Abstract
There is currently significant interest in engineering the two-celled C4 photosynthesis pathway into crops such as rice in order to increase yield. This will require alterations to the biochemistry of photosynthesis in both mesophyll (M) and bundle-sheath (BS) cells, but also alterations to leaf anatomy. For example, the BS of C4 species is enlarged compared with that in C3 species. Because cell and nucleus size are often correlated, this study investigated whether nuclear endoreduplication is associated with increased differentiation and expansion of BS cells. Nuclei in the BS of C4 Cleome gynandra were tagged with green fluorescent protein. Confocal laser-scanning microscopy and flow cytometry of isolated nuclei were used to quantify size and DNA content in BS cells. The results showed a significant endoreduplication in BS cells of C. gynandra but not in additional C4 lineages from both the monocotyledonous and dicotyledenous plants. Furthermore, in the C3 species Arabidopsis thaliana, BS cells undergo endoreduplication. Due to this significant endoreduplication in the small BS cells of C3 A. thaliana, it was concluded that endoreduplication of BS nuclei in C4 plants is not linked to expansion and differentiation of BS cells, and therefore that alternative strategies to increase this compartment need to be sought in order to engineer C4 traits into C3 crops such as rice.
Highlights
From about 30 million years ago, plants using C4 photosynthesis evolved, and they populate more than 60 independent lineages of angiosperms (Christin et al, 2013)
In the C3 species Arabidopsis thaliana, BS cells undergo endoreduplication. Due to this significant endoreduplication in the small BS cells of C3 A. thaliana, it was concluded that endoreduplication of BS nuclei in C4 plants is not linked to expansion and differentiation of BS cells, and that alternative strategies to increase this compartment need to be sought in order to engineer C4 traits into C3 crops such as rice
The C4 pathway involves the reactions of photosynthesis being divided between two compartments in the leaf, and this leads to CO2 being concentrated in bundle-sheath (BS) chloroplasts in full C4 plants that contain the primary carboxylase Ribulose Bisphosphate Carboxylase Oxygenase (RuBisCO)
Summary
From about 30 million years ago, plants using C4 photosynthesis evolved, and they populate more than 60 independent lineages of angiosperms (Christin et al, 2013). The C4 pathway involves the reactions of photosynthesis being divided between two compartments in the leaf, and this leads to CO2 being concentrated in bundle-sheath (BS) chloroplasts in full C4 plants that contain the primary carboxylase Ribulose Bisphosphate Carboxylase Oxygenase (RuBisCO). The subsequent reduction or transamination of oxaloacetate to organic four-carbon acids such as malate and aspartate generate high concentrations of these metabolites, and this drives their diffusion into the second compartment. In species that use the classic two-celled pathway, the high flux of metabolites between mesophyll (M) and BS cells is dependent on close contacts between these cell types, and typically this results in a stylized arrangement of each vein being surrounded by a ring of BS cells, which in turn is inside a ring of M cells, resulting in socalled Kranz anatomy (Hatch and Slack, 1966). The BS cells of C4 plants contain many chloroplasts to increase the Abbreviations: BS, bundle sheath; CLSM, confocal laser-scanning microscopy; DAPI, 4′-6-diamidino-phenylindole; GFP, green fluorescent protein; M, mesophyll
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