Molecular Plant Virology

Volume II: Replication and Gene Expression

Jeffery W. Davis

• Publisher: CRC Press
• Year: 2018
• ISBN:
  9781351091688
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Botany
• Index: Biological Sciences

In calling this series Molecular Plant Virology, I had in mind aspects of plant virology of interest to biochemists, molecular geneticists, biophysicists, genetic engineers, or, collectively, molecular biologists. At the same time, the intention was to provide up-to-date reviews, by expert contributors, on current research topics in plant virology of interest and referential use to virologists and plant biologists. The selected topics are pitched mainly at a research level, but with sufficient introduction and cross-referencing to enable graduate students to enter this fascinating field and, hopefully, not get lost.

Chapter 1

EARLY EVENTS IN PLANT VIRUS INFECTIONS

John G. Shaw^*

* This review was completed in September, 1983.

TABLE OF CONTENTS

I.	Introduction
II.	Scope of This Chapter
III.	Stages in the Early Events
IV.	Other Virus-Host Systems
	A.	Animal Viruses
		1.	Adsorption
		2.	Entry
		3.	Uncoating
	B.	Bacterial Viruses
V.	Early Events in Plant Virus Infections
	A.	Experimental Systems
	B.	Experimental Procedures
	C.	Mechanically Inoculated Leaves
		1.	Attachment and Entry
		2.	Uncoating
	D.	Protoplasts
		1.	Attachment and Entry
		2.	Uncoating
	E.	Other Approaches
VI.	Concluding Remarks
Acknowledgments
References

I. INTRODUCTION

Of all the steps in the development and expression of a plant virus infection, undoubtedly the least understood are those which come first, i.e., the so-called early events. Our knowledge of the events in infected cells, during which active viral gene expression, replication, and progeny virion accumulation occur, is considerably more detailed than our understanding of what precedes these events. This gap continues to increase with time and, in recent years, there have been successively fewer publications dealing with the subject of this chapter. While it is surely not necessary to point out the vital importance of the first stages in virus infections, it is puzzling that so little effort has been devoted to events without which the dynamic activities involved in the production of the new virus particles and in the development of some very serious diseases will not occur.

It is the purpose of this article to bring together some of the information that bears upon the early events in the establishment of virus infections of plants. This will not result in the offering of new concepts, profound insights, or inspired suggestions because, at least to this reviewer, the subject is so complex and so poorly understood that few such expressions of wisdom are readily apparent. I should, in fact, warn the reader at the outset of what may seem a decidedly gloomy treatment of the subject. This mood, if apparent, should be regarded as an expression of the necessity to emphasize those aspects of an important subject about which far too little is known and, yet, about which rather dogmatic but clearly unsubstantiated statements are sometimes made. It is also hoped, of course, that this account will remind readers of the pressing need for more energetic and inspired investigation of the subject.

II. SCOPE OF THIS CHAPTER

For probably any virus-host system, the early events in the establishment of infection can be thought of as those essential activities which occur during the period from the first contact between virion and host cell to the point where the parental viral nucleic acid can begin to function in translation, transcription or integration into the host genome. This definition can become complicated in referring to infections resulting from cell-to-cell spread of virus in tissues. In such cases, the early events may be quite different from those in cells that are inoculated directly. Thus, this article will focus upon the early events in cells, single or in tissue, which are the first to become infected after inoculation.

Questions concerning the mechanisms and activities involved in the earliest stages of plant virus infections will be seen to remain largely unresolved. It may thus be appropriate to list here some of the more apparent reasons for this lack of information:

1. The early events in infection of crop plants, in their natural settings, occur on a scale which cannot at present be effectively investigated. Somewhat artificial experimental systems and procedures must therefore be used for the study of these events.

2. The requirement for large numbers of virus particles to ensure the infection of significant numbers of cells leads to what is the greatest problem concerning these types of investigations — the difficulty of distinguishing the particles and their constituents which play an active role in the establishment of infection from those which do not.

3. The vast majority of plant viruses are of small size and simple structure. This makes them very difficult to detect and describe by electron microscopy in the early stages of infection.

4. The apparent requirement for wounding of cells or tissues in order to initiate infections results in a much less definitive interaction of plant virus particles and cells than does the adsorption of animal and bacterial virus attachment structures to cell receptors.

5. A lack of investigative effort is the major contributing factor to our poor understanding of the early events in plant virus infection. While other aspects of plant virology receive intensive investigation and generate great excitement, the initial stages in the viral life cycle continue to be studied on an almost casual basis with a paucity of fresh approaches. In addition, only one plant virus has been used to any extent in such investigations.

It is hopefully obvious that most of the important virus infections of plants do not occur in laboratories and greenhouses. Rather, they occur in fields, pastures, orchards, and forests usually with very little role played by human beings. They occur, in many cases, because an insect, in the course of seeking to nourish itself, has acquired virus particles from an infected plant and has subsequently introduced them into a different plant. They also occur after transmission by other invertebrates and by fungi and pollen.

There is virtually no information on the early events in such natural situations. For rather obvious reasons, it has been necessary, in order to obtain even the most meager insights into the processes involved in these events, to utilize systems and procedures which are undoubtedly quite different from those which occur under natural conditions. This review will deal with information that has come from the use of these experimental systems. At the same time, it is hoped that some appreciation of the probable artificiality inherent in such studies will be maintained.

Most of this chapter will focus upon investigations of the early events in virus infections of leaves and protoplasts and of the interaction of virus particles with subcellular fractions and membranous substances. This will be preceded by brief descriptions of the various stages involved in the early events and some general discussion of the manner in which they have been investigated in plants.

A fairly recent review of early events in plant virus infections has been presented by de Zoeten.¹

III. STAGES IN THE EARLY EVENTS

The early events in virus infections have often been considered as involving three steps: adsorption (or attachment, retention, binding, fixation), penetration (or entry), and uncoating (or disassembly). Some of these terms suggest activities or mechanisms which, for many host-virus systems, have not been adequately demonstrated. Perhaps more worrisome is that the use of these terms frequently suggests a series of discrete stages, often in a given order, in cases where there is inadequate evidence of such stages or where two or more of the activities appear to occur simultaneously or in a different order. Nevertheless, the terms do have wide acceptance and some of them will be used in this chapter.

The initial contact between virus particle and host cell is referred to as adsorption or attachment. The process during which the virion or its nucleic acid and, in some cases, other constituents, passes into the cytoplasm of the cell has been described as entry or penetration. Uncoating is the removal of various components of the mature virus particle and the consequent release of the viral genome and other constituents which play a part in the establishment of infection. For the reasons mentioned above, and from what will follow, it will be apparent that these terms can be used in a rather careless manner in referring to early events in virus infections of plants.

It may be worth mentioning here two other terms which, while not used to refer to events, have often been featured in discussions of plant virus infections. “Infectible sites” are said to be created on the surfaces of leaves as a result of abrasion during the mechanical inoculation procedure. These sites may be converted to “infective centers” if virus particles are present at an appropriate time. Such sites and centers, which are defined in an “operational” sense, obviously exist, but the use of the terms seems inappropriate here and may in fact promote casual, superficial consideration of the locations and mechanisms actually involved during the early events of infection. These terms will not be used in this chapter.

The early events in virus infection may be said to have been completed when the nucleic acid (and, in some cases, other virion constituents) of parental virus particles becomes available for the first reactions leading either to the synthesis of new viral protein and nucleic acid molecules or to its integration into the host genome. For the vast majority of plant viruses, the first activities after the early events are thought to involve translation of at least part of the single-stranded, positive-sense, parental viral RNA (see Chapter 3, this volume). Other types of viruses will have different requirements at this stage. These reactions, and those that follow as the infection proceeds, will be treated in other chapters in this series.

VI. OTHER VIRUS-HOST S...

Table of contents

• Cover
• Title Page
• Copyright Page
• Table of Contents
• Chapter 1 Early Events in Plant Virus Infections
• Chapter 2 Genome Structure and Gene Expression of Plant RNA Viruses
• Chapter 3 Structure, Replication, and Expression of the Bipartite Genome of Cowpea Mosaic Virus
• Chapter 4 Organization and Expression of the Cauliflower Mosaic Virus Genome
• Chapter 5 Replication of Caulimoviruses in Plants and Protoplasts
• Chapter 6 Structure and Function of the DNA Genome of Geminiviruses
• Index