Background

The globalization of agriculture has led to plants being cultivated in areas far from the pathogens that originally coevolved with them. Therefore the evolution of resistance is unlikely to have kept pace with the evolution of pathogen species. If these new pathogen strains migrate to the new cultivation areas, high losses can be expected due to susceptibility. A narrow genetic base like in vegetative propagated plants will enhance this effect [1]. This has been seen historically in banana plantations. In the 1960s, the banana export depended on the Gros Michel cultivar. However, this species was largely eradicated by the race 1 strain of Panama disease that is caused by Fusarium oxysporum f. sp. cubense (Foc). Over a period of just 50 years 40,000 ha became unproductive for banana production [2]. As commercially grown bananas are largely sterile clones with a triploid genome, traditional breeding techniques have proven inadequate to develop resistant cultivars [3].

Foc is a soil-borne pathogen forming fungicide resistant, thick-walled chlamydospores that can survive in the soil for decades. This causes above ground application of fungicides to be ineffective. Foc enters the vascular system through the roots and causes wilting and discoloration by vessel plugging eventually the plant will dye [4] (Figure 1). The fungus secretes phytotoxins, including fusaric acid, that are able to diminish the vitality of plant cells enabling colonization of the plant by Foc [5].

Back in the 1960s the resistant Cavendish cultivar was identified just in time, replacing Gros Michel as the primary banana species for export [6]. In 1992 a new variant of Foc, tropical race 4 (TR4) was discovered in Sumatra and peninsular Malaysia in Cavendish plantations. Levels have since been seen to rise and to spread throughout the world [7] (Figure 2). In 2013 the presence of race 4 was announced in Jordan, leaving only 20% of the Jordan Valley production area disease free. The biggest banana exporting counties located in Latin America, the Caribbean or West Africa are yet to be affected by this destructive disease but the rapid expansion draws a grim picture for the future of the banana [8, 9]. Further spread of tropical race 4 will have devastating effects for Cavendish plantations, and other cultivars, including many cooking and dessert bananas. Considering the high susceptibility of several banana cultivars to this almost uncontrollable disease, it has been estimated that 80% of the worldwide production is threatened [10]. The iGEM team Wageningen 2014 aims to prevent a repetition of the devastating effects that race 1 had in the 1960s and to introduce a system into agriculture that will ensure food safety. This system is called BananaGuard.

References

1. Strange RN., Scott PR. (2005) Plant Disease: A Threat to Global Food Security. Annual Review of Phytopathology Vol. 43: 83-116
2. Stover RH (1972) Banana, Plantain and Abaca Diseases. Wallingford, UK: CAB International, 316 pp
3. Ortiz, R., Ferris, RSB., & Vuylsteke, DR. (1995). Banana and plantain breeding. In Bananas and plantains (pp. 110-146). Springer Netherlands.
4. Dong, X., Ling, N., Wang, M., Shen, Q., & Guo, S. (2012). Fusaric acid is a crucial factor in the disturbance of leaf water imbalance in Fusarium-infected banana plants. Plant Physiology and Biochemistry, 60, 171-179.
5. Berestetskiy, A. O. (2008). A review of fungal phytotoxins: from basic studies to practical use. Applied Biochemistry and Microbiology, 44(5), 453-465.
6. Ploetz, RC. (2005) Panama disease, an old nemesis rears its ugly head: part 1, the beginnings of the banana export trades. Plant Health Prog. doi, 10, 1094.
7. Ploetz, RC. (1994) Panama disease: Return of the first banana menace." International Journal of Pest Management 40(4): 326-336.
8. García-Bastidas F., Ordóñez, N., Konkol, J., Al-Qasim, M., Naser, Z., Abdelwali, M., Kema, G. H. J. (2014). First report of Fusarium oxysporum f. sp. cubense Tropical Race 4 associated with Panama disease of banana outside southeast Asia. Plant Disease, 98(5), 694-694.
9. Ploetz, RC. (2006) Panama disease, an old nemesis rears its ugly head: part 2, the cavendish era and beyond. Plant Health Progress
10. Ploetz, RC. (2005) Panama disease, an old nemesis rears its ugly head: part 1, the beginnings of the banana export trades. Plant Health Prog. doi, 10, 1094.