Functional characterization of phosphoenolpyruvate carboxykinase-type C4 leaf anatomy: immuno-, cytochemical and ultrastructural analyses.
OBJECTIVE
Species having C4 photosynthesis belonging to the phosphoenolpyruvate carboxykinase (PEP-CK) subtype, which are found only in family Poaceae, have the most complex biochemistry among the three C4 subtypes. In this study, biochemical (western blots and immunolocalization of some key photosynthetic enzymes) and structural analyses were made on several species to further understand the PEP-CK system. This included PEP-CK-type C4 species Urochloa texana (subfamily Panicoideae), Spartina alterniflora and S. anglica (subfamily Chloridoideae), and an NADP-ME-type C4 species, Echinochloa frumentacea, which has substantial levels of PEP-CK.


RESULTS
Urochloa texana has typical Kranz anatomy with granal chloroplasts scattered around the cytoplasm in bundle sheath (BS) cells, while the Spartina spp. have BS forming long adaxial extensions above the vascular tissue and with chloroplasts in a strictly centrifugal position. Despite some structural and size differences, in all three PEP-CK species the chloroplasts in mesophyll and BS cells have a similar granal index (% appressed thylakoids). Immunolocalization studies show PEP-CK (which catalyses ATP-dependent decarboxylation) is located in the cytosol, and NAD-ME in the mitochondria, in BS cells, and in the BS extensions of Spartina. In the NADP-ME species E. frumentacea, PEP-CK is also located in the cytosol of BS cells, NAD-ME is very low, and the source of ATP to support PEP-CK is not established.


CONCLUSIONS
Representative PEP-CK species from two subfamilies of polyphyletic origin have very similar biochemistry, compartmentation and chloroplast grana structure. Based on the results with PEP-CK species, schemes are presented with mesophyll and BS chloroplasts providing equivalent reductive power which show bioenergetics of carbon assimilation involving C4 cycles (PEP-CK and NAD-ME, the latter functioning to generate ATP to support the PEP-CK reaction), and the consequences of any photorespiration.
Voznesenskaya
Elena V
EV
Laboratory of Anatomy and Morphology, V. L. Komarov Botanical Institute of Russian Academy of Sciences, Prof. Popov Street 2, 197376, St. Petersburg, Russia.
Franceschi
Vincent R
VR
Chuong
Simon D X
SD
Edwards
Gerald E
GE
D015153:Blotting, Western; D007150:Immunohistochemistry; D008291:Malate Dehydrogenase / Q000032:analysis / Q000378:metabolism; D046529:Microscopy, Electron, Transmission; D008954:Models, Biological; D015388:Organelles / Q000378:metabolism / Q000648:ultrastructure; D019764:Phosphoenolpyruvate Carboxykinase (ATP) / Q000032:analysis* / Q000378:metabolism; D010788:Photosynthesis / Q000502:physiology; D018515:Plant Leaves / Q000737:chemistry / Q000166:cytology / Q000201:enzymology*; D006109:Poaceae / Q000166:cytology / Q000201:enzymology* / Q000648:ultrastructure
D008291:Malate Dehydrogenase; C046446:malate dehydrogenase-(oxaloacetate-decarboxylating) (NAD+); D019764:Phosphoenolpyruvate Carboxykinase (ATP)