Plants and fungi get viruses too, often fatally. However, one virus is not only harmless to a major crop but it converts a serious threat into a protector, a finding that could be a game-changer for the fight against important crop diseases.
Canola, sometimes known as rapeseed, produces more than just a popular cooking oil. Plenty of animal feed and much of the world's biodiesel come from the pretty yellow flowers. Like any crop, canola is subject to various pathogens, and its vulnerability increases the more the crop is grown in large-scale monocultures. Canola's biggest threat is a fungus called Sclerotinia sclerotiorum that can kill the plant in days through stem rot. No resistant canola strains are known.
Professor Daohong Jiang of Huazhong Agricultural University, China, has found the best way to control the fungus may not be through extensive suppression measures, as has been standard up until now. Instead, it can be tamed with the virus SsHADV-1.
When S. sclerotiorum is infected with the virus, it loses its deadly effect on canola plants and instead forms a mutually beneficial relationship. The fungus lives inside the plant without killing it, but bolsters the immune system and enhances its growth.
In Molecular Plant, Jiang and co-authors report treating canola plants with SsHADV-1 and reducing stem rot by two-thirds. Moreover, canola carrying this double infection put on 18 percent more weight in the same conditions as uninfected counterparts. This leads to 7-15 percent more seed production, an important fact from the grower's perspective.
Not only does SsHADV-1 benefit the plant and the farmer, Jiang thinks it is probably good for the fungus as well. “The fungus now recognizes the plant as 'home' instead of killing it," he said in a statement. It grows more slowly in the short term but gets to survive much longer. The paper calls this a three-way symbiosis since the virus wins as well, but if you count the farmers it is really four-way.
Beneficial viruses have been identified before but proven hard to spread. However, "If you treat the seed with virus-infected fungus, the fungus will grow with the plant throughout its life," said Jiang. "Just like how we vaccinated our kids when they were born, the protection is life-long."
Indeed, SsHADV-1-infected fungus strains may spread in the field, benefiting farms that neighbor those where treatments are applied. Protecting canola is certainly a useful achievement, but fungal infections on some other crops are greater threats to humanity. A third of food crops worldwide are destroyed by a fungal disease. Without wheat rusts, rice blasts, and banana plant diseases, global hunger would be far rarer.
The question then is whether SsHADV-1 could teach these plant pathogens to live in harmony with their hosts. Crop pathogens are so diverse it is very unlikely any one virus could affect them all, and mycoviruses usually have a narrow range, but Jiang noted S. sclerotiorum also infects beans and sunflowers, making them a good place to start for further research. Other fungal diseases may be immune to SsHADV-1, but Jiang hopes to inspire other scientists to seek its counterparts.