Fusarium xylarioides, a soil-borne fungus causes “Coffee wilt disease” that has a history of causing significant damages to coffee crops. There were outbreaks of the disease in 1920s which were managed appropriately. However, the disease re-merged in due course leading to outbreaks that caused severe crop damages. Earlier studies had indicated that the causative fungal specie may have evolved by acquiring genes from related specie. A study published on 5 December 2024 has confirmed that the re-emergence of outbreaks of coffee wilt disease was due to horizontal gene transfers from the related fungal species Fusarium oxysporum to the causative specie Fusarium xylarioides which allowed the causal fungal species to evolve and acquire suitable traits to infect the crops leading to re-emergence of outbreaks and damage to coffee plants.
In genetic engineering, new gene or DNA is artificially transferred into an organism’s cell using vectors such as plasmids or modified viruses for introducing new capability to an organism.
In nature, gene transfer or transmission of genetic information takes place in reproduction vertically from parents to offsprings down generations. This is a common feature in eukaryotes which enables them to acquire variation for adaptation and evolution. In prokaryotes such as in bacteria, however, genetic materials are transferred horizontally (or laterally) between organisms of the same generation without involving reproduction. This is called horizontal gene transfer (HGT) and is the only way bacteria can acquire new genes to adapt to negative selection pressures and evolve for survival. This can happen by transfer of DNA from the environment and its integration into bacterial chromosome or plasmid (transformation). Genes can also be transferred horizontally from one bacterium to another by bacteria-infecting virus or bacteriophages (transduction), or by direct horizontal transfer of genes from a donor bacterial cell to a recipient cell via sex pilus (conjugation).
Though predominantly observed in prokaryotes, horizontal gene transfer is associated with eukaryotes as well. Endosymbiosis is known to have played a role in eukaryotic evolution through bacteria–eukaryote gene transfer. Several instances of eukaryote –eukaryote gene transfer have been documented.
The phenomenon of horizontal gene transfer is significant for it contributes to evolution. For example, This is responsible for development of antibiotic-resistant/multidrug resistant strains of bacteria which is a major public health issue. In agriculture, role of horizontal gene transfer between related fungal species has long been suspected in re-emergence of outbreaks of coffee wilt disease.
Coffee Wilt Disease
Coffee is an important commercial crop. Its global market size is estimated to be about $223 billion. The coffee plant belongs to genus Coffea. It has many species, but Arabica and Robusta species are the most popular accounting for most of the global production. Coffea arabica accounts for 60–80% of the world’s coffee production, while Coffea canephora (commonly known as Coffea robusta) accounts for about 20–40%.
Coffee wilt disease is caused by a soil-borne fungus Fusarium xylarioides which gains entry through roots of the crop to colonise the xylem degrading the cell walls for nutrients. It blocks water uptake leading to wilting of the plants. The related fungus Fusarium oxysporum also is a soil-borne pathogen that spreads through infested soil and is responsible for wilting disease in many crops such as Panama disease in bananas, tomato vascular wilt etc. F. oxysporum lives on other plants (such as banana) intercropped with coffee for shade but shares coffee as a host with F. xylarioides.
Since the 1920s, the coffee crops in Africa have suffered periodic outbreaks of wilt disease with damaging impact on coffee production and farmers’ livelihood, particularly in Ethiopia and central Africa. The earliest outbreaks in the 1920s were successfully managed using appropriate means however the disease re-emerged in 2000s. Did the causative fungus Fusarium xylarioides undergo evolution after the initial outbreaks in 1920s so as to enhance ability to infect coffee plants leading to re-emergence of outbreaks? There were indications from studies that F. xylarioides acquired genes to enhance ability to infect.
Historical genomics study published in 2021 supported the idea that the arabica and robusta coffee plants partly acquired distinct effector genes via horizontal transfer from F. oxysporum. Effector genes encode molecules involved in disease establishment. These genes are expressed throughout the lifecycle of fungi to support disease process.
In the recent study published on 5 December 2024, researchers conducted comparative genomic analysis of 13 historic strains of F. oxysporum to understand how wilt disease causing fungus evolved and adapted to its host coffee plants. It was found that F. Xylarioides has four distinct lineages: one adapted to arabica coffee plants, one adapted to robusta coffee plants, and two historic lineages that lived on related coffee species. Further, these strains had acquired critical genes from the related F. oxysporum, which enabled the disease-causing F. xylarioides to breakdown the cell walls of coffee plants to cause wilt disease. The eukaryote-eukaryote horizontal gene transfer from F. oxysporum to F. xylarioides allowed the former to infect coffee plants effectively making re-emergence of Coffee wilt disease possible.
This understanding of how the disease is caused may be helpful in fine-tuning agricultural practices and managing plant diseases more effectively.
***
References:
- University of Colorado Denver. Horizontal Gene Transfer – activity-guide. Available at https://www.ucdenver.edu/docs/librariesprovider132/a-sync_sl/genetics/upload-2/bacterial-genetics/horizontal-gene-transfer-activity-guide.pdf
- Keeling, P., Palmer, J. Horizontal gene transfer in eukaryotic evolution. Nat Rev Genet 9, 605–618 (2008). https://doi.org/10.1038/nrg2386
- Peck, L.D., et al. Historical genomics reveals the evolutionary mechanisms behind multiple outbreaks of the host-specific coffee wilt pathogen Fusarium xylarioides. BMC Genomics 22, 404 (2021). Published: 04 June 2021. DOI: https://doi.org/10.1186/s12864-021-07700-4
- Peck L.D., et al. Horizontal transfers between fungal Fusarium species contributed to successive outbreaks of coffee wilt disease. PLoS Biology. Published: 5 December 2024. DOI: https://doi.org/10.1371/journal.pbio.3002480
***
