The so-called Rice Yellow Mottle Virus (RYMV) is to blame for massive crop losses in Africa, particularly among small-scale farmers. A study team from Heinrich Heine University Düsseldorf (HHU) and the French National Study Institute for Sustainable Development (IRD) used genome editing to create rice lines that are immune to the disease. The rice varieties developed by the researchers and described in the Plant Biotechnology Journal are a first step toward the ability to build resistant regionally adapted elite cultivars for small-scale African food producers.
RYMV is an RNA virus transmitted through insects and direct leaf-to-leaf contact. In Africa, where the majority of producers farm areas of land measuring less than one hectare, the virus typically destroys ten to one hundred percent of rice harvests. This creates a life-threatening situation for the poorest farmers.
There is currently no effective virus defense. “The only real protection is to develop rice varieties that possess a resistance gene against RYMV, which would make the plant invulnerable,” says Dr Yugander Arra, main author of the study published in Plant Biotechnology Journal.
A research team from the Institute for Molecular Physiology at HHU (headed by Professor Dr Wolf B. Frommer) and the Institut de recherche pour le développement (IRD) in Montpellier, France, has developed such resistant rice lines.
It is, in a sense, an archetype that was useful for its forefathers but now causes disastrous crop losses during droughts. It appears expedient to turn off this gene, and there are no noticeable adverse effects.
Dr Eliza Loo
There are currently three resistance genes known; mutations in just one of these genes, RYMV1, 2, or 3, are sufficient to confer resistance. Rymv2, a resistant variant, is seen in African rice (Oryza glaberrima) types that yield poorly. RYMV2, also known as CPR5.1, encodes an essential protein found in the cell nucleus’ pores. The deletion of the single CPR5 gene copy in the model plant Arabidopsis thaliana results in a broad spectrum of resistance to viruses, bacteria, and fungus. However, growth is severely restricted, and the plants display spontaneous lesions, resulting in low yield. So it was critical to determine whether rymv2 resistance could be transmitted to other rice kinds without causing problems.
In Africa, other high-yield rice varieties based on the Asian species Oryza indica are mainly used and these do not have the resistance gene. Inserting the relevant gene is however not a particularly promising approach as the descendants of such “inter-species” hybrids are highly sterile and therefore cannot reproduce and pass on the resistance easily.
Using the CRISPR/Cas genome editing technology, the researchers have demonstrated that mutations in the RYMV2 gene may be created in an Asian rice variety, rendering it resistant to the virus in a manner comparable to the African variant. In the following step, the goal is to alter relevant African elite varieties in the same way, making them available to African small-scale farmers. The international research partnership “Healthy Crops,” which is led by HHU, aims to help these farmers.
Background: Plants have genetic mechanisms that were beneficial to survival in the early stages of evolution but are today more likely to be harmful. Maize is an excellent illustration of this. When dry conditions exist during fertilisation, a gene causes the kernels to be aborted. This gene-induced characteristic was beneficial to the wild perennial predecessors of today’s maize plants, but it has a negative impact on the production of annual plants currently utilized in agriculture.
The situation is similar with the rice examined here. Professor Frommer: “This resistance trait is attributable to the loss of a gene function that is not essential. If we switch the gene off completely, the plants behave normally. However, as a result of the loss of the gene function, they are resistant to the virus.”
Dr Eliza Loo, Healthy Crops Group Leader, adds: “It is, in a sense, an archetype that was useful for its forefathers but now causes disastrous crop losses during droughts. It appears expedient to turn off this gene, and there are no noticeable adverse effects.”
Surprisingly, switching off the nearly related CPR5.2 gene, as well as the two genes RYMV2 and CPR5.2, does not cause problems, at least under greenhouse settings. It is worth noting that the deletion of CPR5.2 does not result in RYMV resistance. Everything shows that altering the RYMV2 gene is a promising strategy for controlling rice disease in Africa.
