Transmission through seeds: The unknown life of plant viruses

Plant viruses are not exceptions and evolved various ways to achieve transmission. The best characterized is plant-to-plant horizontal transmission by direct contact, mechanical means (lawn and farm equipment, grazing animals, etc.) or by vectors (arthropods, nematodes, fungi, oomycetes, etc.). While very important for the epidemiology of most plant viruses, horizontal transmission is far from being the only way for virus dispersal. Parent-to-offspring vertical transmission through seeds has been known for a century [1], and for some plant viruses such as persistent (also known as cryptic), viruses is the only way to infect new hosts [2]. However, many aspects of the virus journey to reach the reproductive organs and its life within seeds remain poorly understood. This knowledge gap can be perhaps attributed to a combination of factors: Due to the low virus titer in the seed, detection is often difficult. Also, studies on seed transmission require long-term experiments, which is frequently a limiting factor for researchers. Consequently, until recently, it was considered as a transmission mode of lesser relevance. For instance, 35 years ago, only 100 plant viruses were described as seed-transmitted, normally at very low rates [3]. This vision is currently changing. Methods for virus detection in seeds and seedlings (virus detection in seeds does not necessarily entail transmission to the progeny, as virus could be present in the testa/endosperm but not in the embryo) have been progressively improved in specificity and sensitivity [4]. In parallel, the number of viruses described as seed-transmitted increased 6-fold, with some reaching 100% of infected seeds [2,5]. Moreover, it has been shown that vertically transmitted persistent viruses are predominant in wild plants, and they are also common in crops [2]. By answering 5 basic questions about plant virus seed transmission, here I aim to provide arguments supporting why we should care about this mode of transmission and to summarize current knowledge and future avenues of research on this subject.