The Plants’ Trojan Horse

During an infection, the exchange of toxins occurs between pathogens and host cells, along with the involvement of complement proteins, bacterial adhesins, or fungal lectins. In the case of viral infections, one can observe the binding of viral antigens on the protein capsule to receptors on the host cell's surface membrane. This interaction allows the viral RNA and enzymes, such as reverse transcriptase, to take control of the host cell. Reverse transcriptase copies the viral RNA into a DNA copy, which is then translocated to the nucleus. Subsequently, the viral DNA is transcribed to produce new mRNA, instructing the rough endoplasmic reticulum to synthesize viral proteins and assemble new viral particles. This phenomenon is characteristic of retroviruses, and for those studying Year 12 Biology, a more in-depth explanation of this process will be covered in the immunity topic when examining HIV. Recently, it was discovered that fungi (which are classified as eukaryote and are ‘living’, unlike the viruses aforementioned) inject sRNAs into host cells to silence host genes that trigger an immune response by alerting nearby cells of an infection through the synthesis and release of signalling chemicals (see ref 2.). On the other hand, the reverse of this process - the transfer of RNAs from the host cell and into the pathogen - is less understood.

However, a study published a month ago (see ref 1.) proves that in response to infection by Botrytis cinerea (which causes grey mould disease on more than 1400 plant species, including the strawberries you left in your fridge for a month past their expiry date), Arabidopsis thaliana (which is the first plant to have its genome sequenced, and is used to explore the effects on/ responses of plants as this species has been researched in great detail) secretes mRNA thatis specific to fungi.

The results indicate the use of extracellular vesicles to transport the mRNA for endocytosis, where the mRNA then infiltrates the fungi (by diffusing across the phospholipid bilayer making up the cell-surface membrane by being packaged in lipid “bubbles” – see ref 1.), where the mRNA interferes with the fungal polysomes used for translation of the genetic material of the fungi. This prevents further fungi replication and limits growth of this infection to surrounding plant tissue and highlights how this RNA defence mechanism discovery is a new line of specific defence used by plants (that is different to the nonspecific mechanism of antimicrobial chemical secretion for example) This strongly supports the theory of “crosskingdom RNAi” (see ref 3.), which explores the co-evolutionary arms race between hosts and microbes, specifically looking at the transfer of RNAs to supress the host immune response by the pathogen and the transfer of RNAs to prevent the pathogen replicating any further by the host.

As the global population expands swiftly, the need for food is on the rise. Unfortunately, the impact of climate change, resulting in more frequent extreme weather events, has disrupted the production of essential crops like rice, especially in flood-prone coastal regions like India. Future research into how plants use mRNA to defend against fungi infections could allow us to genetically modify the mRNA, making them specific to a wider range of plant disease-causing pathogens and making these crops more resistant to disease and increasing future yield.

Keywords:

Antigen: a toxin or other foreign substance which induces an immune response in the body, especially the production of antibodies RNA: ribonucleic acid, a nucleic acid present in all living cells. Its principal role is to act as a messenger carrying instructions from DNA for controlling the synthesis of proteins, although in some viruses RNA rather than DNA carries the genetic information

Rough Endoplasmic Reticulum: series of connected flattened sacs, part of a continuous membrane organelle within the cytoplasm of eukaryotic cells, that plays a central role in the synthesis of proteins. The rough endoplasmic reticulum (RER) is so named for the appearance of its outer surface, which is studded with proteinsynthesizing particles known as ribosomes Endocytosis: the taking in of matter by a living cell by invagination of its membrane to form a vacuole.

Endocytosis: the taking in of matter by a living cell by invagination of its membrane to form a vacuole.

References [cited 2024 Jan 15]

1. Wang S, He B, Wu H, Abreu-GoodgerC, Birch P, Jin H, et al. Plant mRNAs move into a fungal pathogen via extracellular vesicles to reduce infection Home: Cell Press [Internet]. Cell.com. Published 2024 Dec 15 Available from: https://www.cell.com/cellhost-microbe/fulltext/S1931- 3128(23)00470-5#%20

1. WeibergA, Wang M, Lin FM, Zhao H, Zhang Z, KaloshianI, et al. Fungal Small RNAs Suppress Plant Immunity by Hijacking Host RNA Interference Pathways. Science [Internet]. 2013 Oct 3;342(6154):118–23. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/ PMC4096153/

1. Wang M, WeibergA, Lin FM, Thomma BPHJ, Huang HD, Jin H. Bidirectional crosskingdom RNAi and fungal uptake of external RNAs confer plant protection. Nature Plants. 2016 Sep 19;2(10). Available from: Bidirectional cross-kingdom RNAi and fungal uptake of external RNAs confer plant protection | Nature Plants1

Synoptic links :

GCSE: 4.3 – Infection & Response (looking at plant responses to infection by fungi and how this process of RNA injection into a foreign cell resembles the replication mechanism used by viruses)

A-Levels: 3.1.3 - Lipids (packaging of RNA into lipid “bubbles” that diffuse across fungi cell-surface membrane) 3.1.5.2 –DNA Replication (how a change in the mRNA template of the host cell results in the rough endoplasmic reticulum of the host cell producing viral proteins) 3.2.3 – Transport across cell membranes (endocytosis) 3.2.4 –Cell recognition and the immune system (exploring how viruses fuse only to specific cells, how the immune system response is triggered and how to suppress it, and the role of antigens/ antibodies)