Differentiation of C4 photosynthesis along a leaf developmental gradient in two Cleome species having different forms of Kranz anatomy.

In family Cleomaceae there are NAD-malic enzyme-type C4 species having different forms of leaf anatomy. Leaves of Cleome angustifolia have Glossocardioid-type anatomy with a single complex Kranz unit which surrounds all the veins, while C. gynandra has Atriplicoid anatomy with multiple Kranz units, each surrounding an individual vein. Biochemical and ultrastructural differentiation of mesophyll (M) and bundle sheath (BS) cells were studied along a developmental gradient, from the leaf base (youngest) to the tip (mature). Initially, there is cell-specific expression of certain photosynthetic enzymes, which subsequently increase along with structural differentiation. At the base of the leaf, following division of ground tissue to form M and BS cells which are structurally similar, there is selective localization of Rubisco and glycine decarboxylase to BS cells. Thus, a biochemical C3 default stage, with Rubisco expression in both cell types, does not occur. Additionally, phosphoenolpyruvate carboxylase (PEPC) is selectively expressed in M cells near the base. Surprisingly, in both species, an additional layer of spongy M cells on the abaxial side of the leaf has the same differentiation with PEPC, even though it is not in contact with BS cells. During development along the longitudinal gradient there is structural differentiation of the cells, chloroplasts, and mitochondria, resulting in complete formation of Kranz anatomy. In both species, development of the C4 system occurs similarly, irrespective of having very different types of Kranz anatomy, different ontogenetic origins of BS and M, and independent evolutionary origins of C4 photosynthesis.