Gene scissors “silence” genes
The researchers had the Cas9 gene scissors cut the GBSS and PTST genes of the cassava plant at the embryogenic stage, thereby changing the sequence of the plant’s genetic code. Both genes are involved in the production of amylose. If they are defective, the cassava plant is no longer able to produce it.
Bull and his research team cultivated several particularly promising plant lines in a glasshouse. The researchers then studied them to determine the amylose content of their storage roots. Some of the lines was found to have produced no amylose at all: the starch in the roots of this modified cassava contained only amylopectin.
These ‘waxy’ (amylose-free) cassava roots join a list of other globally important crop varieties, such as maize and potato, which have similar traits.
Crossing eliminates foreign DNA
To remove the foreign genetic material they had introduced into the cassava, the plant scientists crossed two individual plants of a transgenic amylose-free cassava line together. Cassava carries two copies of each of its chromosomes; the foreign DNA had been inserted into only one of two identical chromosomes in these individuals. On that basis, one in four of the progeny of this crossing would be free of foreign DNA.
“Without flowers, we would not have been able to perform the cross to remove the foreign material,” Bull says. However, the plants retained the ability to produce only amylose-free starch. Bull explains, “This means that in the first generation of progeny, the trait we wanted remained but the foreign DNA could be completely crossed out.” The tricky part was getting the cassava to flower and produce seeds. This plant rarely flowers in nature, and almost never in a glasshouse environment. Cassava is generally propagated not through sexual reproduction but rather via stem cuttings, which are genetically identical.
