Pathogenesis-Related Proteins (PRs) with Enzyme Activity Activating Plant Defense Responses

Abstract

Throughout evolution, plants have developed a highly complex defense system against different threats, including phytopathogens. Plant defense depends on constitutive and induced factors combined as defense mechanisms. These mechanisms involve a complex signaling network linking structural and biochemical defense. Antimicrobial and pathogenesis-related (PR) proteins are examples of this mechanism, which can accumulate extra- and intracellular space after infection. However, despite their name, some PR proteins are present at low levels even in healthy plant tissues. When they face a pathogen, these PRs can increase in abundance, acting as the first line of plant defense. Thus, PRs play a key role in early defense events, which can reduce the damage and mortality caused by pathogens. In this context, the present review will discuss defense response proteins, which have been identified as PRs, with enzymatic action, including constitutive enzymes, β-1,3 glucanase, chitinase, peroxidase and ribonucleases. From the technological perspective, we discuss the advances of the last decade applied to the study of these enzymes, which are important in the early events of higher plant defense against phytopathogens.

Keywords: plant protection; plant–pathogen interaction; postformed mechanism; preformed mechanism; signaling pathways.

Figure 1

Initial plant defense response in plant–pathogen interaction. At first, the pathogen is recognized on the plant cell wall surface. Then, the elicitors activate a signaling network, where defense genes are activated to produce PR proteins that accumulate and act in the degradation of the pathogenic cell (i.e., β-1,3-gluc and chit; the degradation products of these enzymes can also act as elicitors). The oxidative buster mediates the generation of ROS in an attempt to limit the spread of the pathogen. PRs (i.e., β-1,3-glucanases, chitinases, PRXs, PR10 together with Phyx) are able to induce a hypersensitivity response to prevent the spread of the pathogen to other tissues, releasing elicitors that induce the plant’s defense mechanism. PR10, RBPs and RIPs are also produced by the plant, mediating virus infection. NB-LRR and PR10 act together in gene defense induction. PRX, peroxidase; PRs, pathogen-related proteins; Chit, chitinase; ROS, reactive oxygen species; Phyx, phytoalexin; β-1,3-gluc, β-1,3-glucanases; Ethy, ethylene; JA, jasmonic acid; LRR-RLK, leucine-rich repeat receptor-like protein kinase; NB-LRR, nucleotide-binding and leucine-rich repeat; RBPs, RNA-binding proteins; RIPs, ribosome-inactivating proteins.