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United States Department of Agriculture

Agricultural Research Service

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Research Project: Examination of Factors Influencing Virus Disease in Floriculture Crops

Location: Application Technology Research

2012 Annual Report

1a. Objectives (from AD-416):
The objectives of this cooperative research project are: 1) to study interactions among Pelargonium flower break virus (PFBV) protein; 2) to examine the effects of silicon on virus infection; and 3) to determine the effects of silicon on copper (Cu) iron (Fe), and manganese (Mn) toxicity in ornamentals.

1b. Approach (from AD-416):
Pelargonium flower break virus (PFBV) is the most common viral pathogen of geraniums. PFBV impairs growth and physical appearance, reducing their salability. To control PFBV, we are identifying interactions among the viral proteins, with the ultimate goal of interfering with these associations, thereby inhibiting the PFBV life cycle. Using yeast two-hybrid and affinity column pull-down approaches we are identifying protein interactions. We are writing two manuscripts on these interactions and will be testing viral protein binding molecules for their ability to interfere with PFBV infection. An advantage of this technology is that it could be employed quickly and easily by growers. We recently discovered that addition of soluble silicon (Si) delays Tobacco ringspot virus (TRSV) systemic symptoms in Nicotiana tabacum. TRSV infects a variety of food and ornamental crops, so our work has implications for both the agriculture and floriculture industries. The mechanism(s) by which Si caused this delay in viral symptoms is/are unclear. We will be investigating the mechanism(s) behind this “Si effect” by examining the accumulation of virus in inoculated leaves of plants treated with Si by ELISA and RT-PCR. We will also be studying the effects of Si on viral systemic spread by a leaf excision assay. Finally, we will be monitoring the expression of plant defense enzymes and genes in TRSV-inoculated and systemic leaves provided with Si. TRSV-infected tobacco leaves show a higher level of Si than in uninfected plants and this correlates with the delay in systemic symptom formation. To identify the potential signaling pathway(s) leaves on tobacco plants will be sprayed with a variety of molecules involved in responses to pathogens (such as salicylic acid, jasmonic acid, ethylene, etc.). The plants will then be monitored for Si uptake into leaves. We will also identify putative Si transporters in Nicotiana and apply this to ornamental crops. Si also helps plants resist abiotic stress as well, as reported in our first Si paper. Si alleviated the toxic effects of high levels of Cu (copper) in Arabidopsis. We are now exploiting this success to alleviate Cu toxicity in ornamental plants. The effects of Si on Cu toxicity are being investigated with zinnia (a Si accumulator) and snapdragon (a Si non-accumulator). We are also extending these studies to other metals (such as iron; Fe) in Si accumulator and non-accumulator plants. To perform this work, we examine: symptoms, fresh and dry weight, elemental analysis, and phenylalanine ammonia lyase (a stress enzyme) activity. Plant Fe metabolism is different from that for Cu. So if Si alleviates toxicity of both metals, this suggests a useful general strategy for helping plants cope with metal stress. In total, data generated from our studies will provide useful information regarding the use of Si for the alleviation of both abiotic and biotic stress to help inform growers to make better decisions regarding Si fertilization.

3. Progress Report:
Copper (Cu) is an essential micronutrient required by all plants, but higher than required doses of the metal can be toxic to plants. Cu toxicity is a problem because the element is a common component of fungicides as well as certain manure-based fertilizers. Previous work with the model plant Arabidopsis thaliana indicated that silicon (Si) could help overcome Cu toxicity effects. Therefore, these studies were extended to ornamental plants. Zinnia, a high Si accumulator, and Snapdragon, a low-accumulator, were selected. Si alleviated Cu toxicity in both plant species but it was more effective in Zinnia (the high Si accumulator). Hence, providing Si to three dicot species in different plant genera alleviated Cu toxicity. This suggests that the application of Si to treat Cu toxicity in plants is of wide utility and could be of great benefit to growers. Interestingly, the leaf Si concentration in Snapdragon was much higher in the plants challenged with toxic Cu levels than under control levels. Thus, in Snapdragon, typically a low Si accumulator, the foliar levels can become elevated when the plants were challenged with the stress of Cu toxicity. This work was published in the Journal of the American Society of Horticultural Science (ASHS), received an award for the most outstanding publication in floriculture by ASHS, and led to further investigate the phenomena with a more well-characterized system, Tobacco. Like the Snapdragon studies above, it was also discovered that Tobacco plants infected with TRSV and supplemented with Si showed higher foliar levels of Si than plants supplemented with Si alone. Thus, infection with a pathogen caused a stress that stimulated foliar Si uptake. Since Cu toxicity in Snapdragon caused increased foliar Si uptake as well, Tobacco was tested to see if it would respond to Cu toxicity in the same way. Preliminary data indicate that indeed exposing Tobaccos to elevated Cu causes the leaves to acquire more Si than those not exposed to phytotoxic doses of the metal. These data suggest several things: 1) low-accumulators can acquire higher foliar Si levels than expected, if provided with an appropriate stress; 2) the plant must have a way of sensing high foliar Cu levels; 3) a signaling pathway must exist that tells the roots to send more Si to the leaves and 4) the roots must either increase their Si acquisition mechanisms from the soil or they release the element from internal stores (i.e., the vacuole) to permit the elevation of leaf Si levels. To describe this new phenomenon, the term Stress-Induced Si Accumulation (SISA) was coined. Two aspects of SISA are currently being explored; namely, the uptake of Si and the potential signal that regulates Si acquisition by leaves. To study the uptake side, possible Si transporters are being examined. Two potential genes in Tobacco that possess characteristics consistent with other known transporters have been discovered and are now being tested. To examine the potential signal involved in regulating SISA, several hormones have been tested that are involved in disease signaling in plants and have identified a potential candidate. Future work will confirm these results and to determine if this can be exploited to help plants acquire Si more efficiently. This work could help growers since the increased leaf Si should permit the plant to more effectively deal with stress. Si does not always provide a benefit to plants and these cases are as important to study as the beneficial cases. As published earlier, Si could help plants better deal with TRSV infection. However, there are related plants that are used as ornamentals and these are susceptible to a variety of viral infections. A common virus is Cauliflower mosaic virus (CaMV). To determine if Si influenced CaMV infection, an experiment was performed on a model plant commonly used in the lab. Results suggest that Si helps CaMV to more efficiently infect turnips than plants not supplemented with the element. Since this system shows a negative effect of Si (enhancing viral infection) rather than a beneficial effect, it warrants study and illustrates potential limitations for the use of Si in aiding plant propagation. Several selections from a national plant germplasm center were tested for viruses. It was found that many of the plants were infected with Pelargonium Flower Break Virus (PFBV). Several of the plants were infected with Pelargonium line pattern virus (PLCV) as well, either alone or in co-infections with PFBV. PLCV and PFBV are related but distinct viruses, both belonging to the Carmovirus family. While evaluating these plants, it was noticed that viral symptoms on Pelargoniums were affected by a variety of factors including seasonality and general plant health. On several accessions, viral symptoms were most prevalent from late Fall to early Spring and then disappeared, only to reappear in Fall again. In addition, symptoms also disappeared on new leaves that formed on cuttings prior to rooting, but they reappeared once new roots were established. This indicates that just because plants do not show symptoms at one time of the year does not necessarily mean that they are uninfected. The fact that plants could be infected but asymptomatic could be a serious problem as the virus may spread to other hosts. This project helps to address parent project objective 1: Evaluate plant nutritional requirements to optimize production and enhance quality; and Sub-Objective 1b; Determine the uptake accumulation, and potential benefit of silicon in ornamental crops and explore the potential for its use as a buffer to Cu toxicity and alternative approach to pathogen control. Milestone 1a: complete screening of 10 crops for potential Si uptake.

4. Accomplishments

Last Modified: 06/28/2017
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