Pollen grains carry hundreds of plant viruses

It’s not just pollen on a spring breeze. Just as some human viruses spread when humans reproduce, plant viruses can use pollen to move from flower to flower. A study in Nature Communication shows how abundant pollen-borne viruses are and suggests that human activity may help them spread.

University of Pittsburgh evolutionary ecologist Tia-Lynn Ashman and colleagues used genetic sequencing to catalog viruses on wildflower pollen from four different environments: California prairies, coastal California, an agricultural area of Pennsylvania and Appalachia. The team found 22 known viruses, some of which have serious effects on crops. They also found evidence of hundreds of viruses that scientists had never seen.

The findings match microbiology findings, says University of Florida plant virologist Amit Levy, who was not part of the study: “There’s just a lot more virus everywhere than expected.”

The team also discovered an interesting correlation. Flowers from the agricultural site carried genome fragments from more than 100 different viruses, while flowers from the California prairies (where human activity is lowest among the areas studied) had only about a dozen. The other sites had intermediate viral diversity. The researchers hypothesize that the homogeneity of plants in crop fields could encourage more viruses to inhabit these areas – once a virus evolves to infect a crop, it finds many compatible hosts.

Although this link is preliminary, Levy says it makes sense that industrial agriculture could spawn plant pathogens. With plants grouped together, “there is no social distancing between cultures”.

Ashman wonders if bees, which farmers often keep, could also be exacerbating the spread of the plant virus in agricultural areas. Bees are less fussy about which plants to visit than most native bees, potentially carrying viruses between wildflowers and crops.

Hernan Garcia-Ruiz, a virologist at the University of Nebraska-Lincoln who was not involved in the new study, says it caught his attention because the authors found a plethora of viruses even in plants that didn’t seem not sick. But these microbes may not be as benign if they are passed from wild plants to crops. Garcia-Ruiz cites sugarcane mosaic virus, a serious pathogen of sugarcane and corn that lurks in wild grasses between harvest seasons. “As soon as the maize is available, the insects bring the virus back into the maize,” he says.

Ashman agrees that it’s important to understand the effects of viruses on a variety of plants, especially if humans are encouraging the spread from natural habitats to agriculture and vice versa. As a scientific hypothesis, she finds this prospect “tantalizing” – but “perhaps frightening”.