By: Akshith Ragukumar
MicroRNA (miRNA) plays an important role in plant communication. miRNA trafficking has been described between the plants and the host for communication. It is found that the presence of miRNA in the phloem sap helps transport the miRNA to the entire plant and plays an important role in long distance signaling. It is interesting to me that the plants can take up the miRNA from the neighboring plants and the plant-to-plant miRNA transfer was discovered in the well known miR156/SPL modules.
Figure 1: Images of the control plant on (b) and ds-miR156 treated plant is on (c). Magnified seedlings from b (d) and c (e) displaying adventitious roots (in red). (f) represents the transcript level of SPL3, SPL9 and SPL10 extracted from roots (source).
For a long time, miRNAs were believed to be transcriptional regulators that are generated and active in a single cell or a collection of nearby cells that are linked by plasmodesmata. However, the discovery of miRNAs in the phloem sap suggested that these molecules may act as long distance signaling molecules and be transmitted throughout the entire plant. Analyses demonstrating the various miRNA compositions in phloem sieve elements and adjacent tissues provided support for these hypotheses. Therefore, particular miRNAs may be added to phloem sap, which is then delivered to the various plant organs.
miRNA in gene silencing:
miRNA are 20–22 nt long, which control the posttranscriptional gene silencing to control gene expression in the majority of eukaryotes. Messenger RNA (mRNA) degradation or its translation inhibition are two ways that RNA interference (RNAi) modifies gene expression. Translation inhibition can still occur with moderate levels of sequence complementarity, but mRNA target slicing needs high levels of complementarity between the miRNA and the target region. However, some plant miRNAs with perfect or nearly perfect complementarity to a single target site can primarily suppress translational mRNA production. Since target mRNA degradation is the preferred RNAi pathway in plants due to the wide sequence complementarity between plant miRNAs and their targets, this phenomenon is known as ‘plant miRNAs’ (Loreti, E. and Perata.,2022). Plant development activities, including the patterning of the embryo, meristem, leaf, and flower, as well as the plant’s reaction to biotic and abiotic stressors, depend heavily on miRNAs.
The identification of host-induced gene silencing (HIGS) has led to the creation of innovative tools for controlling plant diseases. Gene-targeting sRNAs or dsRNAs for pathogens or pests are expressed in genetically modified plants in HIGS methods. These sRNAs are delivered into the pathogen or pest and use cross-kingdom RNA trafficking to quiet the virulence genes, hence enhancing plant disease resistance. In order to fight diseases like nematodes, viroids, viruses, insects, and fungus, numerous plant species have been modified, ranging from laboratory plants to commercial crops.(Loreti, E. and Perata.,2022)
miRNA signaling from plant to plant:
It has been reported that, between the parasitic plants and their host, the trans-species miRNA trafficking occurs naturally. It can be seen from the experiment with the parasitic plant Cuscuta that it uses haustoria to obtain water and nutrients from its host plant. When Cuscuta campetris parasitize Arabidopsis and tobacco, a significant number of 22 nt long miRNAs are induced at the haustorium. In order to promote the synthesis of secondary siRNAs and the consequent degrading of host mRNAs, these miRNAs can commandeer the silencing apparatus of host plants (Loreti, E. and Perata.,2022). A role for miRNAs in plant-to-plant communication is possible given that some miRNAs are mobile molecules within the plant and can move between the plant and its pathogen as well as between parasitic plants and their host plants.
Figure 2: sRNA trafficking between parasitic plants and their host (Source)
MiRNAs are selectively absorbed by the roots before being transferred into the xylem. Fluorophore labeled miR399 was used to demonstrate this, showing the ability of a chemically synthesized pure miRNA to penetrate circulatory tissue and travel to distant tissue. It is believed that the exosome mediated transport was necessary. But this and other data shows that the possibility that exosome mediated transport of exogenous miRNAs to plant roots is not necessary.
Conclusions:
In conclusion, plant communication is greatly influenced by miRNA between the host and the plants. Its been shown that the presence of miRNA in the phloem sap facilitates the delivery of miRNA to the entire plant and is crucial for long-distance transmission. The ability of plants to absorb miRNA from nearby plants is very fascinating. miRNA has a significant impact on plant-host communication since it supports both physiological and pathological processes such development, cell differentiation, and proliferation.
Citations and useful links:
Loreti, E. and Perata, P. (2022), Mobile plant microRNAs allow communication within and between organisms. New Phytol, 235: 2176-2182. https://nph.onlinelibrary.wiley.com/doi/full/10.1111/nph.18360
Marek Marzec, MicroRNA: a new signal in plant-to-plant communication, Trends in Plant Science, Volume 27, Issue 5, 2022, Pages 418-419, ISSN 1360-1385, https://www.sciencedirect.com/science/article/pii/S136013852200005X
Betti, F., Ladera-Carmona, M.J., Weits, D.A. et al. Exogenous miRNAs induce post-transcriptional gene silencing in plants. Nat. Plants 7, 1379–1388 (2021). https://www.nature.com/articles/s41477-021-01005-w