Minisymposium 24: Metabolic Engineering

Abs # 38004: Introduction of maize C₄ photosynthesis genes in rice: expression, regulation and the effects on photosynthetic physiology and grain yield

Presenter:	Murphy, L R, murphyl@wsu.edu
Authors	Murphy, L R (A)   Jiao, D M (B)   Zhang, Y  (B)   Kuang, T  (C)   Cho, D  (D)   Yang, J  (E)   Franceschi, V R (A)   Ku, M SB (A)
Affiliations:	(A): Washington State University, Pullman, WA, U.S.A. (B): Jiangsu Academy of Agricultural Sciences, Nanjing, China (C): Chinese Academy of Sciences, Beijing, China (D): Kangwon University, Kangwon, Korea (E): Yangzhou University, Yangzhou, China

C₄ plants are able to overcome photorespiration and photosynthesize more efficiently through the C₄ pathway and Kranz leaf anatomy that work together as a “CO₂ pump” to supply Rubisco (ribulose 1,5-bisphosphate carboxylase/oxygenase) with enriched CO₂, thus edging out competitive O₂. This mechanism gives C₄ plants an advantage over C₃ plants that fix CO₂ via the conventional C₃ pathway: high photosynthetic capacity, faster growth rate and high water and nutrient use efficiency. In addition, several enzymes involved in the C₄ pathway are known to play key roles in plant defense mechanisms against adverse conditions. It is believed that introduction of some of the C₄ traits to C₃ plants may lead to a higher photosynthetic capacity and an enhanced tolerance to stress. The C₃ plant rice was successfully transformed with genes encoding key enzymes of the C₄ pathway: phosphoenolpyruvate carboxylase (PC) and pyruvate, Pi dikinase (PK) from the C₄ plant maize. The maize genes were independently introduced into rice and expressed in the proper cellular compartments (cytosol and chloroplast, respectively) at high levels. Conventional hybridization was used to integrate these genes into the same plants (CK). Direct fixation of atmospheric CO₂ via the C₄ pathway remains low in these plants. However, relative to the wildtype (WT) PC, PK and CK transgenic plants show higher photosynthetic rates, higher tolerance to photo-oxidation, and up to a 35% increase in grain yield in controlled environment and field experiments. The higher photosynthetic capacity may be attributed to a combination of increased stomatal conductance and enhanced tolerance to photo-oxidation. Under photo-oxidative conditions, transgenic rice plants expressing high levels of maize PC show the least damage to PSII, as monitored by chlorophyll a fluorescence. Both PC and carbonic anhydrase increased by more than 2-3 fold within 4 hr of treatment, as did scavenging enzymes (e.g. superoxide dismutase, peroxidase). These results suggest that PC plays a key role in defense mechanism against photo-oxidation. The increased grain yields in the transgenic rice plants have been associated mainly with an increased panicle number per plant with comparable or slightly higher fertility than WT. Taken together, our studies demonstrate that photosynthesis and growth by C₃ plants can be largely improved by overexpressing the key enzymes of the C₄ pathway even though a full C4 cycle is not yet operative.