Monday, April 22, 2013

Who's responsible?

Did you know that termites are actually incapable of consuming plant material on their own? It's true: they rely on a host of bacterial helpers that reside in their gut to break down their food for them. In turn, the bacteria depend on the host for this supply of raw energy (3). This is a classic example of mutualistic symbiosis, where two organisms benefit from each others' presence (in nature, this can be in the form of chemical aid as mentioned above, or may come in some other form, like protection/shelter).

Why should we care? Well, to better understand cause and effect relationships in nature, we need to be aware of all the components that are playing a role. For example, imagine if there was such a scenario where the flu virus actually infected and resided within helpful bacteria that lived in our lungs, and not us directly (and the cause of sickness were due to the death of these bacteria). If this imaginary flu-carrying bacterium turned out to be unable to survive on skin and other common surfaces but ultra-resilient as an airborne pathogen, then our whole idea of flu transmission would be wrong, and health policy would need be reformed; people would be far less concerned with washing their hands and sanitizing surfaces, and would start wearing masks. To take full advantage of our surroundings, we need to fully understand our surroundings.

Such may be the case with the Tomato yellow leaf curl virus (TYLCV). As its name might suggest, the acquisition of this disease by tomatoes causes a curling/shriveling of the leaf, a yellow discoloration, stunting of plant growth, and more (2), and can result in complete loss of tomato crop; this, consequently, leads to disastrous economical impacts. In a study by Su et al., it was found that the incidence of TYLCV infection correlated strongly with the presence of a bacterial symbiont of whitefly Bemisia tabaci. B. tabaci is known to transmit this virus, but what we did not know previously is that symbiont Hamiltonella is likely responsible for its intense transmission.

Hamiltonella is a secondary symbiont that is sometimes found in B. tabaci, where is not required for its survival (unlike the primary symbiont), but still may play other important roles in the host. In a series of parallel experiments, major results of the study essentially showed that whiteflies containing Hamiltonella had more of the TYLCV, lost less of the virus over time, and had hugely greater transmission efficiency of the virus to plants both in absolute amounts and when compared to whiteflies that did not have Hamiltonella. Ultimately, it was shown that this symbiont has a significant effect on the acquisition, retention, and most importantly, transmission of TYLCV. Consequently, it is not so much B. tabaci as it is Hamiltonella to blame for the significant loss of tomato crop worldwide, and would be reasonable to focus efforts of elimination on the symbiont in crop treatments (in addition to the whitefly) in order to attenuate this long-standing problem.

Even with modern implementations to curb economical loss caused by this virus including introduction of more resistant crop varieties and growing tomatoes in greenhouses, damages are estimated at greater than $300 million in Europe and the U.S., and the situation is far worse in developing countries where these methods are too costly to employ (1). This idea of vector transmission is important. Yes: a world without pizza sauce, bloody mary's, and bruschetta would be no world I'd want to live in. But more importantly, the study demonstrates that there is more to the story than meets the eye. This could apply to many other situations in nature. Viruses are small, and the ways they cause disease are not always so clear; by understanding the full picture, we as the human population will be better equipped to focus our research efforts in such a way that will make our treatments more effective and efficient.

Primary article:

Su, Q. et al. Insect symbiont facilitates vector acquisition, retention, and transmission of plant virus. Sci. Rep. 3, 1367; DOI:10.1038/srep01367 (2013)

See article here 

Secondary articles:

1) Efrat, Glick, Yael Levy, and Yedidya Gafni. "The Viral Etiology of Tomato Yellow Leaf Curl Disease – A Review." Plant Protect. Sci. 45.3 (2009): 81-97.

2) Simone, G. "Tomato Yellow Leaf Curl Virus Management for Homeowners." TYLCV Home Management. University of Florida, n.d. Web. 2013. <http://ipm.ifas.ufl.edu/agriculture/vegetables/tomato/tylcv_home_mgmt.shtml>.

3) "Termite Gut." Microbe Wiki. N.p., n.d. Web. <http://microbewiki.kenyon.edu/index.php/Termite_gut>.

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