A special protein in rice to protect it against viral infection

Srabani Roy Chowdhury (MAKAUT, WB)

Plants possess many mechanisms to safeguard themselves from different pathogenic infections. There is a two-tiered system deployed by plants, as their defense response. They are-

• Pathogen-Associated Molecular Pattern (PAMP) triggered immunity (also called PTI). It is considered to be a weak immune response, brought into action by plasma membrane-localized pattern recognition receptors. They perceive and recognize specific molecular patterns like damage-associated molecular patterns (DAMPs), microbe pathogen-associated molecular patterns (MAMPs), etc.
• Effector Triggered Immunity (also called ETI). It is a race-specific disease immune response mediated by the R gene.

There are various plant cell receptors present on the cell surface. They can be Receptor-like Kinases (RLK) and Receptor-like Proteins (RLP). Receptor-like Kinases have a transmembrane domain, an extracellular ligand-recognition domain (leucine-rich repeats or LRR), and an intracellular kinase domain. Receptor-like Proteins lack the kinase domain. So, they associate with other proteins with a kinase domain. Both RLP and RLK are equally necessary for the plant’s innate immunity development.

OsRLP1 and OsSOBIR1:

Viral inoculation assays in rice plants show that OsRLP1 and OsSOBIR1 present in rice take part in immunity against viral infection. Rice Black Streaked Dwarf Virus or RBSDV is a pathogen that mainly attacks rice and maize. It is transmitted by a small brown planthopper. It causes growth abnormalities like dwarfism. Darkening of leaves and reduction in yield can be seen in infected rice plants. OsRLP1 is a LRR gene of an RLP and OsSOBIR1 is a LRR gene of RLK. OsRLP1 interacts with OsSOBIR1. It is found to activate Mitogen-Activated Protein Kinases or MAPKs which trigger the gene expression related to PTI.

What did the study find?

Studies provide insights about the receptor-like protein associated with its adaptor kinase that induces immunity in rice against viral infection.

• RT-qPCR assays confirmed that there was a significant increase in the up-regulation of the OsRLP1 gene in an RBSDV infected plant.  OsRLP1 encodes a protein consisting of 980 amino acids. It had an N-terminal signal peptide, a transmembrane domain, and multiple LRR domains. However, it lacked a C-terminal cytoplasmic kinase catalytic domain.
• RT-qPCR and western blot assays of the OsRLP1 gene in transgenic plants were also conducted. The results showed that they were not only less susceptible but also had a reduced viral load compared to the wild type. Thus, increased resistance against the RBSDV was present.
• RNA high-throughput sequencing was studied to prove the requirement and mechanism of the OsRLP1 gene for defense against RBSDV infection.
• Analysis of differentially expressed genes revealed high defense response, acquired resistance, peroxidase activity, and chitin metabolism. This proves that OsRLP1 regulates the gene expression of the defense system and affects the interaction between the RBSDV and the rice plant.
• The expression of PTI genes was studied and to check whether the OsRLP1 gene induces PTI response or not. Results showed significant PTI gene expression.

Receptor-like Proteins lack the cytoplasmic kinase domain and require to attach to some other component to fulfill the need for the cytoplasmic domain. Protein homology studies state that an LRR receptor-like kinase of rice named OsSOBIR1 contains kinase activity. It is a plasma membrane-associated kinase. Co-immunoprecipitation assays showed strong interaction of OsRLP1 and OsSOBIR1 in protoplasts of rice. This interaction also participates in activating PTI response. Studies also show that the removal of the OsSOBIR1 gene makes the plant more susceptible to RBSDV infection.

Conclusion:

Receptor-like Proteins play an important role in defense against viral infections in plants. The studies concluded that OsRLP1 along with OsSOBIR1 is needed for activation of PTI response against antiviral diseases. The interaction of OsRLP1 and OsSOBIR1 induces the formation of a signaling receptor complex which helps in the activation of PTI response and provides resistance against viral infection. This study provides primary evidence of an RLP associating with its adaptor kinase OsSOBIR1 to induce immunity against RBSDV infection.

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Reference-

1. Zhang, H., Chen, C., Li, L., Tan, X., Wei, Z., Li, Y., Li, J., Yan, F., Chen, J., & Sun, Z. (2021). A rice LRR receptor-like protein associates with its adaptor kinase OsSOBIR1 to mediate plant immunity against viral infection. Plant Biotechnology Journal, pbi.13663. https://doi.org/10.1111/pbi.13663

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