**Abstract**

Various pathogenic microorganisms (such as fungi, bacteria, viruses and nematodes) affect plant viability and productivity. However, plants combat these pathogens by inducing their defense mechanism to sustain their fitness. The aggregation of pathogenesis-related (PR) proteins in response to invading pathogens is a crucial component of a plant's self-defense mechanism. PR proteins induce innate resistance in plants through fungal cell wall disintegration, membrane permeabilization, transcriptional suppression, and ribosome inactivation. Earlier studies have demonstrated their crucial role in determining resistance against phytopathogens, making them a promising candidate for developing disease-resistant crop varieties. Plant genetic engineering is a potential approach for developing disease-resistant transgenic crops by employing several PR genes (thaumatin, osmotin-like proteins, chitinases, glucanases, defensins, thionins, oxalate oxidase, oxalate oxidases like proteins/ germin-like proteins and LTPs). Furthermore, the overexpression of PR proteins enhances the resistance against phytopathogens. As a result, this chapter gives an overview of PR proteins, including their classification, functional characterization, signaling pathways, mode of action and role in defense against various phytopathogens. It also highlights genetic engineering advances in utilizing these genes singly or synergistically against various phytopathogens to impart disease resistance. Various challenges faced with the products of transgenic technology and synergistic expression of different groups of PR proteins were also discussed.

**Keywords:** biotic stress, pathogen-related proteins, plant genetic engineering, plant defense signaling, disease-resistant crops

### **1. Introduction**

With the rapid expansion in the world population, the area under cultivation has decreased [1]. Moreover, biotic stress has been a significant challenge for farmers since the dawn of agriculture. Global yield loss due to plant pathogens is estimated at 16% [2]. To overcome the economic loss in agricultural production, most research in this field focuses on protecting crops against pathogens, insect pests and nematodes. Crop production and productivity can be enhanced by significant breakthroughs in agricultural practices such as cultural controls, pesticide application, crop rotation, and plant breeding.

On the other hand, Pathogens frequently escape chemicals through strong selection and evolution, resulting in crop loss due to infection. Altering the genetic architecture of crops through breeding programmes is another option for crop protection, but it is a labor-intensive and time-consuming operation. In theory, genetic engineering, which refers to the use of biotechnology to alter an organism's genetic material directly [3], is a potential tool for improving disease resistance. Furthermore, genetic engineering can overcome the limitations of traditional breeding technology, including the introduction/alteration of specific genes with minimum undesirable changes to the rest of the genome; cross-species exchange of genetic material; and introduction of variations/genes into asexually propagated crops like bananas [4]. As a result, research studies have been directed toward the genes that impart long-term resistance to many pests or pathogens and are safe for consumption.

In plants, tolerance and susceptibility to a particular pathogen are determined by a complex interaction of signals and responses corresponding to specific environmental conditions. So, the major difference between resistant and susceptible varieties is the ability to recognize an invading pathogen and further activate host defense mechanisms. Plants have evolved various defense mechanisms, including activating both constitutive and inducible defense responses to combat the diseases. When pathogens are detected, immune receptors in plants recognize specific molecules that signal the activation of effective defense responses. Despite extensive research, details of host defense mechanisms that limit pathogenic infections have yet to be elucidated. The majority of defense responses are characterized by the transcriptional activation of a large number of genes (>1% of the genome), many of which have unknown functions [5, 6]. Pathogen identification activates signaling pathways that result in the formation of reactive oxygen species (ROS), protein kinases, phytohormones, phytoalexins, phenolic compounds and pathogenesis-related (PR) proteins, and eventually a hypersensitive response (HR). Production and accumulation of PR proteins, which are low molecular weight proteins, in plants during pathogen attack is vital [7]. In most plant species, nineteen families of PR proteins (PR-1 to PR-19) have been identified to date. The role of PR proteins in plant defense includes altering the integrity of pathogen and activating other defense pathways through the generation of elicitors.

Due to improvements in transformation techniques and isolation of numerous pathogenesis-related genes, plants can now be engineered to have effective and broad-spectrum resistance against pathogens. The transgenic approaches using PR genes have been proven to be efficient for obtaining pathogen resistance in plants [8, 9]. Several transgenic plants have been developed that offer varying degrees of protection against certain fungal and oomycete diseases.

This chapter overviews the PR proteins, including their classification, activation as defense signaling indicators, and mode of action against the pathogens. It also highlights the success and challenges of the transgenic approach using PR genes for disease resistance.


*Pathogenesis-Related Proteins and Their Transgenic Expression for Developing Disease-Resistant… DOI: http://dx.doi.org/10.5772/intechopen.106774*


*Case Studies of Breeding Strategies in Major Plant Species*

**Table 1.**

*Classification and properties of PR proteins.*

*Pathogenesis-Related Proteins and Their Transgenic Expression for Developing Disease-Resistant… DOI: http://dx.doi.org/10.5772/intechopen.106774*
