Plant microRNAs: Sentinels in Plant Innate Immunity
Authors: Era Vaidya Malhotra, Anshul Watts, Nimmy M. S.

Plants have an elaborate immune system to defend themselves of invading pathogens. The plant innate immune system provides immunity either by pathogen-associated molecular pattern (PAMP) recognition by plant pattern recognition receptors (PTI: PAMP triggered immunity) or pathogen effector protein recognition by plant resistance genes (ETI: effector triggered immunity). Plants recognise certain highly conserved pathogen molecular motifs (PAMPs), by a set of cell surface receptors known as pattern recognition receptor (PRRs). Recognition of PAMPs leads to activation of MAP kinase signaling pathways and a general defense response is initiated within the plant system. Some pathogens suppress the PTI response and release some effector proteins into the plant cells. The second layer of plant defense comes into play in such situations, where some resistance (R) proteins encoded by resistance genes recognise the pathogen effectors and lead to a rapid and effective defense response.

MicroRNAs (miRNAs) are non-coding endogenous small RNA molecules, nearly 20 â€"24 nt in length, which play a critical role in post-transcriptional regulation of genes. Most of the mature miRNAs as well as the miRNA targets are evolutionarily conserved. MicroRNAs govern a wide array of plant growth and development processes and are also involved in the plants’ response to different biotic and abiotic stresses. Several plant miRNAs have been found to play a role in both PTI as well as ETI responses. The first microRNA discovered to be playing a role in plant defense was the Arabidopsis miR393. MiR393 is a part of the PTI response and is induced in response to a bacterial PAMP flg22. It targets the auxin transporters, leading to their degradation, thus suppressing auxin signalling in the plant cells and thereby suppressing bacterial growth. Several other miRNAs having a similar auxin targeting role and playing a part in PTI have been discovered, such as miR167 and miR160.

MiRNAs as part of the induced PTI act by targeting certain genes that are required for/stimulate pathogen growth. As is the case with miR393, such microRNAs once induced, negatively regulate the accumulation of the mRNA of their target genes. These target genes are host genes which augment pathogen spread within the host cells, and their suppression by miRNAs restricts pathogen proliferation. Several other studies have reported the induction of certain microRNAs which target genes that negatively regulate defense response, thus permitting defense response genes to be expressed. Also, there are several reports of suppression of microRNAs which target genes that positively regulate defense response on pathogen attack, thus again helping in smooth functioning of the defense cascade. Examples of such up- or down- regulated microRNAs include miRNAs miR159 and miR319.

As more and more research is carried out to understand the molecular mechanisms of plant immune response, role of microRNAs is slowly being discovered. Several evidences are emerging which suggest that these pathogen-responsive small RNAs regulate gene expression reprogramming on encountering a potential pathogen. Some miRNAs may bring about an immune response by post-transcriptional gene silencing, translational repression or transcriptional gene silencing by direct DNA methylation or chromatin modification.

References:

1. Li Y, Zhang Q, Zhang J, Wu L, Qi Y, Zhou J. 2010. Identification of microRNAs involved in pathogen-associated molecular pattern-triggered plant innate immunity. Plant Physiology, 152(4): 2222-2231.
2. Baldrich P, San Segundo B. 2016. MicroRNAs in rice innate immunity. Rice, 9:6 doi: 10.1186/s12284-016-0078-5.
3. Li ZU, Jing X, Chen Z, Yu Y, Li QF, Zhang, YC, Zhang, JP, Wang CY, Zhu XY, Zhang W, Chen YQ. 2016. Large-scale rewiring of innate immunity circuitry and microRNA regulation during initial rice blast infection. Scientific Reports, doi:10.1038/srep25493.




About Author / Additional Info:
Scientist at National Research Centre on Plant Biotechnology (NRCPB), Pusa Campus, New Delhi