Phytophthora infestans

Phytophthora infestans, Destroyer of Potatoes

Krystina Braue

Phytopthora infestans isn’t a common household name, unlike sexier microbes such as Salmonella, or E. coli, or Syphilis (okay, the microbe is Treponema pallidum, but that isn’t exactly household knowledge either). Research and media outlets are often focused on microbes significant to human health because those are of the “most immediate concern.” We hear a lot about “food-borne pathogens,” but rarely do we hear much about “food pathogens,” even though plant diseases often cause as much economic and physical damage as human disease. For its cool microbiology, effect on historical sociology in the United States, and its status as a bioterrorism threat, P. infestans, causative agent of late blight disease in potatoes, deserves a little more respect.

P. infestans is commonly linked to fungi based on its filamentous growth pattern, but it is actually a different type of eukaryote called an oomycete [1]. It does share some characteristics with fungi in addition to its growth pattern like absorbing nutrients and spore reproduction. However, as an oomycete, it is more similar to organisms like algae or diatoms, with which P. infestans shares cell wall composition (glucans instead of chitin) and “diploid vegetative states” (unlike fungi, which are haploid when in vegetative forms) [1]. Oomycetes are part of the higher classifications of Stramenopiles and Chromalveolata, unlike fungi which are in a different group entirely called Ophisthokonta [1]. Oomycetes are further differentiated from fungi by their past endosymbiosis with photosynthetic organisms like algae or cyanobacteria [1]. Evidence of this relationship still exists in the P. infestans genome even though the endosymbiont is no longer there [1].

Figure 1: P. infestans sporangia [1]

P. infestans has both sexual and asexual life cycles and is diploid throughout both with the exception of gamete formation in sexual reproduction [1]. The asexual life cycle is associated with P. infestans pathogenicity- short generation times as well as a high level of sporangia differentiation both contribute considerably (Figure 1) [2]. The ball-like structure on the end of the stem (called an aerial hyphae) in Figure 1 contains the sporangia zoospores that can be released aerially and disseminated at a devastating pace (productive field to zero vegetation in less than a week, see Figure 2) [1]. One unique thing about oomycete spores is that the spores are fully metabolically active when mature, as compared to characteristically desiccated fungal spores [2]. This quality contributes to the pathogenicity of P. infestans because the sporangia are able to control germination to coincide with favorable environmental conditions [2]. Metabolically active spores also mean that P. infestans does not require environmental stresses to sporulate (because proteins required for sporangia formation are already present before the spores are fully developed), and can thus undergo sporulation cycles very efficiently [2]. Sporangia initiate the infection by releasing zoospores that not only fling through the air, but actually swim along waterways towards potential hosts using their two flagella [2]. Upon reaching the host, it encysts and starts the cycle all over again [2]. Other evidence of P. infestans’ spiteful nature is that it doesn’t just target one aspect of the potato plant- it absolutely decimates the potato from the leaves down the stem and into the potato tuber itself [1].

Figure 2: Potato field infected by

P. infestans over one week [1]

So why should we care about a potato pathogen? Keep in mind that even though P. infestans doesn’t do nasty things to humans, the potato is fourth on the list for largest global food sources [3]. P. infestans-associated late blight is one of the worst potato pathogens we have encountered [4]. Over one hundred and fifty years ago, it wiped out the potato crop in Ireland, killing over one million people due to starvation and resulting in one of the largest population migrations in modern history [4]. This pathogen had a profound physical effect on the Irish population (killing one in eight) and a profound sociological effect on the United States as it struggled to accommodate the largely Catholic, largely poor Irish immigrants fleeing starvation [4]. Despite the significance of P. infestans, little research has gone into methods of protecting our current potato crop. Beyond generalized fungicides that are often too expensive for poor countries, the ones most dependent on the crop, very little stands in the way of widespread P. infestans infection [4]. Current infections already translate into global yearly economic losses exceeding six billion dollars due to infection, on top of fungicidal expenses [3].

Adding to the nastiness of P. infestans: as a sporulating yeast it is easily weaponized, something about which the United States has been long concerned [5]. Luckily for us (especially considering the amount of mashed potatoes recently consumed with Thanksgiving turkey), molecular biological methods are leading to alternatives to promoting resistance of potatoes to P. infestans. This has primarily involved transferring R (resistance) genes from potato species, like Solanum bulbocastanum, known to not be vulnerable to P. infestans, to susceptible species domesticated for agriculture [4]. Requirement of these methods underscores the importance of non-human pathogens like P. infestans, and further development and implementation of these methods will hopefully lead to increased security of global food resources both from natural infection and potential biological warfare.

References
[1] W. Fry, “Phytophthora infestans: the plant (and R gene) destroyer,” Molecular plant pathology, vol. 9, no. 3, pp. 385–402, 2008.
[2] H. S. Judelson, R. D. Narayan, A. M. V. Ah-Fong, and K. S. Kim, “Gene expression changes during asexual sporulation by the late blight agent Phytophthora infestans occur in discrete temporal stages.,” Molecular genetics and genomics : MGG, vol. 281, no. 2, pp. 193-206, Feb. 2009.
[3] B. J. Haas et al., “Genome sequence and analysis of the Irish potato famine pathogen Phytophthora infestans.,” Nature, vol. 461, no. 7262, pp. 393-8, Sep. 2009.
[4] J. Song et al., “Gene RB cloned from Solanum bulbocastanum confers broad spectrum resistance to potato late blight.,” Proceedings of the National Academy of Sciences of the United States of America, vol. 100, no. 16, pp. 9128-33, Aug. 2003.
[5] “Chemical and Biological Weapons: Possession and Programs Past and Present,” 2008. [Online]. Available: http://cns.miis.edu/cbw/possess.htm. [Accessed: 10-Nov-2011].