Preface

Chapter 8 **Use of RNA Domains in the Viral Genome as Innate Immunity Inducers for Antiviral Strategies and Vaccine**

Chapter 9 **Gene Constellation of Influenza Vaccine Seed Viruses 213**

Chapter 11 **Viral Counter Defense X Antiviral Immunity in Plants:**

Miguel R. Rodríguez Pulido, Francisco Sobrino, Belén Borrego and

Alessandra Tenório Costa, Juliana Pereira Bravo, Rodrigo Kazuo Makiyama, Alessandra Vasconcellos Nunes and Ivan G. Maia

**Improvement 193**

Ewan P. Plant and Zhiping Ye

**Mechanisms for Survival 251**

Chapter 10 **Vaccines and Antiviral Agents 239**

Margarita Sáiz

**VI** Contents

Hongxuan He

Viruses are essentially itinerating genomes conveniently packaged in protein shell struc‐ tures, sometimes surrounded by lipid membranes derived from the host cells they highjack in order to reproduce. In fact, being able to introduce a discrete number of genes in a cell, viruses made their enormous contribution to the exploration of mechanisms of genome rep‐ lication and gene expression long before the advent of recombinant DNA technology. Stud‐ ies on phages that infect bacteria built a significant body of knowledge in the early years of molecular biology. In parallel, active research was oriented at viruses that cause diseases in humans, other animals and plants (one chapter of this book deals with citrus tristeza virus). In particular, medical virology has been by far the preeminent area of interest. Efforts have been made aimed at curing and preventing viral diseases that caused large numbers of vic‐ tims long before their viral etiology was recognized. In Europe 400,000 people died annually of smallpox in the 18th century. Based upon observation and empirical trials a vaccination procedure was devised to prevent this dreadful infection that caused high morbidity and mortality. Vaccination was likely practiced in Africa, India, and China long before the 18th century, when it was introduced to Europe. Edward Jenner's work represented the first sci‐ entific attempt to control an infectious disease by the deliberate use of vaccination. Strictly speaking, he did not discover vaccination but was the first person to confer scientific status on the procedure and to pursue its scientific investigation.

Biological sciences have gone a long way since then. Molecular biology of viruses (after all, viruses are more or less complex associations of macromolecules) and studies on virus-host interactions have provided a wealth of knowledge that helps designing different prevention strategies aimed at innate and adaptive immune responses. Several chapters of this book focus on viral protein complexes, gene expression, nucleotide sequences and genetic conste‐ lations in viral populations related with the design and production of new immunogens, and establishment of vaccination schemes to prevent viral diseases.

Drifting away from the human victims of viral diseases, it is worth mentioning that the prosperous silk industry in China drove attention to an economically relevant viral infection of the silk worm (Bombyx mori) starting a chase for the pathogen that affected the silk pro‐ duction. Three chapters in this book deal with viruses that belong to the family responsible for the economical losses of the silk industry: Baculoviridae. The studies on baculoviruses range from pathogenesis to viral genomics and gene expression, and most of the members are regarded more as friends than threats; the biology of baculoviruses has been harnessed for diverse applications such as microbial pest control, protein expression and gene trans‐ duction. Alternative gene transduction strategies are dealt with in the chapter summarizing different gene delivery systems. Advances in molecular virology have paved the way to al‐ ternative novel vaccine and gene therapy strategies.

In summary, this book is only a small collection of chapters dealing with examples of RNA and DNA viruses, and issues such as how these "gene packages" have learnt to take advant‐ age of their hosts, molecular recognition events that hosts may use to counterattack the vi‐ ruses, and how researchers have developed strategies to use viruses or their parts as tools for different purposes.

#### **Dr. Víctor Romanowski**

**Chapter 1**

**The Complex Genetics of** *Citrus tristeza* **virus**

The 2000 x 11 nm long bipolar flexuous filamentous particles of *Citrus tristeza virus* (CTV) (genus *Closterovirus*, family *Closteroviridae*) (Figure 1) contain a single-stranded positive-sense RNA genome of 19.3 kb, which is encapsidated in two different capsid proteins that coat the opposite ends of the virions [1, 2]. CTV is the largest identified RNA virus infecting plants and the second largest worldwide after the animal *Coronaviruses*. The virus is phloem limited and it is transmitted by aphids (*Hemiptera: Aphididae*) (Figure 1), and mechanically by graft propagation of virus-infected plant tissues. CTV isolates from different hosts and areas display great variability either biologically or genetically. There are wild CTV isolates that consist basically of a main genotype and its quasispecies, but others could contain a mixture of strains (groups of viral variants with similar sequence) that differ in symptomology and in viral transmission efficiency by aphids. These CTV strains could bear divergent CTV genotypes. Additionally, wild isolates are also composed by a population of defective RNAs (D-RNAs)

The *Tristeza* syndrome, induced by CTV, has devastated entire commercial citrus industries around the world, since it has caused the death of hundred million trees worldwide. In point of fact, this virus is present in most of the citrus producing areas infecting nearly all species, cultivars and hybrids of *Citrus* spp. and related genera. Phenotypically, CTV induces differ‐ ent grade and wide range of symptoms in*Citrus* species. In effect, depending on the virus isolate and the variety/rootstock combination, CTV strains can cause different syndromes in the field like 'decline' (QD) or 'stem pitting' (SP). Some CTV isolates induce a third syndrome, in glasshouse conditions, that is referred as 'seedling yellows' (SY). Furthermore, CTV causes a myriad of different symptom combinations in indicator plants depending on the CTV strain, or the mixture of strains, present in the plant host indexed. Remarkably, there are mild CTV strains that cause a complete lack of symptoms in almost all species and varieties of citrus, including those present in the citrus orchards, even though these mild viruses multiply to high titers [4, 5].

> © 2013 Albiach-Marti; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

that could change by aphid or graft transmission or by host passage [3].

Additional information is available at the end of the chapter

Maria R. Albiach-Marti

http://dx.doi.org/10.5772/56122

**1. Introduction**

Instituto de Biotecnología y Biología Molecular (IBBM, UNLP-CONICET) Facultad de Ciencias Exactas, Universidad Nacional de La Plata Argentina
