Mapping the genes of wild bananas provides insights for disease resistance
Recent RTB-funded efforts are mapping the genes of bananas to breed varieties that are resistant to common diseases, as part of wider strategies to combat the diseases. All cultivated bananas are susceptible to banana Xanthomonas wilt disease (BXW), and many dessert bananas are susceptible to Fusarium wilt disease. However, some wild bananas are resistant to these diseases. Understanding the genes that defend plants from these diseases is a first step to breeding healthier, more productive banana varieties.
BXW is menacing banana and plantains across East Africa, where these crops are major staple foods. BXW is caused by a bacterium named Xanthomonas campestris. Most cultivated varieties of banana and plantain are susceptible to BXW. Fortunately, some varieties of a wild banana species, Musa balbisiana, are resistant to BXW, but their fruits are small, full of seeds and not edible. So, a key strategy for managing the disease would be to breed cultivated bananas with wild ones, bringing in the genes for disease resistance, but without dragging in unwanted genes that would compromise the fruit quality. The first step is to identify those bacteria-fighting genes.
Recent work funded by RTB has mapped these crucial genes for resistance, by comparing the wild banana, Musa balbisiana, with a cultivated variety, Pisang Awak, which is very susceptible to BXW. The project, led by Leena Tripathi at IITA, found about 30 stress-related genes that were differentially expressed in the wild banana in response to bacterial infection.
Within 12 hours of the pathogen attack, some of the resistance genes identify the invader, and other genes express responses to fight it off, for example by reinforcing the cell walls, inducing rapid calcium fluxes and oxidation bursts around the infection site. In other words, these genes activate the plant’s basal defense (its innate, first line of protection against pathogens.). Other genes program the death of infected banana cells, robbing the disease of its toehold in the plant.
This research project has mapped the candidate genes for resistance on the wild banana. Future plant breeders may be able to bring the traits for disease resistance from wild bananas to the cultivated ones that farmers grow in Africa by using modern biotechnological tools, says Tripathi.
In another area, is the Fusarium wilt disease, caused by the fungus Fusarium oxysporum, that is threatening the world’s banana supply. Recently, a new form of the fungus, tropical race 4, has been spreading in Asia and has reached Africa and Latin America. Cavendish, the most widely grown banana variety, is very susceptible to tropical race 4. RTB has supported an alliance of researchers in Asia, Africa and Latin America to share knowledge and information to combat the disease. These researchers are helping to carry out surveillance on the spread of the disease, especially in Africa, by developing and implementing detection tools for the disease, and screening germplasm for resistance. Information and communication technology (ICT) tools are being developed to help identify the disease in farmers’ fields.
Many commercial banana varieties are susceptible to Fusarium wilt, yet some wild bananas are resistant, including Pahang, a type of the wild species, Musa acuminata. So, research was carried out to understand the genetic basis of immunity of this hardy, wild banana.
This project, led by the Yunnan Academy of Agricultural Sciences and Bioversity International, has shed some light on candidate genes for Fusarium resistance. The Fusarium fungus is abundant in the soil, and commonly enters the plant through the roots. The wild banana fights off the disease by accumulating hydrogen peroxide in root cells, strengthening the cell walls and making secretions that inhibit spore formation and fungal growth.
This work identified a few candidate genes in the wild banana species, which code for responses to Fusarium infection. This helps us to better understand the genetic basis for disease resistance in bananas, says Alberto Cenci, an expert in banana genetics at Bioversity International.
To find the genes that code for these responses, scientists compared expressed genes in the wild, resistant Pahang, with those of a susceptible commercial variety, “Brazilian.” Before being infected with the pathogen, both banana varieties expressed differences in thousands of genes: Pahang expressed 3,867 genes at higher levels than those in Brazilian. However, Brazilian expressed 2,452 genes at higher levels than in Pahang. Since these differences were observed before infection, Pahang probably has many defense genes expressed all the time, which is called “constitutive defense.” One day after infection, genes in the wild Pahang code for eight proteins to be produced, but this was not so for the susceptible Brazilian banana. Pahang has many resistance genes primed before infection, and then others which are induced by the infection, but faster than with Brazilian. So, the fungus can overcome the defenses of the susceptible variety, which seem to respond too little, too late.
The results obtained with BXW and Fusarium wilt are both examples of cutting-edge science being used to understand disease resistance in bananas and plantains, and to help plant breeders to produce new crop varieties that resist these diseases.