Pathogenesis-related protein

Introduction

Pathogenesis-related proteins (PR proteins) are a group of plant proteins that are induced by various pathogens, such as viruses, bacteria, and fungi, as well as by different stress conditions like wounding, temperature extremes, and heavy metal toxicity. They are part of the plant's systemic acquired resistance (SAR) mechanism, a type of plant defense against disease.

Classification

PR proteins are classified into 17 families (PR-1 to PR-17) based on their amino acid sequences, serological relationships, and biological activities. Each family is characterized by unique structural and functional properties. For instance, PR-1 proteins are markers of SAR and are often associated with fungal resistance, while PR-2 proteins are known for their β-1,3-glucanase activity, which is crucial in the degradation of fungal cell walls.

Structure and Function

PR proteins are generally small, with a molecular weight ranging from 6 to 43 kDa. They are rich in glycine, cysteine, and aromatic amino acids. Some PR proteins have enzymatic activities, such as chitinases (PR-3, PR-4, PR-8, and PR-11), β-1,3-glucanases (PR-2), and proteases (PR-6). These enzymes can degrade the cell walls of pathogens, thereby inhibiting their growth and spread.

Other PR proteins, like PR-1, PR-5, and PR-12, do not have enzymatic activities but can still inhibit pathogen growth. PR-1 proteins, for instance, can bind to sterols in fungal cell membranes, leading to membrane disruption and cell death. PR-5 proteins, also known as thaumatin-like proteins, can permeabilize fungal cell membranes. PR-12 proteins, on the other hand, can bind to and sequester iron, a nutrient essential for pathogen growth.

Regulation

The expression of PR proteins is regulated at the transcriptional level by various signaling molecules, such as salicylic acid (SA), jasmonic acid (JA), and ethylene (ET). SA is particularly important in the induction of PR-1, PR-2, and PR-5 proteins, while JA and ET are crucial for the expression of PR-3, PR-4, and PR-12 proteins. These signaling molecules are produced in response to pathogen attack and activate the transcription of PR genes through the action of specific transcription factors.

Role in Plant Defense

PR proteins play a crucial role in plant defense against pathogens. They can directly inhibit pathogen growth through their enzymatic activities or through other mechanisms like membrane disruption and iron sequestration. Moreover, some PR proteins can induce the production of reactive oxygen species (ROS) and nitric oxide (NO), which are toxic to pathogens.

In addition to their direct antimicrobial activities, PR proteins can also enhance the plant's defense response. For instance, they can stimulate the production of defense-related signaling molecules like SA, JA, and ET. They can also induce the expression of other defense-related genes, leading to a heightened state of defense readiness known as priming.

Applications in Agriculture

Due to their role in plant defense, PR proteins have potential applications in agriculture. They can be used as biomarkers for disease resistance in plant breeding programs. Moreover, the genes encoding PR proteins can be transferred to crop plants through genetic engineering to enhance their resistance against pathogens. For instance, transgenic plants expressing chitinase and β-1,3-glucanase genes have been shown to exhibit increased resistance against fungal pathogens.

References

1. Van Loon LC, Rep M, Pieterse CM. Significance of inducible defense-related proteins in infected plants. Annu Rev Phytopathol. 2006;44:135-62. 2. Sels J, Mathys J, De Coninck BM, Cammue BP, De Bolle MF. Plant pathogenesis-related (PR) proteins: A focus on PR peptides. Plant Physiol Biochem. 2008;46(11):941-50. 3. Breen S, Solomon PS. The world according to GARP transcription factors. Front Plant Sci. 2018;9:1712.