Pathogenesis related proteins are the class of proteins produced by plants against pathogen attack. These proteins play a crucial role in plant defence against pathogen. Expression of PR genes is not only due to pathogen attack but also takes place by abiotic stress such as wounding, osmotic stress, UV light and oxidation stress.
Several plant parts such as root flowers, and cultured cells are committed for active PR gene expression. Signal transduction in plants defence is directly related to PR genes. A number of molecules derived from pathogen during host-pathogen interactions are directly responsible for the synthesis of PR proteins.
Several elicitors have known to trigger PR proteins in plant. Some of the classic example of elicitors is chitin fragments, and glucan derived from fungal cell walls. Certain fungal species release peptide, glucoproteins, and oligosaccharides from bacteria and Avr-proteins from bacterial and fungal pathogen elicite resistance response.
One of the well characterised elicitor is glucan elicitor (GE) released from Phytopthora sojae cell wall by β-1, 3 glucanase. Infection of parsley plant by the pathogen phytopthora sojae resulted in opening of ion channels, expression of PR and other defence genes. In addition, pathogen phytopthera sojae secreted 42 kDa glycoprotein and shown to exhibit elicitor activity.
Another class of elicitor is the polypeptide encoded by pathogen avirulence (avr) gene. In this mechanism a pathogen contains a particular avr gene is recognised by the host plant that carries a corresponding resistance (R) genes and trigger disease resistance mechanism.
Activation of PR genes is influenced by various noticeable pathways. In response to infection plant often exhibit enhanced production of reactive oxygen species (ROS), salicylic ads (SA) and ethylene. Jasmonate acts as signal molecule in the induction of PR genes. These are also serve as secondary signals to activate plant defence i.e., Jasmonate activate genes encoding defensins, protease inhibitor and basic PR proteins.
The gaseous harmone ethylene and Jasmonate together induces expression of osmotin, PRI-b in tobacco and pin 2 in tomato respectively. Regulation of PR genes often takes place at transcriptional level. Number of cis- regulatory elements facilitating pathogen induced PR genes expression has been characterized. These are W-box, GCC box, PR box and G-box.
Promoter of number of tobacco basic PR genes contain GCC box, recognised as ethylene responsive element. Most of the PR genes are known to contain GCC box with 5′ AGCC GCC sequence. The 140 bp GCC box present in the osmotin promoter is responsible for most of the stimuli. Similarly PRI- and PR2 genes contain W-box and also present in tobacco CHN50, asparagus AOPRI and potato PR-10.
Overexpression of PR Proteins:
Most effective and desirable strategy for disease resistance in plants has been the deployment of resistant genes into commercially acceptable cultivars. Transformation of plants with resistant genes provides disease resistance. For example, the ‘R’ genes pto, and Xa2 of tomato confer resistance to fungal and bacterial pathogens on tomato and rice, respectively.
However, the resistance showed by these R genes is specific to particular host. Isolation of genes for resistance to several pathogens facilitates gene pyramiding strategies for the practical deployment of resistance genes. The relative merits of the two strategies can be analysed for the deployment of ‘R’ genes to pathogens of vegetable crops like Brassica.
