Cassava (Manihot esculenta Crantz) and sweetpotato (Ipomoea batatas), two major root crops worldwide, are characterized by the favorable nature of high efficiency of utilization of light, temperature and water resources. They are also productive, rich in starch and suitable for poor soils unable to support other grain crops. Therefore, they have been considered the most promising bioenergy crops in China. Technologies to solve major constraints that affect their production and bioindustrial applications are demanding. Due to the biological nature of cassava and sweetpotato, including the vegetative propagation mode, high heterozygosity and inbreeding depression, as well as low fertility and seed setting, conventional breeding efforts to address the constraints to their production have only met with limited success. Our objectives focus on the development of novel cassava and sweetpotato that have better yield, more diversified starch types, higher abiotic stress tolerance and more suitable for bioenergy production via biotechnology, thereby greatly contributing to the sustainable development of tropical agriculture and biomass-based renewable energy development.
The major activities in our ongoing research include:
1)Isolation and identification of genes related to sucrose/starch biosynthesis and accumulation using functional genomics tools.
2)Molecular mechanisms of abiotic stress resistance in cassava and sweetpotato.
3)Improvement of viral disease resistance by the application of RNA interference technology.
4)Promotion of genetic improvement by establishment of efficient genetic transformation systems for elite cultivars in cassava and sweetpotato.
Several major achievements of our program have been made over the last three years. To engineer cassava for more productive and increased resistance to drought, we have applied the autoregulatory leaf senescence inhibition system in cassava and performed the first field trial of transgenic plants in Hainan Island, China. In the two-year-term experiments, the SAG12::IPT transgenic cassava showed a significant delay of leaf senescence and increased drought tolerance. Therefore, we are able to demonstrate the feasibility of increased photosynthesis effectiveness by prolonging the leaf life in cassava, possibly leading to a higher yield of its starchy storage roots. Another achievement is the starch modification in cassava and sweetpotato. Starch is composed of amylose and amylopectin, which are mainly synthesized by granule-bound starch synthase (GBSSI) and starch branching enzyme (BE), respectively. Amylose content in cassava and sweetpotato storage roots arranges from 15% to 25%. To produce amylose-free or less amylose starch, we have developed transgenic cassava plants in which the GBSSI expression was reduced or completely silenced by homologous small RNAs generated from hp-RNA expressing constructs. Transgenic cassava plants demonstrated significant alternation of starch composition in harvested storage roots. The new starch types from transgenic cassava will provide novel raw materials for bioindustrial applications.
In sweetpotato, we have also developed an efficient genetic transformation system suitable for many elite cultivars. Transgenic plants have been produced for engineering the crop with modified starch, improved tolerance to salinity stress and increased resistance to viral disease.
