Cassava, also known as manioc, is a staple food for nearly a billion people and an important source of raw materials. It secures an income for small farmers, especially in Africa. Cassava is an undemanding crop; it does not need fertilizer and grows even in dry areas.
However, cassava cultivation is affected by many pests and diseases. Cassava mosaic disease (CMD) in particular often damages the crop. CMD is caused by DNA geminiviruses that are transmitted to plants by sap-sucking whiteflies (Bemisia tabaci) and can destroy entire fields and decimate yields.
Cassava mosaic disease is a particularly serious problem in Africa and India. However, the virus is now also spreading in cassava fields in Southeast Asia. Growers and farmers urgently need CMD-resistant cassava cultivars.
A research consortium tracks down the resistance gene
Help may be on the way thanks to a discovery by an international research team led by Wilhelm Gruissem, professor of plant biotechnology at ETH Zurich. Working with several resistant and susceptible West African cassava cultivars, the team used elaborate and time-consuming genomic analyzes to identify the gene responsible for specific resistance to cassava mosaic virus.
The resistance was originally discovered by farmers in West Africa, who observed that while the majority of their cassava plants in the fields had died due to virus infection, a few plants survived. This caught the attention of researchers, who then attempted to identify the cause of this resistance.
In their study, which has just been published in Communication Nature, the team led by Wilhelm Gruissem shows that resistance is caused by a single gene which is the blueprint for a DNA polymerase – an enzyme responsible for DNA replication in a cell. However, DNA polymerase not only replicates DNA, but also performs “proofreading” to correct errors in the sequence of DNA building blocks that can occur during replication. And it is precisely this enzyme that geminiviruses need to replicate their own DNA and therefore reproduce.
Is DNA polymerase not working properly?
Since cassava has a double set of chromosomes, the plant has two copies of each gene. If one copy of the DNA polymerase gene is mutated, the viruses cannot multiply and the infection stops. In plants susceptible to the disease, however, both copies of the DNA polymerase gene lack the mutation that gives rise to resistance to CMD.
“We don’t yet know exactly how the resistance mechanism works,” says Gruissem. “This is something that will need to be investigated in future studies. But he suspects the mutations affect an area of the enzyme responsible for correcting errors in DNA replication. These changes could affect the functioning of DNA polymerase, preventing it from correcting errors in viral DNA replication; these errors end up preventing the virus from replicating and spreading in the plant.
Targeted virus-resistant breeding with genome editing
By identifying the gene responsible for so-called CMD2 resistance, researchers are playing an important role in improving food security in tropical and subtropical regions. The gene they identified now serves as a genetic marker for plant breeders, indicating whether or not resistance is present in their plants.
For economic and agronomic reasons, it is not possible to export stems of virus-resistant cassava plants from West Africa to Asia for field propagation. This means that Asian breeders must find another way to introduce resistance into their plants. “One possibility is to use modern CRISPR-Cas technology to precisely edit the DNA polymerase gene and activate disease resistance,” says Gruissem.
The research project involved researchers from ETH Zurich as well as the Donald Danforth Plant Science Center in St. Louis, the University of California, Los Angeles and the National Crops Resources Research Institute in Uganda. A substantial portion of the research funding was provided by the Bill & Melinda Gates Foundation.
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Materials provided by ETH Zurich. Note: Content may be edited for style and length.