport chain (Perales et al., 2004;Sunderhaus et al., 2006). Although not active in vitro, gCA has been shown to bind inorganic carbon (Martin et al., 2009), affect complex I levels, plant growth, and gas-exchange rates when deleted (Perales et al., 2004;Soto et al., 2015), and cause plant sterility when ectopically overexpressed ). Arabidopsis has eight aCA genes, but only aCA1, aCA2, and aCA3 appear to be expressed in leaf tissue. aCA1 has been reported to be localized to the c...
Species have different strategies for loading sugars into the phloem, which vary in the route that sugars take to enter the phloem and the energetics of sugar accumulation. Species with passive phloem loading are hypothesized to have less flexibility in response to changes in some environmental conditions because sucrose export from mesophyll cells is dependent on fixed anatomical plasmodesmatal connections. Passive phloem loaders also have high mesophyll sugar content, and may be less likely to exhibit sugar-mediated down-regulation of photosynthetic capacity at elevated CO concentrations. To date, the effect of phloem loading strategy on the response of plant carbon metabolism to rising atmospheric CO concentrations is unclear, despite the widespread impacts of rising CO on plants. Over three field seasons, five species with apoplastic loading, passive loading, or polymer-trapping were grown at ambient and elevated CO concentration in free air concentration enrichment plots. Light-saturated rate of photosynthesis, photosynthetic capacity, leaf carbohydrate content, and anatomy were measured and compared among the species. All five species showed significant stimulation in midday photosynthetic CO uptake by elevated CO even though the two passive loading species showed significant down-regulation of maximum Rubisco carboxylation capacity at elevated CO. There was a trend toward greater starch accumulation at elevated CO in all species, and was most pronounced in passive loaders. From this study, we cannot conclude that phloem loading strategy is a key determinant of plant response to elevated CO, but compelling differences in response counter to our hypothesis were observed. A phylogenetically controlled experiment with more species may be needed to fully test the hypothesis.
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