Background

The globalization of agriculture has led to plants being cultivated far from the pathogens that coevolved with them. Therefore resistance to novel arising strains is unlikely to evolve. If those strains migrate to the new cultivation area, high losses can be expected due to susceptibility. A narrow genetic base like in vegetative propagated plants will enhance this effect [1]. An example for this dynamic represents the banana species Cavendish, which is currently under threat by the Panama disease. In the sixties already, the world had to face this disease, caused by the fungus Fusarium oxysporum f. sp. cubense (Foc). Back then, the banana export trade was based on a cultivar called Gros Michel. This cultivar was almost completely wiped out by the so called race 1 of Foc. Over a period of just 50 years 40,000 ha became unproductive for banana production [2]. Commercially grown bananas are largely sterile clones with a triploid genome, making traditional breeding for a resistant cultivar difficult [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). During colonization the fungus will secret phytotoxins including fusaric acid that are able to diminish the vitality of plant cells render colonization possible [5].

Back in the sixties the resistant cultivar Cavendish was identified just in time and replaced Gros Michel as the primary banana for export [6]. In 1992 a new variant of Foc, tropical race 4 (TR4) was discovered in Sumatra and peninsular Malaysia in Cavendish plantations and it is on the rise to spread throughout the world [7](Figure 2). In 2013 the presence of race 4 was announced for 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 and other cultivars, including many cooking and dessert bananas. Considering the high susceptibility of several cultivars to this almost uncontrollable disease it was 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 sixties and to introduce a system into agriculture that will ensure food safety.

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.