Submitted to: Plant Biotechnology Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 2, 2006
Publication Date: July 1, 2006
Citation: Ihemere, U., Arias-Garzon, D., Lawrence, S.D., Sayre, R. 2006. Genetic modification of cassava for enhanced starch production. Plant Biotechnology. Vol.4, #4 Pg. 453.465
Interpretive Summary: The starchy roots of cassava are a valuable source of calories for about 600 million people in the developing tropical countries. Cassava is preferred by subsistence farmers in Africa since it can be stored for years underground in case of future famine. Producing a higher yielding cassava plant should help in providing more food for tropical cultures. Genetically engineering cassava with a modified gene for the pivotal step in starch biosynthesis in the roots has resulted in plants that have a two-fold greater top and root mass than unmodified plants. This results in an increase in both number and total yield of cassava roots. Since cassava is propagated through stem cuttings this should result in an improved variety of cassava useful to subsistence farmers.
To date, transgenic approaches to biofortify orphan crops grown by subsistence farmers have been rather limited. This is particularly true for the starchy root crop cassava (Manihot esculenta Crantz). Cassava is a major source of calories for over 250 million persons living in sub-Saharan Africa and ranks fourth among all tropical crops as a source of calories. Cassava has a high photosynthetic rate and one of the highest known rates of sucrose production. When grown under optimal conditions its starch yield exceeds that of many hybrid corn varities. It was our hypothesis that root starch production could be substantially improved by increasing sink strength for carbohydrate. To test this hypothesis we generated transgenic plants with potentialy enhanced starch synthesis and accumulation by expressing a modified bacterial ADP-glucose pyrophosphorylase (AGPase) in roots. AGPase catalyzes the rate-limiting step in starch biosynthesis and the bacterial form of the enzyme has enhanced kinetics relative to the plant enzyme. To facilitate maximal AGPase activity we further modified the E. coli glgC gene (encoding AGPase) by site-directed mutagenesis (G336D) to inhibit allosteric feed-back regulation by fructose-1,6-bisphosphate. To specifically enhance sink strength we selectively expressed the modified glgC gene in roots using the tuber-specific and carbohydrate-regulated patatin promoter of potato. Total AGPase activity in three independent transgenic plants ranged from one to two-fold higher than control plants. Significantly, AGPase activity was directly correlated with root and total plant biomass accumulation. Transgenic plants expressing the highest levels (1.7 X wild type) of root AGPase activity had two-fold greater top (leaf and stem) and root biomass levels than wild-type plants grown under greenhouse conditions. These results demonstrate that targeted modification of enzymes regulating source-sink relationships is an effective strategy for increasing cassava yields.