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...
This review presents an overview of the two ways that cyanobacteria, algae, and plants have adapted to high O2 and low CO2 concentrations in the environment. First, the process of photorespiration enables photosynthetic organisms to recycle phosphoglycolate formed by the oxygenase reaction catalyzed by ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Second, there are a number of carbon concentrating mechanisms that increase the CO2 concentration around Rubisco which increases the carboxylase reaction enhancing CO2 fixation. This review also presents possibilities for the beneficial modification of these processes with the goal of improving future crop yields.
Plants of the salt marsh grass Spartina alterniflora Loisel were collected from North Carolina and grown under controlled nutrient, temperature, and photoperiod conditions. Plants were grown at two different illumination levels; substrate salinity was varied, and leaf photosynthesis, transpiration, total chlorophyll, leaf xylem pressure, and specific leaf weight were measured. Conditions were controlled so that gaseous and liquid phase resistances to CO diffusion could be calculated. Growth at low illumination and high salinity (30 ppt) resulted in a 50% reduction in photosynthesis. The reduction in photosynthesis of plants grown at low illumination was correlated with an increase in gaseous resistance. Photosynthetic rates of plants grown at high salinity and high illumination were reduced only slightly compared to rates of plants grown, in 10 ppt and Hoagland's solution. Both high salinity and high illumination were correlated with increases in specific leaf weight. Chlorophyll data indicate that specific leaf weight differences were the result of increases in leaf thickness. It is therefore hypothesized that photosynthetic response can be strongly influenced by salinity-induced changes in leaf structure. Similarities in photosynthetic rate on an area basis at high, illumination were apparently the result, of increases in leaf thickness at high salinity. Photosynthetic rates were generally quite high, even at salinities close to open ocean water, and it is concluded that salinity rarely limits photosynthesis in S. alterniflora.
Increasing salinity led to substantially higher ratios of mesophyH surface area to leaf area (A"/A) for Phaseolus vulgaris and Gossypium hirsutum and a smaller increase for Atripkx patuls, a salt-tolerant species. The increase in internal surface for CO2 absorption did not lead to higher CO2 uptake rates, since the CO2 resistance expressed on the basis of mesophyli cell wall area (r,,u) increased even more with salinity. The differences among species in the sensitivity of photosynthesis to salinity in part reflect the different A"/A and r
In many wetland species, root aerenchyma is produced by the predictable collapse of root cortex cells, indicating a programmed cell death (PCD). The objective of this study was to characterize the cellular changes that accompany this PCD in the marsh species Sagittaria lancifolia. Structural changes in membranes and organelles were examined during development of root cortex cells to compare with previous examples of PCD. The organization of cortical microtubule (CMT) arrays in root cells from S. lancifolia was also evaluated as a possible predictor of cell lysis. Nuclear fragmentation and condensation were the earliest changes observed in cells undergoing lysis. Breakdown of the tonoplast and other organelles and disruption of the plasma membrane followed. After loss of cytoplasm, cells collapsed to form gas spaces. These results were compared to collapse of root cortical cells of Zea mays and Oryza sativa during aerenchyma development. Changes in the appearance of the cytoplasm of all three species were similar at later stages of aerenchyma development. The relative timing of disintegration of the tonoplast and middle lamella appeared to differ among the three species. Changes in the organization of CMT arrays did not appear to be a predictor of PCD in S. lancifolia. Aerenchyma production in plants involves a type of PCD that is morphologically distinct from PCD described from many animals.
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