[Show abstract][Hide abstract] ABSTRACT: C4 photosynthesis is a complex phenotype that allows more efficient carbon capture than the ancestral C3 pathway. In leaves of C4 species, hundreds of transcripts increase in abundance compared with C3 relatives and become restricted to mesophyll (M) or bundle sheath (BS) cells. However, no mechanism has been reported that regulates the compartmentation of multiple enzymes in M or BS cells. We examined mechanisms regulating CARBONIC ANHYDRASE 4 (CA4) in C4 Gynandropsis gynandra. Increased abundance is directed by both the promoter region and introns of the G. gynandra gene. A 9-nucleotide motif located in the 5' untranslated region (UTR) is required for preferential accumulation of GUS in M cells. This element is present and functional in three additional 5' UTRs and six 3' UTRs where it determines accumulation of two isoforms of CA and pyruvate,orthophosphate dikinase in M cells. Although the GgCA4 5' UTR is sufficient to direct GUS accumulation in M cells, transcripts encoding GUS are abundant in both M and BS. Mutating the GgCA4 5' UTR abolishes enrichment of protein in M cells without affecting transcript abundance. The work identifies a mechanism that directs cell-preferential accumulation of multiple enzymes required for C4 photosynthesis.
[Show abstract][Hide abstract] ABSTRACT: C₄ photosynthesis allows increased photosynthetic efficiency because carbon dioxide (CO₂) is concentrated around the key enzyme RuBisCO. Leaves of C₄ plants exhibit modified biochemistry, cell biology, and leaf development, but despite this complexity, C₄ photosynthesis has evolved independently in at least 45 lineages of plants. We found that two independent lineages of C₄ plant, whose last common ancestor predates the divergence of monocotyledons and dicotyledons about 180 million years ago, show conserved mechanisms controlling the expression of genes important for release of CO(2) around RuBisCO in bundle sheath (BS) cells. Orthologous genes from monocotyledonous and dicotyledonous C₃ species also contained conserved regulatory elements that conferred BS specificity when placed into C₄ species. We conclude that these conserved functional genetic elements likely facilitated the repeated evolution of C₄ photosynthesis.
[Show abstract][Hide abstract] ABSTRACT: Cells associated with veins of petioles of C3 tobacco possess high activities of the decarboxylase enzymes required in C4 photosynthesis. It is not clear whether this is the case in other C3 species, nor whether these enzymes provide precursors for specific biosynthetic pathways. Here, we investigate the activity of C4 acid decarboxylases in the mid-vein of Arabidopsis, identify regulatory regions sufficient for this activity, and determine the impact of removing individual isoforms of each protein on mid-vein metabolite profiles. This showed that radiolabelled malate and bicarbonate fed to the xylem stream were incorporated into soluble and insoluble material in the mid-vein of Arabidopsis leaves. Compared with the leaf lamina, mid-veins possessed high activities of NADP-dependent malic enzyme (NADP-ME), NAD-dependent malic enzyme (NAD-ME) and phosphoenolpyruvate carboxykinase (PEPCK). Transcripts derived from both NAD-ME, one PCK and two of the four NADP-ME genes were detectable in these veinal cells. The promoters of each decarboxylase gene were sufficient for expression in mid-veins. Analysis of insertional mutants revealed that cytosolic NADP-ME2 is responsible for 80% of NADP-ME activity in mid-veins. Removing individual decarboxylases affected the abundance of amino acids derived from pyruvate and phosphoenolpyruvate. Reducing cytosolic NADP-ME activity preferentially affected the sugar content, whereas abolishing NAD-ME affected both the amino acid and the glucosamine content of mid-veins.
Full-text · Article · Nov 2009 · The Plant Journal
[Show abstract][Hide abstract] ABSTRACT: Cells associated with veins of C(3) species often contain significant amounts of chlorophyll, and radiotracer analysis shows that carbon present in the transpiration stream may be used for photosynthesis in these cells. It is not clear whether CO2 is also supplied to these cells close to veins via stomata, nor whether this veinal photosynthesis supplies carbon skeletons to particular metabolic pathways. In addition, it has not been possible to determine whether photosynthesis in cells close to veins of C(3) plants is quantitatively important for growth or fitness. To investigate the role of photosynthesis in cells in and around the veins of C(3) plants, we have trans-activated a hairpin construct to the chlorophyll synthase gene (CS) using an Arabidopsis thaliana enhancer trap line specific to veins. CS is responsible for addition of the phytol chain to the tetrapyrolle head group of chlorophyll, and, as a result of cell-specific trans-activation of the hairpin to CS, chlorophyll accumulation is reduced around veins. We use these plants to show that, under steady-state conditions, the extent to which CO2 is supplied to cells close to veins via stomata is limited. Fixation by minor veins of CO2 supplied to the xylem stream and the amount of specific metabolites associated with carbohydrate metabolism and the shikimate pathway were all reduced. In addition, an abundance of transcripts encoding components of pathways that generate phosphoenolpyruvate were altered. Leaf senescence, growth rate and seed size were all reduced in the lines with lower photosynthetic ability in veins and in cells close to veins.
Full-text · Article · Apr 2009 · The Plant Journal
[Show abstract][Hide abstract] ABSTRACT: C(4) photosynthesis involves alterations to leaf development, cell biology and biochemistry. Different lineages of C(4) plants use varying mechanisms to generate the C(4) pathway. Although the biochemistry of C(4) photosynthesis was described around 20 years ago, the phylogenetic distance between Arabidopsis and the traditional C(4) models has not facilitated the transfer of knowledge from Arabidopsis research to understanding C(4) systems. We show that Cleome, a genus closely related to Arabidopsis, contains species spanning a developmental progression from C(3) to C(4) photosynthesis. The majority of species we assessed are C(3) plants but have increased venation in leaves. Three C(3) species have both increased venation and enlarged bundle sheath cells, and there is also a tendency to accumulate proteins and transcripts needed for C(4) photosynthesis. Cleome gynandra shows all the characteristics needed for efficient C(4) photosynthesis, including alterations to leaf biochemistry, cell biology and development, and belongs to the NAD-dependent malic enzyme subtype. Combined with its phylogenetic proximity to Arabidopsis, the developmental progression from C(3) to C(4) photosynthesis within the genus provides a potentially excellent new model to increase our understanding of C(4) photosynthesis, and provide insights into its evolution.
Full-text · Article · Oct 2007 · The Plant Journal
[Show abstract][Hide abstract] ABSTRACT: The genus Cuscuta (dodder) is composed of parasitic plants, some species of which appear to be losing the ability to photosynthesize. A molecular
phylogeny was constructed using 15 species of Cuscuta in order to assess whether changes in photosynthetic ability and alterations in structure of the plastid genome relate to
phylogenetic position within the genus. The molecular phylogeny provides evidence for four major clades within Cuscuta. Although DNA blot analysis showed that Cuscuta species have smaller plastid genomes than tobacco, and that plastome size varied significantly even within one Cuscuta clade, dot blot analysis indicated that the dodders possess homologous sequence to 101 genes from the tobacco plastome. Evidence
is provided for significant rates of DNA transfer from plastid to nucleus in Cuscuta. Size and structure of Cuscuta plastid genomes, as well as photosynthetic ability, appear to vary independently of position within the phylogeny, thus supporting
the hypothesis that within Cuscuta photosynthetic ability and organization of the plastid genome are changing in an unco-ordinated manner.
Full-text · Article · Oct 2005 · Journal of Experimental Botany