Boosting plant defenses using silicon fertilization

Boosting plant defenses using silicon fertilization

Summary

This project integrates field and laboratory experiments to test the protective effect of Si against herbivorous insects on different crops. Our experiments indicate that, upon damage, Si supplementation enhances the activity of protein defensive enzymes, total phenolics and the production of glandular trichomes in tomato plants.  Maize, tomato and soybean plants supplemented with silicon accumulated more Si on their leaves than their non-supplemented controls. Likewise Si supplementation affected the foliar concentration of iron and aluminum in maize. The foliar concentration of P, K, Mg, S, Mn, Fe, Cu, B, Al, Na, and Zn was affected by fall armyworm (Spodoptera frugiperda) herbivory in maize, tomato and soybean plants in a specific manner. Our bioassays showed that fall armyworm caterpillars gained less weight when fed on maize plants supplemented with Si compared with those supplemented with lime. In soybean and tomato plants previously exposed to herbivory, fall armyworm caterpillars gained less weight compared with untreated controls. Field experiments did not show any effect of Si supplementation on yield. The results of this study suggest a beneficial effect of Si supplementation in the enhancement of plant immunity but not in yield. 

Objectives/Performance Targets

1) To determine the effect of Si fertilization on yield and plant resistance to herbivores under greenhouse and field conditions.

1a). To test the effect of Si supplementation in the enhancement of plant defense responses upon herbivory in greenhouse conditions.

Tomato plants supplemented with Si (2 mM and 5 mM potassium silicate) and not Si-supplemented controls (2 mM and 5 mM potassium chloride) were grown in greenhouse conditions and exposed to Helicoverpa zea caterpillars three weeks after transplanted. 10 days later, tissue from the newer leaves was harvested to test for the activity of defensive proteins (polyphenol oxidase (PPO), peroxidase and trypsin protease inhibitor activity) and total phenolics. For all plant treatments, the activity of PPO, peroxidase, trypsin inhibitor, and the total phenolics content was significantly higher on plants exposed to herbivory compared with undamaged controls. When the activity of these proteins was compared only among plants treated with caterpillars, there were not significant differences among treatments. A significantly higher content of total phenolics was found on plants supplemented with 5 mM Si compared with plants supplemented with 5 mM KCl (t = -2.72, p-value = 0.026; N= 5).

1b). To test the effect of Si supplementation on plant resistance to herbivores and yield in field conditions.

In summer 2015, tomato, corn and soybean plants were planted at the Russell E. Larson Agricultural Research Center (Rock springs, PA), and fertilized at the moment of transplanting with either two different doses of calcium silicate (3gr and 1gr) or their respective controls (3gr and 1gr of calcium carbonate). The presence of insect damage was monitored every week during 3 months. Insects did not affect tomato or corn plants during the whole season, only a few aphids were observed in tomato by the end of August. The presence of aphids was not affected by the treatments. Three different insect species were found in soybean plants: first, the silver spotted skipper caterpillar (Epargyreus clarus) was found by the end of June; second, the soybean looper (Pseudoplusia includens) was found by mid July; lastly, Japanese beetles (Popillia japonica) were the most predominant insect found during July and August. The distribution and damage of these insects was not affected by the plant treatments. 

The effect of Si treatments on yield was measured by harvesting and weighing mature tomato fruits, corn ears and soybean pots. No significant differences were found between treatments for the total weigh (gr) of tomatoes (ANOVA, F_3,76 = 0.25 p value = 0.861; N=20 plants per treatment)  or soybeans (ANOVA, F_3,12 = 0.66 p value = 0.595; N = 4 plots per treatment of 6 plants each) per plant. No data was obtained for corn due to extensive raccoon damage.

In summer 2016, Maize, tomato and soybean were grown and supplemented with two different doses (4 and 2 pounds per square feet) of either calcium silicate or calcium carbonate. No insect damage was observed in soybean or tomato. In maize, damage of European corn borer and corn earworm were observed at cob maturity with no significant differences among treatments. The statistical analyses of yield did not found significant differences among treatments in maize (ANOVA, F_3,12 = 0.69 p value = 0.576; N = 4 plots per treatment of 10 plants each) and soybean (ANOVA, F_3,36 = 2.16 p value = 0.11; N = 10 plots per treatment of 6 plants each )(Figure_1, Figure_2). Tomato plants were heavily pruned by rabbits and not reliable data was obtained.

2) To determine the effect of silicon fertilization and insect damage on the density of trichomes and mineral composition of plant leaves.

Tomato plants supplemented with Si (2 mM and 5 mM potassium silicate) and not Si-supplemented controls (2 mM and 5 mM potassium chloride) were grown in greenhouse conditions and exposed to Helicoverpa zea caterpillars three weeks after transplanted. 10 days later the newer fully expanded leaf was harvested to count the number of glandular trichomes in a 0.24 mm² area and the remaining leaves dried for mineral analysis. For all treatments, the number of trichomes was higher on plants exposed to herbivory than in the corresponding undamaged controls. Among the plants treated with caterpillars, the number of trichomes was significantly higher on plants supplemented with 5 mM Si compared with plants supplemented with 5 mM KCl (t = -5.66, p-value < 0.0001; N = 6), but there were not significant differences between plants treated with 2 mM Si, 2 mM KCl and 5 mM Si.

Maize plants supplemented with Si accumulated significantly more Si on their leaves than not Si-supplemented controls (ANOVA, F_7,48 = 51.2 p value = 0.0000: N = 7 plants per treatment) (Figure 3). Tomato plants supplemented with Si accumulated significantly more Si on the leaves than the not supplemented controls (ANOVA, F_7,46 = 17.97 p value = 0.0000: N = 6-7 plants per treatment) (Figure 4), which is surprising since tomato is not a Si accumulator plant. Fall armyworm herbivory did not affect the accumulation of Si in these plants. Soybean plants supplemented with Si accumulated significantly more Si on their leaves than the not Si-supplemented controls and fall armyworm herbivory appear to suppress Si accumulation (ANOVA, F_7,48 = 50 p value = 0.0000: N = 7 plants per treatment) (Figure 5).

Si supplementation increased the foliar concentration of iron and aluminum in maize. The foliar concentration of P, K, Mg, S, Mn, Fe, Cu, B, Al, Na, and Zn was affected by fall armyworm (FAW) (Spodoptera frugiperda) herbivory in maize, tomato and soybean plants in a specific manner. FAW feeding increased the accumulation of Fe in maize but decreased its accumulation in soybean. FAW feeding decreased the accumulation of Na in maize but increased its content in soybean. The content of Zn was lower in tomato and soybean plants fed by FAW. Likewise FAW feeding decreased the levels of Mn compared with undamaged controls, for more details see Table_1 and Figure_6, Figure_7, Figure_8, Figure_9, Figure_10, Figure_11.

3) To determine the effect of Si-fertilized plants on caterpillar growth, mandible wear and integrity of the insect’s gut protective membrane.

3a). To test the effect of Si-fertilized plants on caterpillar weight gain.

Maize, tomato and soybean plants supplemented with Si (2 mM and 5 mM potassium silicate) and not Si-supplemented controls (2 mM and 5 mM potassium chloride) were grown in greenhouse conditions and exposed to FAW caterpillars three weeks after transplanted. 10 days later the new leaves were detached and used to feed 3-day old FAW caterpillars. These caterpillars were weighed 6 days later and the weight gain calculated. In maize plants, FAW caterpillars gained less weight when fed on plants supplemented with 3 g of Si compared with their corresponding controls (ANOVA, F_3,62 = 3.32 p=0.025 0.0001: N= 8-10) (Figure_12). In soybean and tomato plants, caterpillars gained less weight when fed on plants previously exposed to herbivory independent on the Si treatment (Figure_13, Figure_14).

3b). To test the effect of Si-fertilized plants on caterpillar mandible wear and integrity of their gut membrane. 

The experiments of this activity have been performed but the images have not been taken.

Accomplishments/Milestones

Most of the experiments proposed have been already performed, but there are still some samples to analyze. The field experiments turn out to be problematic due to damage to crops by larger animals. The 2016 field season was extremely dry during May to July, conditions that affected our experiments. In general, most of the activities originally programmed for this year were accomplished. We requested an extension of one more year to properly finish the project.

Impacts and Contributions/Outcomes

Si is known to accumulate in plant tissues where it plays an important role in maintaining plant structure and rigidity. It also enhances physical protection against pathogens and insect herbivores. This project is exploring the use of Si as an enhancer of plant physical and biochemical defenses in response to herbivory. So far, our results show that Si supplemented plants accumulate more Si than not supplemented controls. Upon herbivory, Si supplemented tomato plants seems to produce higher number of glandular trichomes and total phenolics than their corresponding controls. Furthermore, young caterpillars gained less weight when fed on maize plants supplemented with Si compared with plants supplemented with lime. So far, our results indicate that Si supplementation may enhance plant immunity in accumulators and non Si-accumulator plants like tomato. The outcomes of this project have the potential to incorporate an additional practice for IPM programs and reduce the use of pesticides.  

During 2016, preliminary results from this project were presented to the public and scientific community at 4 different events:

1) 58^th Annual Maize Genetics Conference. March 17-20, 2016 Jacksonville, Fl. Acevedo, Flor; Peiffer, Michelle; Luthe, Dawn; Felton, Gary. Fall armyworm herbivory affects silica accumulation in corn and rice.

Abstract
Plants, especially grasses, accumulate large amounts of silicon dioxide in their leaves. Silica deposition increases the strength and plant rigidity, but it is also a critical component of defense against both abiotic and biotic stresses including herbivores. This research determined the effect of plant silica deposition on insect herbivory and its inducibility. We used corn, rice and the fall armyworm (FAW), Spodoptera frugiperda (Lepidoptera: Noctuidae) as a model system. The inducibility of silica was tested by exposing the plants to herbivory for 24 hours and measuring the Si accumulation in the new regrowth tissues using the Molybdenum yellow method. The effect of silica on FAW mandible durability was evaluated by examining the mandibles of caterpillars fed on corn, rice and artificial diet using backscatter X-ray spectroscopy. Our results show that FAW herbivory induces silica accumulation in rice plants by ~37.2 % compared with undamaged controls. Furthermore, feeding on plants with high silica content destroys most of the mineralized area of the FAW mandibles (mainly composed of Zn, Na and Cl). We conclude that silica accumulation is an inducible plant defense mechanism that causes wear on the insect mandibles and likely hampers their ability to feed on plant tissues. This research was funded by the Northeast Sustainable Agriculture, Research and Education program (NESARE).

2) College of Agricultural Sciences and Gamma Sigma Delta Research Exhibition. The Pennsylvania State University and Gamma sigma Delta Honor Society. University Park, PA. USA. Acevedo FE. And Gary Felton “Beyond strengthening the leaf surface – Silicon enhances herbivore-induced plant defense responses”

3) The 19th Annual Environmental Chemistry and Microbiology Student Symposium. Acevedo FE. And Gary Felton “Boosting plant defenses using silicon fertilization” April 8 – 9, 2016. The Pennsylvania State University. University Park, PA. USA.

4) 6th Annual PSU Sustainable Cropping Systems Symposium. April 1, 2016. Penn State University, University Park, PA. Acevedo FE. And Gary Felton “Boosting plant defenses using silicon fertilization

Beyond strengthening the leaf surface – Silicon enhances herbivore-induced plant defense responses

Abstract

Insect herbivory induces the production of defensive plant compounds that can poison or reduce the growth of their attackers.  Previous studies have shown that plants supplemented with silicon (Si) have increased tolerance to both abiotic and biotic stresses including herbivores.  This tolerance has been mainly associated with the accumulation of Si bodies in plant tissues, but the effect of Si on other plant resistant mechanisms has been explored in only a few systems.  This study tested the effect of Si supplementation on herbivore-induced defense responses in tomato and corn plants. Si-supplemented plants and their respective controls (without Si) were exposed to herbivory by the fall armyworm (FAW) larvae Spodoptera frugiperda.  The plant defense response was tested by counting the density of trichomes and by measuring the activity of two defensive proteins, polyphenol oxidase (PPO) and trypsin proteinase inhibitor (trypsin PI).  Our results show that tomato plants supplemented with Si had higher number of glandular trichomes and higher PPO activity when fed upon by the FAW larvae compared with non-supplemented controls.  In corn, Si-treated plants also had higher activity of trypsin PI upon FAW feeding but the density of trichomes did not change.  We conclude that Si boosts defenses upon insect herbivory in both Si-accumulators and non Si-accumulator plants.  Si supplementation may be a promising practice to reduce the use of pesticides in agricultural systems.  This research was funded by the Northeast Sustainable Agriculture, Research and Education program (NESARE).

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