Negative Effects Of Pesticides On The Environment – In recent years, the expansion and implementation of next-generation sequencing and the reduction of costs have raised interest in phytobiome studies that have allowed the ecological interactions that regulate the holobiont to be dissected. Indeed, herbal plants are associated with a great diversity of microorganisms in all their parts. Crop microbiota affects plant phenotype, growth, yield and quality by contributing to plant disease resistance, plant adaptation to abiotic stresses and plant nutrition. The link between land plants and microbes developed at least 460 million years ago, as suggested by fossil evidence of the earliest land plants, indicating the essential role of microbes for plants. Recent research shows that plants actively recruit beneficial microorganisms to facilitate their adaptation to environmental conditions. Cultivation methods and disease control measures can affect the structure and functions of the plant microbiome. Both pesticide applications and biological control agents can alter biodiversity within the phytobiota and suppress beneficial functions. However, to date, the effects of disease control measures on phytobiota and their possible side effects on plant growth, productivity and crop quality remain a neglected field of study. This paper summarizes the known effects on phytobiota, providing evidence for the role of the plant microbial community in determining the overall effectiveness of applied control measures and suggests that future studies on plant disease control also consider microbe-mediated effects on plant fitness.
Plants live in close association with a dynamic phytobiota, which consists of macrobiota as well as microbiota, especially microbes that inhabit the soil in which plants grow. The importance of soil microbial communities is well recognized for their role in plant performance, including enhancing plant growth, vigor and fitness (Marschner and Rumberger, 2004; Lau and Lennon, 2011; Chaney and Baucom, 2020), as well as nutrient acquisition (Chaparro et al. al., 2012; Reverchon et al., 2015), associated with the presence of rare taxa (Hol et al., 2010) and the attraction of pollinators, predators and parasites of herbivores (Wagner et al. , 2014; Berg et al., 2016; Etemadi et al., 2018). Plants modulate the rhizosphere microbiota (Berendsen et al., 2012; Kwak et al., 2018) and selectively recruit beneficial microbes (Park and Ryu, 2021) thanks to signals, including root exudates, volatile compounds (VOCs), and phytohormones shape the phytobiota by recruiting, rejecting and coordination of interactions (Dicke, 2016). Microorganisms associated with plants form complex networks with other members of the same species, as well as with different species, genera, families, and even domains of life (Wassermann et al., 2019a). Cooperation between plants and microorganisms, as well as interactions within the microbiome, require intensive communication that regulates community assembly (Agler et al., 2016; Venturi and Keel, 2016).
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Negative Effects Of Pesticides On The Environment
The homeostatic balance between microbe-microbe and host-microbe interactions is critical for plant health. Disturbance of this balance is often called microbial dysbiosis and may represent an important mechanism of disease (Martins dos Santos et al., 2010; Sekirov et al., 2010; Kemen, 2014). For example, under high humidity conditions, several immunocompromised Arabidopsis mutants show leaf tissue damage associated with altered phyllosphere microbiota composition, somewhat similar to the dysbiosis that occurs in human inflammatory bowel disease (Chen et al., 2020). Importantly, bacterial community transplantation experiments have shown that the application of a dysbiotic leaf bacterial community to healthy plants leads to tissue damage, indicating that in this case dysbiosis is the cause of the observed symptomatology (Chen et al., 2020). Nevertheless, it is worth mentioning that stress-induced deviation from eubiosis is not always associated with reduced plant performance (Paasch and He, 2021).
Pesticides Explained: The Toxic Chemicals In Up To 70% Of Produce
Phytobiota protect crops from diseases through different modes of action: i) indirect effects mediated by activation of plant-induced systemic resistance (ISR), resistance to pathogen infections transmitted from roots to plant tissues (Pieterse et al., 2014; Conrath et al., 2015 .), ii) exclusion of pathogens in a niche by competition for nutrients and space (Spadaro and Droby, 2016), and/or iii) direct interactions with pathogens through hyperparasitism (invasion and killing of mycelia, spores and resting structures) or antibiosis (production antimicrobial secondary metabolites) (Raaijmakers and Mazzola, 2012; Ghorbanpour et al., 2018). Finally, some fungal viruses are used to induce hypovirulence (Milgroom and Cortesi, 2004; Double et al., 2018) and microbial antagonists can act via inactivation of enzymes involved in pathogenic infections (Elad, 2000) or enzymatic degradation of pathogenic structures (Köhl et al. ., 2019).
One easily overlooked, yet crucially important, aspect to consider when studying phytobiota is that pathogenic species are themselves part of all microbiota (Kamada et al., 2013). This is actually a key aspect of epidemiology since the presence of a pathogen does not necessarily lead to infection and disease is only caused under certain inductive conditions (Scholthof, 2007). In the context of dysbiosis, the concept of pathobiota, which integrates pathogenic agents within biotic environments, has been established and applied to several pathosystems (Baltrus, 2017). Analyzes of the pathobiota have often revealed that pathogens do not act independently, and multiple pathogens may be involved in severe dysbiosis (Singer, 2010; Lamichhane and Venturi, 2015; Berg et al., 2021). This is the case of Fusarium, a very dynamic disease caused by a complex of different Fusarium species, whose composition is geographically dependent, and each of them can respond differently to a changing climate (Yli-Mattila, 2010; Vaughan et al. , 2016). Even if dominant, highly virulent F. graminearum is only one member of the species complex (Ioos et al., 2005; Xu et al., 2005) and control strategies against F. graminearum have recently been found to be hampered by the presence of poorly pathogenic F. poae, demonstrating the complexity of developing control strategies against plant diseases caused by multiple pathogens (Tan et al., 2021). A similar scenario was observed for phytopathogenic bacteria. Pseudomonas syringae pv. actinidiae is always present in a syndemic relationship with Pseudomonas syringae pv. syringae and Pseudomonas viridiflava, two other pathogens of kiwi fruit, suggesting the establishment of a pathogenic consortium leading to a higher capacity of pathogenesis (Purahong et al., 2018). For these reasons, new disease control strategies should target the entire phytobiota and possible pathogen consortia within it rather than individual pathogens to develop suppressive microbial communities that can limit several pathogens at the same time.
Our current knowledge of communication within the phytobiota is still largely based on interactions involving two or three components, and is often measured under controlled conditions. However, a further level of complexity in the study of phytobiota refers to the eukaryotic organisms that interact with plants, each of which has its own internal and external microbiota, influencing interactions between plants and their microbial communities (Cusano et al., 2011; Khaitov et al. al. ., 2015.). The intense communication among members of the plant holobiont and the observations that signals can be co-opted, modified or even destroyed by another member of the community reinforce the need for system-level analysis of communication mechanisms to exploit phytobiota manipulation for crop improvement ( Figure 1 ).
Figure 1 Role of phytobiota and effects of disease control measures on phytobiota (current situation). The panel on future developments highlights various strategies to achieve a long-term and sustainable approach to disease control based on phytobiota manipulation.
Vital Soil Organisms Being Harmed By Pesticides, Study Shows
Chemical pesticides are widely used in intensive agriculture and are still the most effective, reliable and economical means of reducing losses caused by pests and diseases (Huang et al., 2021a). From this point of view, pesticides and fertilizers are essential to ensure food security for an ever-growing world population. Globally, more than 2 million tons of pesticides are used annually (Sharma et al., 2019). In Europe, awareness of the risks of pesticides has led to the development of policies for their sustainable use (Directive 2009/128/EC) and actions, such as the Farm2ForK Strategy, which aims to reduce pesticide use by 50% by 2030. Indeed, in the last decade, the use of pesticides in the EU decreased by 10.2% with the lowest total sales (333,500 tons) recorded in 2019. Fungicides, bactericides and herbicides are still the main groups of pesticides currently used (EUROSTAT – European Statistics, 2021). Growing concern about the impact of pesticides on human health and the environment has prompted research into sustainable alternatives, as well as extensive study of the direct and indirect effects of their use on non-target organisms and the ecosystem services they provide (Ramakrishna et al., 2019). Soil microbiota is crucial for ecosystem functions (Karas et al., 2018). Since synthetic fertilizers and pesticides have a long persistence in the soil, in the last decade, several studies have investigated the impact of pesticides and fertilizers on soil microflora and microbial fertility, which have a direct impact on environmental quality, as well as plant health and productivity (Hartmann et al. , 2014; Zhu et al., 2016; Huang et al., 2021b). This led the European Food Safety Authority (EFSA) to include soil microorganisms in the environmental risk assessment of pesticides (EFSA Panel on Plant Protection Products and their Residues (PPR), 2010; Karas et al., 2018 ). However, the toxicological assessment of pesticides is based only on their effect on the N mineralization test (OECD, 2000) and therefore does not provide information on the key functions of the soil microbiota (Karas et al., 2018). Despite the growing body of knowledge about the effects of pesticides on soil microorganisms, standardized methods have not yet been developed and most studies have been conducted in the laboratory or using higher exposure levels.
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