Use of plant viruses as bioherbicides: the first virus-based bioherbicide and future opportunities

Until recently, only a few plant viruses had been studied for use as biological control agents for weeds, but none had been developed into a registered bioherbicide. This position changed in 2014, when the US Environmental Protection Agency granted an unrestricted Section 3 registration for tobacco mild green mosaic virus (TMGMV) strain U2 as a herbicide active ingredient for a commercial bioherbicide (SolviNix LC). It is approved for the control of tropical soda apple (TSA, Solanum viarum), an invasive 'noxious weed' in the United States. TSA is a problematic weed in cattle pastures and natural areas in Florida. The TMGMV-U2 product kills TSA consistently, completely, and within a few weeks after its application. It is part of the TSA integrated best management practice in Florida along with approved chemical herbicides and a classical biocontrol agent, Gratiana boliviana (Coleoptera: Chrysomelidae). TMGMV is nonpathogenic and nontoxic to humans, animals, and other fauna, environmentally safe, and as effective as chemical herbicides. Unlike the insect biocontrol agent, TMGMV kills and eliminates the weed from fields and helps recycle the dead biomass in the soil. Here the discovery, proof of concept, mode of action, risk analyses, application methods and tools, field testing, and development of the virus as the commercial product are reviewed. Also reviewed here are the data and scientific justifications advanced to answer the concerns raised about the use of the virus as a herbicide. The prospects for discovery and development of other plant-virus-based bioherbicides are discussed. © 2023 Society of Chemical Industry.

World Spread of Tropical Soda Apple (Solanum viarum) under Global Change: Historical Reconstruction, Niche Shift, and Potential Geographic Distribution

Solanum viarum has become extensively invasive owing to international trade, climate change, and land-use change. As it is classified as a quarantine weed by countries such as the U.S. and Mexico, it is critical to understand the prevailing historical dispersal, ecological niche dynamics, and distribution patterns. We reconstructed the historical invasion countries and analyzed the ecological niche shift of S. viarum. Using MaxEnt based on the conservativeness of ecological niches, we studied variations in the potential geographical distributions (PGDs) of S. viarum in ecosystems and variations in suitability probabilities along latitudinal gradients. The invasion history in six continents involved three phases: lag (before 1980), spread (1980-2010), and equilibrium (2010-present). The ecological niche remains conserved. The area of S. viarum PGDs had increased by 259 km2; the PGDs will expand to reach a maximum in the 2050s, SSP5-8.5. The PGDs of S. viarum will migrate to higher latitudes under the same future climate scenarios. The latitudes subject to high threats range from 20° to 30° in forest and cropland ecosystems, 15.5° to 27.5° (northern hemisphere) and 33.1° to 42.8° (southern hemisphere) in grassland ecosystems, and 20° to 35° in urban ecosystems. Global change has led to an increased threat of S. viarum at high latitudes. These findings provide a theoretical basis to monitor and control S. viarum.

Biotic elicitor-induced changes in growth, antioxidative defense, and metabolites in an improved prickleless Solanum viarum

Solanum viarum serves as a raw material for the steroidal drug industry due to its alkaloid and glycoalkaloid content. Elicitation is well-known for measuring the increase in the yield of bioactive compounds in in vitro cultures. The current study was performed for the accumulation of metabolites viz. solasodine, solanidine, and α-solanine in S. viarum culture using microbial-based elicitors added in 1%, 3%, 5%, and 7% on 25^th and 35^th day of culture period and harvested on 45^th and 50^th days of culture cycle. The treatment of 3% Trichoderma reesei and Bacillus tequilensis culture filtrate (CF) significantly increased biomass, alkaloids/glycoalkaloid content, and yield in S. viarum. T. reesei was found to be the best treatment for enhanced growth (GI = 11.65) and glycoalkaloid yield (2.54 mg DW plant^-1) after the 50^th day of the culture cycle when added on the 25^th day. The abundance of gene transcripts involved in the biosynthesis of alkaloids/glycoalkaloids, revealed by quantitative real-time PCR expression analysis correlates with the accumulation of their respective metabolites in elicited plants. Biochemical analysis shows that elicited plants inhibited oxidative damage caused by reactive oxygen species by activating enzymes (superoxide dismutase and ascorbate peroxidase) as well as non-enzymatic antioxidant mechanisms (alkaloids, total phenols, total flavonoids, carotenoids, and proline). The findings of this study clearly demonstrate that the application of T. reesei and B. tequilensis CF at a specific dose and time significantly improve biomass as well as upregulates the metabolite biosynthetic pathway in an important medicinal plant- S. viarum. KEY POINTS: • Biotic elicitors stimulated the alkaloids/glycoalkaloid content in S. viarum plant cultures. • T. reesei was found to be most efficient for enhancing the growth and alkaloids content. • Elicited plants activate ROS based-defense mechanism to overcome oxidative damage.

Elucidating the Fitness of a Dead-End Trap Crop Strategy against the Tomato Fruitworm, Helicoverpa armigera

Simple Summary

The tomato fruitworm, Helicoverpa armigera Hübner (Lepidoptera: Noctuidae), is a destructive polyphagous insect pest of tomatoes and its control largely depends on chemical pesticides. However, indiscriminate use of chemical pesticides has resulted in the development of resistance and posed serious environmental problems. Alternatively, in search of environmentally friendly pest management techniques, the use of trap crops has recently gained more attention. In this study, we compared different accessions of S. viarum to investigate which of them have the highest potential as a dead-end trap crop for H. armigera. Results from the laboratory experiment showed a significant variation among the S. viarum accessions in terms of H. armigera oviposition, and exhibited a higher level of resistance against H. armigera larvae compared to the tomato plants. Under the semi-field condition, trap cropping of S. viarum significantly reduces the total egg-laying of H. armigera on tomato. This study provides important information about the abilities of S. viarum plants to influence the life parameter of H. armigera and highlighted the possibilities to use S. viarum as a dead-end trap crop for the management of H. armigera. However, the efficacy of S. viarum needs to be tested under large, open-field conditions.

Abstract

Solanum viarum has been proposed as a potential dead-end trap crop for the management of Helicoverpa armigera because of its unsuitability for larval growth and survival despite being overwhelmingly preferred for oviposition. This study delved into the different S. viarum accessions for ovipositional preference and non-suitability for larval growth and survival of H. armigera. Besides, foliage trichomes, acylsugars, and phenolic content of S. viarum plants were assessed and compared with tomato. Since there is no significant variation in the ovipositional preference and larval performance of H. armigera, our result revealed that all those evaluated accessions of S. viarum have the potential to be used as a dead-end trap crop for the management of H. armigera. However, significant variation among the S. viarum accessions in terms of H. armigera oviposition was also evident in a no-choice experiment. Because of high-density glandular trichomes, acylsugars, and phenolic content, S. viarum significantly impaired H. armigera larval growth and survival compared to the tomato. Hence, our study elucidated that the S. viarum plant fits with the criteria for dead-end trap crop, and has the potential as a dead-end trap crop for the H. armigera, which needs to be tested under large, open-field conditions.

Growth and alkaloid production along with expression profiles of biosynthetic pathway genes in two contrasting morphotypes of prickly and prickleless Solanum viarum Dunal

Growth and production kinetics of three important glycoalkaloids viz. α-solanine, solanidine, and solasodine in two contrasting prickly and prickleless plants of Solanum viarum Dunal were evaluated under in vitro conditions. The prickleless plants showed improved accumulation of total glycoalkaloid content [7.11 and 6.85 mg g^-1 dry weight (DW)] and growth (GI = 11.08 and 19.26) after 45 and 50 days of culture cycle, respectively. For higher biomass (91.18 g l^-1) as well as glycoalkaloid (52.56 mg l^-1) recovery, the prickleless plants served as highly profitable platform. All the three studied glycoalkaloids were identified and quantified by mass spectrometry and HPLC. All the three studied glycoalkaloids accumulated in age-dependent manner. The presence of two constituents, i.e., solasodine and solanidine mainly contributed for higher accumulation of total glycoalkaloid content in the prickleless plants. However, the synthesis of α-solanine was highly age specific and could be detected after 4 to 5 weeks of culture cycle in both prickle containing as well as prickleless plants of S. viarum. The higher accumulation of glycoalkaloids in prickleless plants was also supported with the expression analysis of six key pathway enzymes viz. mevalonate kinase (MVK), 3-hydroxy-3-methyl-glutaryl coenzyme A reductase (HMGR), farnesyl diphosphate synthase (FPS), UDP-galactose/solanidine galactosyltransferase (SGT1), UDP-glucose/solanidine glucosyltransferase (SGT2), and cytochrome P450 monooxygenase (CYP). The results indicated that the plants harvested after 45 and 50 days of culture cycle accumulated maximum bioactive in-demand glycoalkaloids in the prickly and prickleless plants of S. viarum Dunal, respectively.