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Climatic Change Influencing Plant-Pathogen Interaction

June 8, 2012

I have been growing a vegetable garden for years now. I am constantly battling off pathogens that are causing harm to my plants.  This year I am determined to figure out how I can win this battle against the pathogens.

Being a biology student, I have been taught to look at the bigger picture. I began considering variables that affect plant success: soil, water, light, CO2, oxygen, nutrition, etc.  I then began to consider variable that may have changed over time. Increasing temperatures with global warming has continued to affect our environment with increasing CO2 and O3.

Concentrations of CO2 and trophospheric O3 have increased markedly since the inception of the industrial revolution, and they will continue to climb well into the 21st century. It is anticipated that by 2050, CO2 concentrations are expected to double the pre-industrial levels, while O3 concentrations are increasing by as much as 2.5% annually (Prather et al., 2001; Vingerzan, 2004; Solomon et al., 2007).

Changes within environmental conditions have been known to exacerbate plant disease symptoms and are implicated in 44% of new disease emergence (Boyer, 1995; McElrone et al., 2001; Anderson et al., 2004). As a result, environmental and climatic changes have potential to alter the incidence and severity of plant disease epidemics and disease pressures on natural and crop plant systems, as well as to reshape the co-evolutionary relationships between plants and pathogens ( Chakraborty, 2005; Burdon et al., 2006; Fargette et al., 2006; Ziska and Runion, 2007; Crowl et al., 2008). Plant development and stress responses, while genetically programmed, are also influenced by environmental conditions. Plants are able to adjust to environmental challenges by tightly and differentially regulating their transcriptomes (Baker et al., 1997; Cushman and Bohnert, 2000; Chen et al., 2002; Yamaguchi-Shinozaki and Shinozaki, 2006). The alterations in molecular mechanisms determine and plant’s ability to respond internal and external signals and to adjust to changing conditions (Eastburn et al., 2011)

In assessment of this trend, I began to look at the next factor: plant pathogens and their interaction with their host. Environmental changes have been known to have a direct effect on pathogens, as well as host plants. Many pathogens require not only a host but specific temperature, moisture, and nutrients.  Runion et al. (1994) and Ziska and Runion (2007) discovered plant pathogens vary in the level of host specificity and in the degree of physiological interactions they have with their plant hosts, depending on their mode of infection, and climate-change factors may affect each pathosystem differently. There are two main types of pathogens. Necrotrophic pathogens, which derive nutrition’s from killed host tissues, and have somewhat limited interactions with the active metabolism of host cells (Schumann and D’Arcy, 2006). Therefore, abiotic factors that cause or accelerate tissue necrosis, such as elevated O3 levels, may favor infection by these types of pathogens. In contrast, biotrophic pathogens, also known as obligate parasites, have extended periods of physiological interaction with their hosts, as they derive nutrients from living cells (Schumann and D’Arcy, 2006). Consequently, factors that alter plant growth, such as elevated levels of CO2, may also alter the colonization of host tissues by biotrophic pathogens through changes in host physiology.

It is well documented that both elevated CO2 and O3 alter plant function, but in opposite ways. In general, photosynthetic capacity, water-use efficiency, growth and yield are positively affected by elevated CO2, but negatively affected by elevated O3 across a wide range of study species. Plant responses suggest that stomatal opening are strongly and consistently restricted by physiology and by wax occlusion under elevated CO2 and/or O3 conditions (Eastburn et al., 2011). Enhanced photosynthetic efficiency under elevated CO2 provides additional carbohydrate supply that strongly and consistently results in increased starch and sugar level in leaf tissue (Eastburn et al., 2011). The enhanced sugar content on the leaves potentially would increase sugar-dependent pathogens.

One of my favorite vegetables I grow in my garden is soybeans; unfortunately they are presenting me with the biggest problem.

Photo: Small, purple soybean flowers, Photo Credit: Wikipedia, Soybean

I have been finding brown spots occurring on them from year to year. I have been able to observe the spots begin on the lower-most leaves and watch the disease progress upward. As I look in this incident, I found that the brown spots are commonly caused by Septoria glycines. Studies have shown that, Septoria glycines progresses rapidly as levels of CO2 increases.

Image Citation: Daren Mueller, Iowa State University, Bugwood.org http://www.forestryimages.org/browse/detail.cfm?imgnum=5465839

As I looked into Septoria glycines, I came across Arabidopsis thaliana an infection that occurs by the biotrophic pathogen Erysiphe cichoracearum. Lake and Wade (2009) found that this infection results in fewer stomata produced on the infected leaf surfaces, and is enhanced in environments with elevated levels of CO2. Elevated levels of CO2 have also been found to be associated with increased colony establishment by a powdery mildew pathogen.  Lake and Wade’s (2009) results suggested that resistant ecotypes may become more susceptible to infection in enhanced CO2 environments. Once more suggesting mixed effects of enhanced CO2 levels on the development of diseases caused by biotrophic pathogens.

Mildew on Grape Leaf. Photo credit: Wikipedia mildew, http://en.wikipedia.org/wiki/Mildew

Climate-change studies over the past decade have increased understanding of how factors such as rising CO2 and O3 levels will impact the development of plant disease epidemics. Results from these studies have shown increased pathogen survival, in some species such as Septoria glycines on soybeans. Not all pathogens and plant hosts will respond the same to climatic changes; thus, more studies need to be done to provide an adequate result.

 

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