Protecting our crops – sustainable approaches for the control of plant-parasitic nematodes

Plant-parasitic nematodes (PPN) are a threat to global agriculture with losses estimated at 100 billion USD per year (Savary et al., 2019). Ranking 3rd as the most serious PPNs in agriculture, root- lesion nematodes (RLNs), Pratylenchus spp., are known by their worldwide distribution and broad host range (Jones et al., 2013). Pratylenchus penetrans is one of the most damaging RLN with over 400 hosts affecting economically important agronomic and horticultural crops (Castillo and Volvas, 2017). In Europe, P. penetrans has been detected in potato cultures and ornamental plants in Portugal and in association olive trees in Spain, Italy, and Turkey. This RLN is classified as an A1 quarantine pest in South America, while in EU, is a regulated A2 non-quarantine pest. Effective control measurements for RLN, and specifically for P. penetrans, are limited due to the lack of natural resistant varieties. The impact of P. penetrans alone and in interaction with other plant pathogens (e.g., bacteria and fungi) in such cultures emphasises an urgent demand for the development of new and sustainable control approaches as alternative to common agriculture chemicals. In this sense, the molecular mechanisms of P. penetrans parasitism are being investigated for the development of alternative and more specific control strategies (Vieira et al., 2019; Vicente et al., 2019)

Editorial: impact of global climate change on the interaction between plants and plant-parasitic nematodes

Global climate change is an important environmental influence on plant ecosystems. Temperature, precipitation, duration and quality of sunlight, availability of nutrients, such as nitrogen, phosphorus and potassium, are determinants of plant growth that are likely to change due to global climate change effects. Climate change-induced high levels of atmospheric CO2 promote plant growth due to increased photosynthesis, at the cost of decreased evaporative cooling. In parallel, elevated CO2, temperature and altered precipitation levels strongly influence the biology of nematodes including plant and insect-parasitic nematodes. Although nematode development occur at a faster rate in warmer soil temperature, it is not yet clear about the precise implications of climate change effects on nematode biology as well as on plant-nematode interaction continuum

PROTECTING OUR CROPS - APPROACHES FOR PLANT PARASITIC NEMATODE CONTROL

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Plant-nematode co-cultures in the screening of sustainable nematicides against soil-dwelling parasitic nematodes of plants

The diseases caused by plant parasitic nematodes are still a serious constraint to modern global crop production. An increasing number of active compounds in commercial nematicidal formulations is being banned from use by common policies of pest management. Farmer communities report a low efficiency for the replacement pesticides, which reflects on crop yield and productivity. Novel sustainable biopesticides are urgently needed to cope with global food demands while respecting the most recent environmental policies. Plant-nematode co-cultures offer a stable biotechnological screening tool able to assess the active compound’s nematicidal activity and its effect on host tissues, simultaneously, in an easily accessible system that simulates natural infection. These systems are being developed and optimized at the Nematology laboratory of INIAV. Preliminary results were obtained for co-cultures of Solanum lycopersicum with the nematode Meloidogyne ethiopica and S. tuberosum with Globodera pallida. Future studies will target other plant parasitic nematodes, e.g., the root lesion nematodes (Pratylenchus penetrans) in transgenic roots of potato and the pinewood nematode (Bursaphelenchus xylophilus) on in vitro pine shoots (Pinus sp.)

Nematicidal activity of oxygen-containing compounds against the root lesion nematode

The root lesion nematode, Pratylenchus penetrans, is a serious threat to agricultural crops worldwide, being extremely difficult to control with common pest management practices. New EU commission policies on the development of plant protection products restrict the application of synthetic nematicides. Aiming at the development of a sustainable and environment friendly approach for nematode control, 20 oxygen-containing compounds were evaluated, at 2 mg / mL, for their: (1) nematicidal activity by direct contact assays for 24 h, (2) minimum time period required to reach > 99% mortality, and (3) nematicidal activity by indirect contact (fumigant) for 24 h. All bioassays were performed using acetone as negative control and Oxamyl (a systemic nematicide) as positive control. Overall, P. penetrans was remarkably resistant to the tested compounds, except for benzaldehyde, carvacrol, 3-octanol and thymol, which were able to achieve > 99% mortality, surpassing the activity of the positive control Oxamyl. For these compounds, after ca. 60 min a 50% mortality was recorded, while at 18 h mortality was already > 99%. Using the indirect contact method, mortality was 3-octanol > thymol > carvacrol). Ongoing research is testing the effectiveness of these compounds against P. penetrans parasitizing potato, one of its main hosts. Simultaneously, the mode of action is being studied using a transcriptomic approach

Compounds produced by plants and bacteria for the control of the plant- parasitic nematode Pratylenchus penetrans

Potato (Solanum tuberosum) is one of the world’s most common agricultural crop for human consumption and considered an excellent source of essential nutrients and vitamins. The productivity of this crop is threatened by several plant-parasitic nematodes, among which the root-lesion nematode Pratylenchus penetrans is considered to have a significant impact. This organism is extremely difficult to eradicate and control procedures are expensive and hazardous to the environment and human health. Natural compounds have gained an increasingly importance as chemical substitutes; however, their effect towards plants is largely unknown. Our main goal is to identify naturally-produced compounds able to control P. penetrans and simultaneously non-harm plant host. The research plan includes: (1) in vitro bioassays of plant and bacterial compounds for biological control of P. penetrans; (2) greenhouse trials to test the selected compounds in potato cultures under nematode presence with evaluation of plant physiological response and volatile profile; and (3) preliminary studies in nematode transcriptomic changes and molecular pathways during the response to the nematicidal compounds. Results will be an add-value to the continuous investigation on environmental-friendly substances for the suppression of plant parasitic nematodes

Control of the root lesion Pratylenchus penetrans - the effect of nematocidal activity of plant-derived compounds

The root lesion nematode, Pratylenchus penetrans, is one of the most harmful plant parasites, responsible for worldwide productivity losses in a significant number of plant hosts. Generally, chemical control relies on synthetic compounds used through fumigation or direct contact, which offers a systemic protection. These control methods are costly and hazardous to the environment and to humans. Phytochemicals may play an important role in nematode control. The nematicidal activity of eight compounds that occur naturaly in plants, from two classes of compounds, was assessed at 2 mg/mL, for 24 h. Bioassays were performed following the standard direct contact methodology. P. penetrans was remarkably tolerant to the tested compounds, with mortality range between 1.0 and 5.8 %. To the best of our knowledge, the nematicidal activity of 4 phenolic compounds (catechin, caffeic acid, gallic acid and gentisic acid) was evaluated for the first time for P. penetrans

Host plant response to the application of nematocidal phytochemicals

Potato (Solanum tuberosum) is one of the world’s most common agricultural crop for human consumption and considered an excellent source of essential nutrients and vitamins. The productivity of this crop is threatened by several plant-parasitic nematodes, among which the root-lesion nematode Pratylenchus penetrans is already considered to have a significant impact. Over the years, due to an increasing need of food resources and absence of knowledge, synthetic nematicides were extremely applied on crop fields. However, new policies, supported by the Sustainable Development Goals, impose serious limits to the application of such products. Volatile organic compounds naturally produced by plants have long been recognized for their nematicidal effectiveness. In previous work, the nematicidal activity of several standards of naturally occurring phytochemicals was in vitro assessed on P. penetrans. Only 4 compounds caused more than 99% nematode mortality, namely benzaldehyde, carvacrol, octanol and thymol. The aim of this work was to determine the most suitable approach to apply the selected compounds in soil in order to maintain their nematicidal effectiveness without compromising host health. Two consecutive trials were performed using potato seed plants (cv. Agria), namely: (i) to evaluate the effect of three solvent agents - 10% (v/v) dimethyl sulfoxide (DMSO), 5 mg/mL TRITON-X, and 1% (v/v) acetone - in plant growth; and (ii) to evaluate the best solvent agent spiked with 2 mg/mL of each selected compound. For both trials, a control treatment with water was considered. Pots were maintained in growth chamber conditions, routinely watered and fertilized. Plant growth related parameters were assessed at collection time. In the 1st trial, potato plants were unable to grow in the presence of 10% DMSO solution or the 5 mg/mL TRITON-X. The potato seeds could germinate only in the presence of 1% acetone. In the 2nd trial, potato seeds could germinate in the presence of the 4 compounds dissolved in 1% acetone. No statistical differences (p>0.05) were recorded between the control treatment and the different compound applications in terms of growth parameters observed. Ongoing research is now accessing the nematicidal activity of each of the four compounds in the presence of the potato plant

Elucidation of the nematicidal mode of action of 3-octanol on the root-lesion nematode Pratylenchus penetrans

The root-lesion nematode (RLN), Pratylenchus penetrans, is one of the most severe plant parasitic nematodes (PPN), responsible for productivity losses in a significant number of plant hosts. The chemical control of these PPNs relies on fumigants and non-fumigants compounds, which offer a systemic protection. Such control methods are costly and hazardous to the environment and to humans. Compounds naturally produced by plants may play an important role in nematode control. The aim of this study is to analyze the molecular response of P. penetrans to the nematicidal agent 3-octanol (C8H18O) using a De novo transcriptome assembly approach. Previous work showed that 3-octanol is a strong nematicide of P. penetrans causing 100% mortality in standard direct contact for 24h. The mode of action of this nematicide was studied at sub-lethal exposure for 30 min. Overall, the aliphatic alcohol 3-octanol induced the activation of detoxification mechanisms of P. penetrans with up-regulation of metabolic pathways related with metabolism of xenobiotics by CP450, steroid hormone synthesis, retinol metabolism, drug metabolism among others, while affecting ascorbate and aldarate metabolism, starch/sucrose metabolism and several signaling pathways related with cellular immune response. Understanding the defense mechanisms of P. penetrans against natural nematicides can lead to development of tailor- made control solutions

Distribution patterns of benthic bacteria and nematode communities in estuarine sediments

Benthic organisms are crucial in the regulation of the ecosystem functions. The interactions between benthic nematodes and sediment bacteria across divergent environmental conditions are poorly understood. The main goal of this study was to understand the spatial distribution patterns and diversity of benthic bacterial com- munities and nematode assemblages of the intertidal sediments in three sampling sites (Navigator, Tro ́ia and Moinho) along Sado Estuary (SW, Portugal). Bacterial communities were described using a 16S metagenomic approach, while nematode assemblages were characterized using morphological identification. Overall, bacterial and nematode communities presented significant diversity between sites (p < 0.05), which is primarily related with the environmental variables (e.g., organic matter and percentage of gravel). The spatial distribution of bacterial communities was in accordance with the ecological conditions of three selected sites at a larger scale than nematode assemblages. Previously described as good ecological indicators, nematode assemblages were separated at sampling site level, suggesting that their response is driven by within site specific factors at a smaller scale. Hence, the present study set a fundamental ground for future research on functional interactions between bacteria and nematodes