Fungal effectors: An apocalypse weapon that threaten millions of plant lives

Fig1. Disease symptoms caused by phytopathogenic fungi with different lifestyles: (a) Botrytis cinerea infecting tomato (b) Sclerotinia sclerotiorum infecting rapeseed (c) Uromyces viciae-fabae infecting bean (d) Blumeria graminis infecting barley (e) Ustilago maydis infecting maize, ( f ) Cladosporium fulvum infecting tomato (g) Colletotrichum higginsianum infecting mustard spinach, and (h) Magnaporthe oryzae infecting rice.(Lo Presti et al., 2015)  

Some of the most common fungi that we know are the ones that we eat, also known as mushrooms, which are just the fruiting bodies of the complex network of fungi and its hyphae present under the topsoil. The other kind of fungus that we come across is the mold that we see on bread, which makes it unfit to eat. Such is the diversity in the fungus, and a similar case is seen in the case of fungi that infect plants. 

Fig2. Necrotrophic fungi such as Botrytis cinerea generally grow subcuticularly and kill epidermal cells by secreting toxic metabolites and proteins. Their hyphae eventually replace large parts of the plant epidermis. Both early and late developmental stages are shown.(Lo Presti et al., 2015) 

Plant infecting fungus can be broadly classified into pathogenic and endophytic. Pathogenic fungus affect the plant in a negative way and hence activate the plant immune response system on the other hand, endophytic fungi live in a sort of symbiotic relationship with the plant. 

Fig3. Obligate biotrophic pathogens such as rust fungi (Uromyces viciae-fabae) undergo a complex series of developmental steps and eventually form a balloon-shaped feeding structure.  (Lo Presti et al., 2015) 

To suppress plant defenses, pathogenic fungi produce secreted proteins called effectors, which are highly regulated since they are only expressed during certain time points post infection. These effectors are specific to hosts and hence facilitate infection on particular host plants. Plants have evolved to combat these effectors using the effector triggered immunity (ETI) pathways. 

Fig4. The obligate arbuscular mycorrhizal root symbiont Rhizophagus irregularis colonizes individual cortical cells with highly branched feeding structures called arbuscules Endophytes can colonize either plant roots (Piriformospora indica) or the aerial plant organs and can grow either intracellularly or intercellularly.(Lo Presti et al., 2015) 

One might wonder why is it an apocalypse weapon if it can be easily prevented by plant defense systems. This is because these effectors are located on relatively flexible regions of the genome. Flexible meaning that these regions are either gained through horizontal gene transfer (HGT) or lost through gene deletion based on the host preference of the particular fungal species. On agricultural land which grow a single species of plants these fungi have devastating effect due to easy HGT thus destroying the entire field of crops.

Fig5. Hemibiotrophic fungi such as Colletotrichum spp. initially develop bulged biotrophic invasive hyphae that later change into thin necrotrophic hyphae. Both biotrophic and necrotrophic phases are shown. Hyphae are shown in blue (nonpathogenic fungi) or violet (pathogenic fungi).(Lo Presti et al., 2015) 

There have been some interesting studies recently in understanding what exactly is happening in the soil. One such study looked into the concept of "cry for help" performed by plants when they are under attack by the fungi. The cool thing is that plants are also able to attract species such as pseudomonas which can suppress the fungi growth. Although, easier said than done, to fill the soil with large number of such bacteria takes many plant generations. But recruiting such beneficial microbes, the soil now remains high quality fungal immune.

Fig6. Figures represent infection assays or from left to right: Mock-inoculated plants were treated with sterile water, plants infected with the wild type strain of the fungus and plants infected fungi where the effect. However, when tomato plants were grown on sterile substrate, no difference could be observed between tomato plants inoculated with wild-type V. dahliae or with the Av2 deletion strain, showing that Av2 only contributes to virulence in the presence of a microbiota. This finding suggests that microbiota manipulation is the genuine virulence function of the Av2 effector, and that the effector lacks plant virulence targets. (Kraege A et al., 2026)

Ofcourse in this post we are not here to talk about how cool plant defenses are but how evil fungi are. Verticillium dahliae is an asexual fungus that infects a huge host range. Tis fungi produces effectors which have antimicrobial properties, as mentioned these effectors are produces at the onset of infection and hence this antimicrobial effect is not for survival but to make sure that "cry for help" effect is completely nullified. Many such effectors were seen to be expressed mostly at the late infection stage when the plant is already wilting and immune response fade.

Fig7.  Relative abundance of bacterial phyla shows increased Proteobacteria abundance in plants inoculated with the resistance gene deletion strain. (Kraege A et al., 2026)

Plants still put a decent fight against them, some plant species do possess a gene that can provide against such fungal activity, however only two genes have been found so far that can confer such defenses and this is present in very few plants.

In conclusion, these small organisms which mostly go unnoticed by a common man has a gigantic impact on plants. Many institutions exist that are working to figure out a way to suppress such organisms to prevent yield loss, but fungi are very diverse and a soil contains a complex network of microorganisms, and hence preventing one could affect the entire soil quality. Many companies are actively working on bringing out fungi resistant breeds of plants that could save a farmers yield. Regardless, some fungi like all other organisms evolving constitutively remain to be a weapon of mass destruction.

REFERENCES
1. Lo Presti, L., Lanver, D., Schweizer, G., Tanaka, S., Liang, L., Tollot, M., Zuccaro, A., Reissmann, S., & Kahmann, R. (2015). Fungal Effectors and Plant Susceptibility. Annual Review of Plant Biology, 66(1), 513–545. https://doi.org/10.1146/annurev-arplant-043014-114623

2. Kraege, A., Punt, W., Doddi, A., Zhu, J., Schmitz, N., Snelders, N. C., & Thomma, B. P. (2026). Undermining the cry for help: the phytopathogenic fungus Verticillium dahliae secretes an antimicrobial effector protein to undermine host recruitment of antagonistic Pseudomonas bacteria. New Phytologist, 249(1), 406-417.