Correct option is D
Explanation-
‘R’ genes (Resistance genes) in plants encode resistance (R) proteins that detect specific pathogens and activate defense mechanisms.
A common structure of many R proteins is the NBS-LRR domain:
1. NBS = Nucleotide Binding Site → involved in ATP/GTP binding and hydrolysis.
2. LRR = Leucine-Rich Repeat → involved in protein-protein interactions, especially recognizing pathogen effectors.
These domains are cytoplasmic and part of a large class of R proteins called NBS-LRR proteins.
Option a - RPS2 protein of Arabidopsis
Full name: Resistance to Pseudomonas syringae 2
Class: NBS-LRR
Recognizes AvrRpt2, a type III effector protein from Pseudomonas syringae.
Function: After recognition, RPS2 triggers a hypersensitive response (HR) to stop pathogen spread.
It contains NBS-LRR domain
Option b - Xa1 protein of rice
Xa1 confers resistance to Xanthomonas oryzae pv. oryzae, the bacterial blight pathogen. It is found in rice, functions through recognition of bacterial effectors. It contains NBS-LRR domain
Option c - N protein of tobacco
Confers resistance to Tobacco Mosaic Virus (TMV). It belongs to TIR-NBS-LRR subclass (TIR = Toll/Interleukin-1 receptor-like domain). It activates localized cell death upon virus detection. It contains NBS-LRR domain .
Option d - Mlo protein of barley
This is very different from others . It is not an NBS-LRR protein. It is a seven-transmembrane domain protein, localized in the plasma membrane. Resistance occurs via loss-of-function mutations in the Mlo gene. These mutations prevent the fungus Blumeria graminis (causing powdery mildew) from colonizing cells.
Thus, Mlo-mediated resistance is recessive, unlike dominant NBS-LRR-based resistance.
Final answer : option d - Mlo protein of barley
