Kranz anatomy is not essential for terrestrial C4 plant photosynthesis

An important adaptation to CO2-limited photosynthesis in cyanobacteria, algae and some plants was development of CO2-concentrating mechanisms (CCM)(1). Evolution of a CCM occurred many times in flowering plants, beginning at least 15-20 million years ago, in response to atmospheric CO2 reduction, climate change, geological trends, and evolutionary diversification of species(2). In plants, this is achieved through a biochemical inorganic carbon pump called C-4 photosynthesis, discovered 35 years ago(3). C4 photosynthesis is advantageous when limitations on carbon acquisition are imposed by high temperature, drought and saline conditions. It has been thought that a specialized leaf anatomy, composed of two, distinctive photosynthetic cell types (Kranz anatomy), is required for C-4 photosynthesis(4). We provide evidence that C-4 photosynthesis can function within a single photosynthetic cell in terrestrial plants. Borszczowia aralocaspica (Chenopodiaceae) has the photosynthetic features of C-4 plants, yet lacks Kranz anatomy. This species accomplishes C-4 photosynthesis through spatial compartmentation of photosynthetic enzymes, and by separation of two types of chloroplasts and other organelles in distinct positions within the chlorenchyma cell cytoplasm.