The following objectives expand my Ph.D. thesis research on TSWV evolution under increased temperature, and on the elucidation of the mechanisms underlying the interactions between phytohormones and plant resistance to TSWV infection.
The specific objectives of this proposal are:
1. To determine the effects of climate change on disease progression
a. What is the effect of temperature on disease progression in tomato?
b. What is the effect of drought stress on disease progression in tomato?
2. To determine the effect of phytohormones on disease progression in tomato
a. What is the effect of exogenous methyl jasmonate/jasmonic acid (JA) on disease progression in tomato?
b. What is the effect of exogenous methyl salicylic acid (SA) on disease progression in tomato?
c. What is the effect of exogenous abscisic acid (ABA) on disease progression in tomato?
The purpose of this project is to provide new tools to alleviate crop disease impact associated with climate change which will in turn increase food security and decrease economic losses. Phytohormones will be explored as a tool for sustainable management of tomato diseases in the Northeast.
Many studies claim that disease pressure is likely to increase in the future, if there continues to be a rise in temperatures and extreme weather events (Anderson et al. 2004), but not many studies have looked at the disease dynamics in specific crops such as tomatoes in a region like the Northeast, that is likely going to be more affected by unpredictable weather patterns and by atypical dry and rain spells, as well as increased winter temperature (Alexander et al. 2015). If increased disease pressure occurs, it will have major impacts on the sustainability of disease management. According to the Environmental Protection Agency, the Northeast is beginning to experience longer, hotter, and drier summers along with warmer winters. Logically, warmer winters and summers could potentially allow pest and pathogen vector (mainly insects) populations to rise and make earlier appearances, leading to an increase in pesticide sprays. In addition to effects on insect vectors, management of viral diseases of crops is also highly dependent on environmental conditions, since viruses are dependent on their plant host’s physiological machinery to multiply and cause disease, and plant hosts in turn depend upon the environment for physiological function.
Viruses in the family Tospoviridae, including Tomato spotted wilt orthotospovirus (TSWV), cause disease on more than 1,090 plant species in 84 families (Parella et al. 2003) and have a negative impact on crop and ornamental production in many areas of the world. These viruses have a potential to cause more damage as climate continues to change since the range of their vectors is going to move and expand. Management of tospoviruses relies mainly on plant resistance and insecticide treatments (Riley and Pappu 2004). Unfortunately insecticide treatments rapidly cause thrips to evolve resistance in the field due to high selective pressures (Wang et al. 2016; Zhao et al. 1995). Climate change may also exacerbate resistance evolution in thrips due to accelerated developmental rate at higher temperatures, so other management tactics may be necessary to adequately reduce disease incidence.
Phytohormones, often referred to as biostimulants in agriculture, broadly, are hormones produced by plants that regulate growth and defense against pathogens (Pieterse et al. 2009) and are promising targets for resistance breeding against plant diseases. Two of them, jasmonic acid (JA) and salicylic acid (SA), are involved as part of a biotrophic pathogen plant response (e.g. plant viruses), and of herbivore or necrotroph response, respectively (Pieterse et al. 2009). Abscisic acid (ABA) is generally produced as response to osmotic stress and drought (Singh et al. 2011), but recently, it has been recognized to have also a role in pathogen and virus defense (Alazem et al. 2014; Alazem et al. 2017; Westwood et al. 2013).
There exists a large body of literature estimating the impacts of climate change, yet in reality we have a major lack of knowledge on what the exact impacts of increased temperatures and drought conditions will have on crop disease management. The Northeastern IPM Center recognizes climate change and pests as one of its five signature programs (http://www. northeastipm.org/grant-programs/stakeholder-priorities/). Their advisory board consists of stakeholders, thus the study of climate change is important to growers who are invested in integrated pest management (IPM). They call for more research on the impact of climate change on the distribution and occurrence of pests. Pathogens can be considered a crop pest and often are vectored by insect pests, so changes in disease pressure due to climate change and building of climate resilience through specific IPM disease management strategies is also imperative.
TSWV provides a great model system to test the effects of climate change on disease development and to deploy novel virus disease management strategies. Tospoviruses, such as TSWV, Iris yellows spot virus of onions and Impatiens necrotic virus, are major viruses that limit crop productivity in field and greenhouse throughout the world, and TSWV, the largest virus problem on tomato in warmer US states such as Florida, California and Georgia, is poised to become a larger problem to growers in the Northeast, since it is vectored by thrips that are present in the Northeast and already cause economic losses, especially in greenhouses.
Abscisic acid is an important factor in pathogen resistance. Preliminary data from our lab show that genes in the ABA pathway respond to TSWV infection and are responsible for disease outcome: i.e. infection vs. no infection. JA and SA are well-known for their roles in plant defenses. However, they are tightly linked with ABA and other phytohormones that respond to environmental variables, and their efficacy may change as the climate does. This study will elucidate if TSWV and other virus pathogens will become a more severe problem in the Northeast due to climate change, and will provide phytohormone-based strategies for virus disease management, at different temperatures and drought stress.
During the first months of this project, we purchased and received materials and supplies necessary to conduct the proposed objectives. Aside from standard lab supplies, we have obtained tomato seeds, the 5 hygrometers for testing soil moisture (Objective 1), and the phytohormones Methyl Jasmonate (Me JA), Salicylic acid (SA), and Abscisic acid (ABA) necessary to conduct Objective 2.
For Objective 1, we have conducted tests to verify the wilting point of tomatoes under different temperature conditions. In order to accomplish this result, we have decreased the amount of water in pots of tomatoes that have reached the two true-leaves stage, and we have monitored the soil water potential and wilting symptoms of healthy plants. These experiments have been performed on sets of 5 plants, and will be repeated 3 times at the 3 different temperatures indicated in the proposal.
We have also conducted the first trial to determine the difference in infection rate of 18 tomatoes sprayed with JA and inoculated with Tomato spotted wilt virus (TSWV), compared to the one of 18 tomatoes sprayed with buffer only and inoculated with TSWV (Objective 2). The infectious status of the inoculated plants was evaluated by recording the presence and severity of symptoms and confirmed by ELISA.
We have been working on Objectives 1. and 2. concurrently. Our preliminary results indicate that tomatoes can withstand relative lack of soil moisture for the time necessary to conduct our experiments (Objective 1). Furthermore, we found that the administration of JA to tomatoes decreases TSWV infection (0 plants were infected by TSWV, compared to 3 in the control group). These results will be confirmed with a second TSWV strains with a higher infectivity rate (already identified and now present in our lab), since the one used for the first trial showed a low ability to infect tomato (3 out of 18 inoculated plants). Having a high rate of infection will allow statistical analysis of our results. Based on these preliminary results, we are now in good position to continue with the remaining part of the experiments planned for this proposal.