[How viruses hitch a ride on pollen to infect plants]

Some plant viruses are transmitted to new host plants via pollen. However, since the first report of pollen transmission of plant viruses in 1918, the fundamental mechanisms underlying this process have remained unresolved. Pollen transmission of plant viruses is classified into two types: vertical transmission via pollen and horizontal transmission via pollen. Recently, we have obtained important insights into these mechanisms. In vertical transmission via pollen, it was suggested that when virus-infected pollen grains germinate, the pollen tubes deliver the sperm cells required for fertilization into the embryo sacs while simultaneously introducing the virus, resulting in the formation of virus-infected seeds. In contrast, in horizontal transmission via pollen, it was suggested that pollen tubes that have accumulated the virus penetrate and elongate within the stigma, establishing the initial virus infection sites. The infection then expanded into the vascular tissues of the stigma and style, and subsequently spread throughout the plant via the phloem. Thus, pollenborne viruses are thought to exploit the sexual reproduction system of seed plants, using pollen as a vector to achieve both horizontal and vertical transmission. In this review, we provide an overview of our research conducted to date on the transmission mechanisms of pollen-borne viruses.