RESUMO
Bienertia sinuspersici performs C(4) photosynthesis in individual chlorenchyma cells by the development of two cytoplasmic domains (peripheral and central) with dimorphic chloroplasts, an arrangement that spatially separates the fixation of atmospheric CO(2) into C(4) acids and the donation of CO(2) from C(4) acids to Rubisco in the C(3) cycle. In association with the formation of these cytoplasmic domains during leaf maturation, developmental stages were analyzed for the expression of a number of photosynthetic genes, including Rubisco small and large subunits and key enzymes of the C(4) cycle. Early in development, Rubisco subunits and Gly decarboxylase and Ser hydroxymethyltransferase of the glycolate pathway accumulated more rapidly than enzymes associated with the C(4) cycle. The levels of pyruvate,Pi dikinase and phosphoenolpyruvate carboxylase were especially low until spatial cytoplasmic domains developed and leaves reached maturity, indicating a developmental transition toward C(4) photosynthesis. In most cases, there was a correlation between the accumulation of mRNA transcripts and the respective peptides, indicating at least partial control of the development of photosynthesis at the transcriptional level. During growth under moderate light, when branches containing mature leaves were enclosed in darkness for 1 month, spatial domains were maintained and there was high retention of a number of photosynthetic peptides, including Rubisco subunits and pyruvate,Pi dikinase, despite a reduction in transcript levels. When plants were transferred from moderate to low light conditions for 1 month, there was a striking shift of the central cytoplasmic compartment toward the periphery of chlorenchyma cells; the mature leaves showed strong acclimation with a shade-type photosynthetic response to light while retaining C(4) features indicative of low photorespiration. These results indicate a progressive development of C(4) photosynthesis with differences in the control mechanisms for the expression of photosynthetic genes and peptide synthesis during leaf maturation and in response to light conditions.
Assuntos
Chenopodiaceae/fisiologia , Regulação da Expressão Gênica no Desenvolvimento , Regulação da Expressão Gênica de Plantas , Luz , Fotossíntese , Folhas de Planta/metabolismo , Dióxido de Carbono/metabolismo , Isótopos de Carbono/metabolismo , Chenopodiaceae/citologia , Clorofila/metabolismo , Peptídeos/metabolismo , Folhas de Planta/citologia , Folhas de Planta/crescimento & desenvolvimento , Proteínas de Plantas/metabolismo , Água/metabolismoRESUMO
Spatial and temporal regulation of phosphoenolpyruvate carboxylase (PEPC) is critical to the function of C(4) photosynthesis. The photosynthetic isoform of PEPC in the cytosol of mesophyll cells in Kranz-type C(4) photosynthesis has distinctive kinetic and regulatory properties. Some species in the Chenopodiaceae family perform C(4) photosynthesis without Kranz anatomy by spatial separation of initial fixation of atmospheric CO(2) via PEPC from C(4) acid decarboxylation and CO(2) donation to Rubisco within individual chlorenchyma cells. We studied molecular and functional features of PEPC in two single-cell functioning C(4) species (Bienertia sinuspersici, Suaeda aralocaspica) as compared to Kranz type (Haloxylon persicum, Salsola richteri, Suaeda eltonica) and C(3) (Suaeda linifolia) chenopods. It was found that PEPC from both types of C(4) chenopods displays higher specific activity than that of the C(3) species and shows kinetic and regulatory characteristics similar to those of C(4) species in other families in that they are subject to light/dark regulation by phosphorylation and display differential malate sensitivity. Also, the deduced amino acid sequence from leaf cDNA indicates that the single-cell functioning C(4) species possesses a Kranz-type C(4) isoform with a Ser in the amino terminal. A phylogeny of PEPC shows that isoforms in the two single-cell functioning C(4) species are in a clade with the C(3) and Kranz C(4) Suaeda spp. with high sequence homology. Overall, this study indicates that B. sinuspersici and S. aralocaspica have a C(4)-type PEPC similar to that in Kranz C(4) plants, which likely is required for effective function of C(4) photosynthesis.