Poster: Metabolic Engineering

Abs # 1019: Effects of the expression of maize C₄ genes in rice on grain yield, photosynthesis, and enzyme activity

Presenter:	Murphy, L. R., murphyl@wsu.edu
Authors	Murphy, L. R. (A)   Ku, M.S.B.  (A)   Franceschi, V. R. (A)
Affiliations:	(A): School of Biological Sciences, Washington State University

To introduce some of the C₄ photosynthetic traits into C₃ plants, rice has been transformed independently with maize genes encoding phosphoenolpyruvate carboxylase (PC) and pyruvate, Pi dikinase (PK). Transgenic rice, grown in a controlled environment (28/22°C, 14 h light at 1000 umol/m²/s), show high levels of expression of PC and PK in the cytosol and chloroplast, respectively. The transgenic plants exhibit a normal phenotype with high fertility (85-95%, compared to 90% in wild-type). Relative to the wild-type, PC and PK transgenic plants show higher photosynthetic rates, higher tolerance to photooxidation, and up to a 35% increase in grain yield. The increased yield has been associated with increased panicle number per plant. Photosynthetic measures have shown that the PC transgenic rice has up to 45% higher maximum photosynthetic rate on a leaf area basis at saturating light and ambient O₂ and CO₂ levels. Additionally, the apparent quantum yield (QY) increases from 0.04 in the wild-type to 0.05 in PC plants. PK plants exhibit up to a 20% increase in the maximum photosynthetic rate with a QY of 0.04. Biochemical studies show that PK is light activated in transgenic rice by up to two-fold, relative to three-fold for the enzyme in maize. PC in transgenic rice does not show the same degree of light activation as seen in maize, but does have reduced sensitivity to malate and increased stimulation by G6P in the light, as compared to the dark.