Comparative Proteomics of Chloroplast Envelopes from C3 and C4 Plants Reveals Specific Adaptations of the Plastid Envelope to C4 Photosynthesis and Candidate Proteins Required for Maintaining C4 Metabolite Fluxes  

Abstract

C 4 plants have up to 10-fold higher apparent CO 2 assimilation rates than the most productive C 3 plants. This requires higher fluxes of metabolic intermediates across the chloroplast envelope membranes of C 4 plants in comparison with those of C 3 plants. In particular, the fluxes of metabolites involved in the biochemical inorganic carbon pump of C 4 plants, such as malate, pyruvate, oxaloacetate, and phosphoenolpyruvate, must be considerably higher in C 4 plants because they exceed the apparent rate of photosynthetic CO 2 assimilation, whereas they represent relatively minor fluxes in C 3 plants. While the enzymatic steps involved in the C 4 biochemical inorganic carbon pump have been studied in much detail, little is known about the metabolite transporters in the envelope membranes of C 4 chloroplasts. In this study, we used comparative proteomics of chloroplast envelope membranes from the C 3 plant pea (Pisum sativum) and mesophyll cell chloroplast envelopes from the C 4 plant maize (Zea mays) to analyze the adaptation of the mesophyll cell chloroplast envelope proteome to the requirements of C 4 photosynthesis. We show that C 3 -and C 4 -type chloroplasts have qualitatively similar but quantitatively very different chloroplast envelope membrane proteomes. In particular, translocators involved in the transport of triosephosphate and phosphoenolpyruvate as well as two outer envelope porins are much more abundant in C 4 plants. Several putative transport proteins have been identified that are highly abundant in C 4 plants but relatively minor in C 3 envelopes. These represent prime candidates for the transport of C 4 photosynthetic intermediates, such as pyruvate, oxaloacetate, and malate.

Elements Required for an Efficient NADP-Malic Enzyme Type C4 Photosynthesis

“…7). Indeed proteomic studies indicate that TPTs were highly abundant in C4 plants (Bräutigam et al, 2008;Majeran et al, 2010), which supports the prediction here that TPTs in both BSCs and MCs are critical for effective C4 photosynthesis. It follows that when the goal of converting a C3 plant to a C4 is to achieve increased A, then these high levels of TPT in both plastid types will also be key.…”

Proteomic Analysis of Chloroplast-to-Chromoplast Transition in Tomato Reveals Metabolic Shifts Coupled with Disrupted Thylakoid Biogenesis Machinery and Elevated Energy-Production Components

“…The spectral-count-based label free has been reported as allowing quantification of protein abundance with similar efficiency to isotope labeling (Zhu et al, 2009). This methodology had been applied in a number of other studies related to the plastid proteome (Bräutigam et al, 2008;Ferro et al, 2010;Demartini et al, 2011).…”

Understanding metabolite transport and metabolism in C4 plants through RNA-seq

“…Instead of the conjunct action of BASS and NHD, the Andropogonae have an alternative transport protein in mesophyll chloroplasts which, to date, has not been characterized at the molecular level ( Figure 1c, Table 1). Candidates for the chloroplast pyruvate importer in maize were suggested based on proteomic comparison of maize and pea chloroplast envelopes: mesophyll envelope proteins (Mep) Mep1-4 (Table 2) [16]. A pyruvate transporter is expected to be highly abundant in maize mesophyll cells.…”

“…For example, abundant expression of TPT [5 ,16] ( Table 1) couples the parts of the Calvin Benson Bassham cycle localized in the mesophyll and bundle sheath [24 ,48]. The shuttle may also transfer ATP and reducing equivalents in the form of triosephosphates [16]. C 4 metabolism creates large pools of the transfer metabolites which need to be protected from the underlying metabolism, for example by reducing entry in the TCA cycle via pyruvate dehydrogenase complex kinase [8].…”

“…Many enzymes involved in the C 4 cycle reside in organelles rather than in the cytosol [4]. Their localization necessitates the presence of transport proteins which connect the reactions of the C 4 cycle to the cytosol [16]. In turn, the cytosols of the PEPC containing mesophyll tissue and the RubisCO containing bundle sheath tissue share a symplastic connection [17][18][19][20].…”