Kranz anatomical and biochemical characterization of C4 photosynthesis in an aphyllous shrub Calligonum comosum (Polygonaceae).

Calligonum is a shrubby and aphyllous C4 eudicot lineage, employing green cylindrical shoots for carbon acquisition; however, comprehensive assessments of C4 photosynthesis within the genus are limited. One species, C. comosum, grows in Jordan, offering an opportunity for evaluating features of C4 photosynthesis in Calligonum. Light microscopy showed that C. comosum exhibits a variation of the classical salsoloid Kranz type (Calligonum variant). Transmission electron microscopy revealed traits typical of the NAD-malic enzyme (NAD-ME) subtype. Bundle sheath cells had numerous large mitochondria and mitochondria:chloroplast ratios were greater than those of mesophyll cells. Moreover, bundle sheath chloroplasts had developed grana, whereas mesophyll chloroplasts were deficient of grana. Enzyme activity and western blotting further supported NAD-ME as the principal decarboxylase. Immunolocalization of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), phosphoenolpyruvate carboxylase (PEPC) and pyruvate-phosphate dikinase (PPdK) showed strict intercellular compartmentation patterns, with Rubisco confined to bundle sheath and PEPC and PPdK to mesophyll. Labeling for PEPC was observed in both the mesophyll and hypodermis, while PPdK was restricted to the mesophyll. We suggest classifying C. comosum as a NAD-ME subtype C4 species and propose a mechanism of C4 photosynthesis, suggesting a tractable C4 model for further insights and studies on the biochemical intricacy of C4 photosynthesis. [ABSTRACT FROM AUTHOR]