Identification of new C3-C4 intermediates species in the genus Heliotropium L. (Boraginaceae).
Muhaidat, Riyadh*,1, Sage, Rowan1, Dengler, Nancy1, 1 University of Toronto, Toronto, ON, Canada
ABSTRACT- Heliotropium section orthostachys contains dozens of C3 and C4 species. Given this diversity of C3 and C4 species in a single genus, we investigated the possibility that some Heliotropium species are intermediate between C3 and C4 photosynthesis. We surveyed for the presence of C3-C4 intermediacy using anatomical studies, enzymatic assays of key enzymes in C3 and C4 photosynthesis (PEP carboxylase and C4-acid decarboxylases) and immunolocalization of Rubisco and glycine decarboxylase. Seven species were examined: H. bovei (an old world C3 that is not in the section orthostachys), H. tenellum, H. procumbens, H. convulvulaceum, H. greggii. H. texanum and H. polyphyllum. H. bovei and H. tenellum are strictly C3 plants. H. texanum and H. polyphyllum are fully-developed C4 species. The rest are C3-C4 intermediates, with weakly-developed Kranz anatomy in H. procumbens, and Kranz-like anatomy in H. greggii and H. convolvulaceum. PEPCase levels were 2-3 times higher in H. procumbens and 5-fold higher in H. greggii and H. convolvulaceum than the C3 species, but were relatively low compared to values in C4 plants. Levels of decarboxylating enzymes showed that the C4 species are of the NADP-ME subtype. Immunolabeling of Rubisco revealed it was distributed throughout leaf chlorenchyma in C3 plants and H. procumbens, totally confined to bundle sheath cells in the C4 species, and had intermediate distribution in H. convulvulaceum and H. greggii. For glycine decarboxylase, labeling occurred in all photosynthetic cells of the C3 species, but exclusively in bundle sheath cells in the leaves of C3-C4 and C4 species. We conclude that these C3-C4 heliotropes represent an early phase of C3-C4 intermediacy. Notably, the appearance of Kranz-like anatomy accompanied by differential biochemical compartmentation is consistent with the hypothesis that enlargement of the bundle sheath tissue and an increase in its metabolic activity are critical initial steps in the evolution of the C4 pathway.
Key words: Kranz anatomy, C4 photosynthesis, Heliotropium
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