| From a photosynthetic point of view, yield of some crops may be further improved by increasing the photosynthetic capacity of the source leaves and/or by increasing partitioning of photoassimilate to organs of economic importance. C4 plants are able to overcome photorespiration, and thus photosynthesize more efficiently, through an additional photosynthetic pathway (C4) and specialized leaf anatomy (Kranz) that work together as a "CO2 pump" to supply Rubisco with enriched CO2 edging out O2 as a competitive substrate. This photosynthetic mechanism endows C4 plants a selective advantage over C3 plants especially in warmer climates and during water deficits: higher photosynthetic capacity, and higher water and nutrient use efficiency. The C4 syndrome is characterized by high activities of C4 enzymes and Kranz anatomy (mesophyll and bundle-sheath cells). Traditionally, Kranz anatomy, which allows for the spatial separation of the C4 biochemical steps, was believed to be required for the C4 CO2 concentrating mechanism in terrestrial plants. However, it is known that C4-like mechanisms can be induced, without the presence of Kranz anatomy, in two submersed aquatic species ( Hydrilla verticillata and Egeria densa). Most recently, the terrestrial C4 plants Bienertia cycloptera, B. sinuspersici, and Suaeda aralocaspica (Chenopodiaceae) from Central Asia have been found to lack Kranz anatomy and function with specialized intracellular compartmentation. Thus, by introducing the necessary genes of the C4 pathway to C3 plants, it may be possible to engineer a C 4-like mechanism and improve photosynthesis without a requirement for Kranz anatomy. In this study, C3 rice was transformed with two genes encoding C4 photosynthetic enzymes (phosphoenolpyruvate carboxylase and pyruvate orthophosphate dikinase) and the photosynthetic, biochemical, and anatomical changes were documented.; Additionally, research with the genus Eleocharis shows that the C4 syndrome may be more plastic than formerly believed. Within the genus, species were shown to possess the full range from C 3 to intermediate to C4. In a species dependant manner, plants changed photosynthetic types whether grown terrestrially or submerged with concomitant changes in the C4 enzyme activities, carbon isotope composition and leaf anatomy. |
