The Cytoplasmic Carbonic Anhydrases βCA2 and βCA4 Are Required for Optimal Plant Growth at Low CO₂

Abstract

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...

Loss of the Chloroplast Transit Peptide from an Ancestral C3 Carbonic Anhydrase Is Associated with C4 Evolution in the Grass Genus Neurachne

“…org/). The different splice forms exhibit varying levels of expression as well as organ-specific expression patterns (Wang et al, 2014) and, in the case of the two proteins encoded by AtbCA4, different subcellular targeting (DiMario et al, 2016). Neurachne spp.…”

“…More likely, these b-CAs, along with the largely chloroplastic CA2b (Supplemental Fig. S3), have maintained their ancestral C 3 roles of providing inorganic carbon for basic housekeeping functions such as amino acid (Raven and Newman, 1994;DiMario et al, 2016) and lipid (Hoang and Chapman, 2002) biosynthesis or regulating stomatal movement (Hu et al, 2010).…”

“…In Gynandropsis gynandra, bCA4 (GgbCA4) transcripts show preferential accumulation in the MC of this C 4 species (Kajala et al, 2012). Transcripts of the bCA4 ortholog from Arabidopsis, a comparatively close C 3 relative, are alternatively spliced, producing two CA isoforms that are targeted to different intracellular locations: AtbCA4.1 localizes to the plasma membrane, while AtbCA4.2 is a cytosolic protein (DiMario et al, 2016). It is not clear whether alternative splicing of GgbCA4 transcripts also occurs, and if it does, which of the GgbCA4 isoforms is associated with the C 4 pathway.…”

The Effects of Pharmacological Carbonic Anhydrase Suppression on Defence Responses of Potato Leaves To Phytophthora Infestans

“…Plants have three types of CAs: α-, β-and γ-type CAs, which are expressed in various tissues and cell compartments [1]. In plants, CAs are mostly widespread in the cytoplasm, chloroplast stroma and in mitochondria [2]. Carbonic anhydrases play a key role in many physiological processes of plants, including photosynthesis, respiration, CO 2 transport and electrolyte secretion among others [1,3].…”

“…Importantly, β-CAs are prevalently expressed CA genes in leaves [2] including βCA2 and βCA3 [7]. Apart from carbon dioxide ixation, it has been documented that β-CAs may contribute to plants' survival under abiotic stress conditions [8,9].…”