A proto-Kranz rice line has thus been generated.Ĭonstitutive Expression of ZmG2 or ZmGLK1 in Rice Induces Greening of Callus and Roots Here we validate that hypothesis and further demonstrate that enhanced chloroplast development leads to enhanced mitochondrial biogenesis, with both types of organelle accumulating metabolic enzymes. On the basis of these findings, we postulated that sustained chloroplast development in the vascular sheath cells of rice might be achieved through the introduction of one of the two heterologous maize GLK genes (either ZmG2 or ZmGLK1 ). However, enhanced chloroplast development in overexpression lines is not sustained beyond the seedling stage, suggesting that ectopic OsGLK1 activity is suppressed by one of the post-transcriptional mechanisms that balance GLK-induced chloroplast development with leaf senescence. GOLDEN2-like (GLK) transcription factors regulate chloroplast development in all land plant species examined, and ectopic expression of OsGLK1 activates chloroplast biogenesis in rice vascular sheath cells. As such, a rational first step toward engineering C 4 rice is to induce proto-Kranz by activating chloroplast and mitochondrial biogenesis in vascular sheath cells. By contrast, there are no proto-Kranz or C 4 species in the sister clade, to which rice belongs. In the grasses, proto-Kranz species exist in the “PACMAD” clade, in which 24 independent origins of C 4 have been identified. In both monocots and eudicots, proto-Kranz anatomy is characterized by increased organelle volume in the bundle sheath and/or mestome sheath cells around the leaf vasculature (collectively referred to as vascular sheath cells), with chloroplasts accumulating the photosynthetic enzyme RuBisCo and mitochondria accumulating the photorespiratory enzyme glycine decarboxylase. Morphological analyses of extant species suggest that Kranz evolved in a stepwise fashion, with “proto-Kranz”- and “C 2”- type anatomies representing intermediate steps along the C 3-to-C 4 trajectory. One of the major challenges in this regard is the need to introduce Kranz anatomy into the rice leaf. A critical step in the evolution of C 4 was thus the functionalization of bundle sheath and/or mestome sheath cells for photosynthesis (reviewed in ).Ī long-term project is underway to introduce the C 4 pathway into the C 3 crop rice, with predictions of up to a 50% yield increase if successful ( ). In most C 4 species, the bundle sheath cells surrounding the vein are the site of refixation, but in some grasses an inner layer of mestome sheath cells with thick suberized cell walls play this role (either instead of or in addition to the bundle sheath cells).

This cellular arrangement, which is referred to as Kranz anatomy, facilitates initial fixation of carbon dioxide in the outer mesophyll cells followed by decarboxylation and refixation by ribulose bisphosphate carboxylase/oxygenase (RuBisCo) in the CO 2-enriched environment of the inner sheath cells (reviewed in ). In most C 4 plants, photosynthetic reactions are compartmentalized between two cell types that are arranged in concentric wreaths around closely spaced veins. The C 4 photosynthetic pathway evolved over 60 times independently in a diverse range of flowering plant species, with trajectories from C 3 to C 4 apparent in a number of lineages. This suite of traits reflects that seen in “proto-Kranz” species, and, as such, a key step toward engineering C 4 rice has been achieved. Increased organelle volume was accompanied by the accumulation of photosynthetic enzymes and by increased intercellular connections. Here we induced chloroplast and mitochondrial development in rice vascular sheath cells through constitutive expression of maize GOLDEN2-LIKE genes. The transition from C 3 to so-called “proto-Kranz” anatomy requires an increase in organelle volume in sheath cells surrounding leaf veins. One approach toward engineering C 4 rice is to recapitulate this trajectory, one of the first steps of which was a change in leaf anatomy. This is an ambitious endeavor however, the C 4 pathway evolved from C 3 on multiple independent occasions over the last 30 million years, and steps along the trajectory are evident in extant species. Because C 4 plants are higher yielding than C 3 plants, efforts are underway to introduce the C 4 pathway into the C 3 crop rice. The C 4 photosynthetic pathway accounts for ∼25% of primary productivity on the planet despite being used by only 3% of species.

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