The Plant Viruses

Pollen is a valuable source of germplasm for breeding and has few associated pests compared with other sources of genetic material. This review seeks to assist the development of appropriate phytosanitary measures by considering the pests that are transmitted by pollen. There are no invertebrates, bacteria, phytoplasmas or spiroplasmas that are pollen-transmitted. Only a limited number of fungal pathogens are associated with the pollen of a restricted number of hosts. In contrast, 39 viruses are pollen-transmitted and a further six are tentatively considered to be pollen-transmitted. The majority of these viruses belong to the Alphacryptovirus, Ilarvirus, Nepovirus or Potyvirus genera. Five viroids have also been identified as being pollen-transmitted.

Plant pathogens transmitted by pollen

Publisher Summary Inclusion bodies can be simply defined as intracellular structures produced de novo as a result of viral infections. They may contain virus particles, virus-related materials, or ordinary cell constituents in a normal or degenerating condition, either single or, more often, in various proportions. By this definition, most, if not all, plant viruses are liable to induce formation of inclusion bodies of some sort. These may be crystalline or amorphous (or both) in the same cell, and may vary in size. Thus, some inclusions are readily seen with the light microscope, as they constitute bulky masses often attaining 10-20 μm along the major axis, whereas others are only visible with the electron microscope. This is a broad interpretation of the concept of inclusion body. Intracellular inclusions arise as a consequence of disturbed cell metabolism following virus infection, but are not necessarily a by-product of this process. In some instances, inclusions constitute a more or less inert accumulation of the virus particles or excess viral material, but there is mounting evidence that, in many cases, inclusion bodies play a primary role in the synthesis and/or assembly of viral components. Therefore, knowledge of their fine structure and composition seems fundamental to an understanding of their probable function. In this chapter, rather than reviewing plant virus inclusions comprehensively, it discusses the most significant findings in this field in the context, whenever possible, of an interpretation of the role of inclusions in the infection process.

Plant Virus Inclusion Bodies

Spinach (Spinacia oleracea) seed from a commercial breeding line suspected of harboring cucumber mosaic virus (CMV) was analyzed for seed transmission of the virus. Initial seed grow-out tests and enzymelinked immunosorbent assay studies indicated that CMV was present in this seed lot at a level of nearly 15%. To verify these results and gain insight into the mechanism of seed transmission, four combinations of crosses between healthy and/or infected parent plants were conducted. None of the spinach seedlings derived from crossing healthy male and healthy female plants contained CMV, whereas a portion of seedlings derived from all of the other three crosses, i.e., healthy male and infected female, infected male and healthy female, and infected male and infected female plants, were infected with CMV. The results demonstrate that CMV is seed transmitted in spinach and indicate that both male and female parent plants can serve as infection sources. Ultrastructural studies, including immunogold labeling, revealed the presence of virus particles in the cytoplasm of ovary wall cells, ovule integuments and nucellus, anther, and seed-coat cells, as well as fine fibril-containing vesicles and electron-dense inclusions of amorphous aggregates in the central vacuoles of these cells. In addition, reverse transcription-polymerase chain reaction (RT-PCR) was used to amplify 860-bp cDNA fragments containing the CMV coat protein (CP) gene from the embryo, endosperm, and pollen tissues of CMV-infected plants. Taken together, these studies indicate that CMV occurs in virtually all spinach reproductive tissues. Analysis of several RT-PCR amplified and cloned CP genes and flanking sequences from parent and progeny plants revealed that the spinachinfecting CMV was a member of subgroup II. Furthermore, cDNA sequencing and restriction endonuclease mapping consistently revealed two sequence variants, designated SP103 and SP104, in most plants analyzed. These data suggest that there may have been mixed infections of two distinct, seed-transmitted CMV variants in spinach.

https://apsjournals.apsnet.org/doi/pdfplus/10.1094/PHYTO.1997.87.9.924

Seed transmission of cucumber mosaic virus in spinach.

The central concept concerning the viruses described in this chapter is that they have in common the following characteristic: the minimum genetic entity which can be derived from the virus, and which can induce an infection in which virions are formed, is two pieces of RNA. The two pieces of RNA, which can be considered to be the two chromosomes composing the genome of the virus, are (for the plant viruses described here) found in distinguishable ribonucleoprotein particles. That is, the genome is distributed in two kinds of virions. The notion of the extracellular phase of a virus life cycle being represented by “virions” rather than “virion” is generally alien to bacterial, animal, and fungal virology. However, probably nearly half of the recognized groups of plant viruses have virions rather than a virion. What advantage does such an arrangement give a plant virus? Why do such viruses find a niche which apparently has not been occupied by a monochromosomal, monovirion virus? A priori, the potential disadvantage of having to bring at least two particles or RNA molecules into a cell, rather than one, is most obvious.

A.F. Yang

Papers

The mechanism of seed transmission of tobacco ringspot virus in soybean