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Summary Plant-parasitic nematodes are devastating phytopathogens that cause substantial economic losses in the agricultural sector. There are numerous management measures available to reduce the threat posed by plant-parasitic nematodes. One of the most effective management strategies is the use of synthetic chemical nematicides. However, many of these chemicals negatively impact the environment and health of non-target organisms, so there is great urgency for finding new environmentally safe and sustainable approaches for managing plant-parasitic nematodes. Plant roots release numerous metabolites that possess defensive properties; thus, it is imperative to understand the root-regulated interactions among plants and nematodes that form the basis for managing plant-parasitic nematodes. The present review focuses on the root metabolites and chemical agents that serve as attractants, repellents, stimulators, or inhibitors for nematodes. They influence plant resistance by modifying plant physiology to resist nematodes and can also inhibit nematode growth and development by secreting antinematode chemicals. The ultimate goal of this review is to focus mainly on the current state of knowledge on the interaction between plant roots and the potential of these compounds in management of plant-parasitic nematodes.

Brassinosteroids (BRs) are steroidal phytohormones that play a multitasking role during the growth and development period as well as stress responses. Plant mutants in BR production or defective signaling result in various physiological as well as phenotypical abnormalities. Presently, various studies have highlighted stress tolerance induction by brassinosteroids in various plant species indicating BRs as a broad spectrum key regulator participating in various molecular processes. The present chapter highlights the studies on BRs regarding their potent role in inducing stress tolerance during viral, fungal, bacterial or nematode attacks in plants. BR's multifaceted role during stress and its potential cross-talk with other phytohormones in adapting and ameliorating the stress have been critically analyzed. The divergence in dosage of BR analogs and their mode of application for management of stress are also discussed which serves as a future area of interest in challenging agriculture, thereby highlighting relevant knowledge gaps.

Hydrogen sulfide (H2S), an important signal molecule, plays a key role in the growth and development of plants. Along with this, it has been found to ameliorate various stresses like salinity, drought, heavy metals, radiation, etc. by regulating the antioxidant levels or via certain other mechanisms. Since UV radiation induces damage to living organisms, including plants, the deleterious effects of such radiation have been observed in the impairment of the growth and development of plants. H2S is found to be involved in the amelioration of radiation stress by promoting the accumulation of certain reactive oxygen species scavengers, UV-absorbing compounds, and other antioxidants and antioxidative enzymes.

Pathogen attack in crop plants has negative impacts on plant biology which ultimately causes reductions in crop yield. Hydrogen sulfide (H2S) in addition to its normal plant physiology functions is known for its roles in the regulation of plant physiology under biotic stress. This H2S-mediated regulation of plant biology under biotic stress is modulated by hormonal crosstalk. In this chapter we will discuss how H2S regulates plant growth under biotic stress by interacting with plant hormones like auxins, abscisic acid, ethylene, jasmonic acid, gibberellins, and salicylic acid.

Chromium (Cr) is an element naturally occurring in rocky soils and volcanic dust. It has been classified as a carcinogen agent according to the International Agency for Research on Cancer. Therefore, this metal needs an accurate understanding and thorough investigation in soil–plant systems. Due to its high solubility, Cr (VI) is regarded as a hazardous ion, which contaminates groundwater and can be transferred through the food chain. Cr also negatively impacts the growth of plants by impairing their essential metabolic processes. The toxic effects of Cr are correlated with the generation of reactive oxygen species (ROS), which cause oxidative stress in plants. The current review summarizes the understanding of Cr toxicity in plants via discussing the possible mechanisms involved in its uptake, translocation and sub-cellular distribution, along with its interference with the other plant metabolic processes such as chlorophyll biosynthesis, photosynthesis and plant defensive system.

24-Epibrassinolide (EBL) is considered the most probable brassinosteroid (BR) candidate that could be used for practical application in agriculture. EBL-induced stress-protective properties were evaluated in in vitro-grown tomato (Solanum lycopersicum L.) varieties Pusa Ruby (susceptible to nematodes) and PNR-7 (resistant to nematodes) during nematode pathogenesis. Sterilized tomato seeds treated with 10−11, 10−9, or 10−7 M EBL and germinated in vitro were inoculated with second-stage juveniles of Meloidogyne incognita [(Kofoid and White) Chitwood]. Whole plant analyses of morphological and biochemical parameters 7 d after inoculation showed significant improvements in plant growth and development for both varieties and a highly significant reduction in the number of galls in the susceptible variety. Increased specific activities of antioxidative enzymes (catalase, ascorbate peroxidase, glutathione reductase, glutathione peroxidase, guaiacol peroxidase, and superoxide dismutase) were observed in EBL-treated seedlings of both varieties, but increases were higher in the resistant variety. A highly significant increase in antioxidants (ascorbic acid content, total flavonoid content, total glutathione content, and total phenolic content) was observed in EBL-treated Pusa Ruby seedlings, whereas in PNR-7, significant increases were found except for total flavonoid content, which increased non-significantly. Confocal microscopic images showed amelioration of stress in roots of EBL-treated seedlings as indicated by the decrease in level of green fluorescence in them as compared to untreated and nematode-inoculated roots.

Meloidogyne spp. are microscopic, obligatory endoparasites with worldwide distribution which cause severe damage to agricultural crops. The present study revealed the nematicidal activity of Streptomyces antibioticus strain M7 against Meloidogyne incognita. The culture supernatant of the isolate caused 100% J2 mortality after 24 h and inhibited egg hatching (only 3%). In addition, the nematicidal activity of actinomycins V, X2 and D purified from strain M7 was also checked. In vitro studies displayed 97.0–99.0% juvenile mortality and 28.0–44.0% egg hatching after 168 h at 240 µg/ml of actinomycin, with LD50 (lethal dose) values of 28–120 µg/ml. In vivo study further validated the nematicidal activity of strain M7, where nematode infested tomato plants treated with culture supernatant/cells/solvent extract showed reduction in root galls and egg masses per plant by 50.0–62.06% and 53.48–76.74%, respectively, and significantly enhanced the shoot length (54.67–76.39%), root length (36.45–64.88%), shoot fresh weight (111–171.77%), root fresh weight (120–163.33%), shoot dry weight (54.45–145.45%), and root dry weight (100–133.3%) over the nematode infested plants treated with water. Furthermore, tomato plants treated with cells/culture supernatant/extract of strain M7 without nematode infestation also showed significant increase in various plant growth parameters. Thus, the outcome of the study revealed the potential of S. antibioticus strain M7 and actinomycins produced from it to be developed as safe nematicidal agents to control the root knot nematodes, and to increase the crop yield.

Pesticides are extensively used in modern agriculture and are an effective and economical way to enhance the yield quality and quantity, thus ensuring food security for the ever-growing population around the globe. Approximately, 2 million tonnes of pesticides are utilized annually worldwide, where China is the major contributing country, followed by the USA and Argentina, which is increasing rapidly. However, by the year 2020, the global pesticide usage has been estimated to increase up to 3.5 million tonnes. Although pesticides are beneficial for crop production point of view, extensive use of pesticides can possess serious consequences because of their bio-magnification and persistent nature. Diverse pesticides directly or indirectly polluted air, water, soil and overall ecosystem which cause serious health hazard for living being. In the present manuscript, an attempt has been made to critically review the global usage of different pesticides and their major adverse impacts on ecosystem, which will provide guidance for a wide range of researchers in this area.

Brassinosteroids (BRs), a class of steroidal hormones, play diverse roles in plant growth, development, signaling and defense against various biotic and abiotic stresses. It is broad spectrum key regulator in plants that participates in various molecular processes. Exogenous application of BRs vanish various constrains in the path of agricultural development. The present book chapter highlights the interaction and crosstalk of brassinosteroids with other phytohormones such as auxins, gibberellins, jasmonic acid, abscisic acid, salicylic acid, polyamines, ethylene and strigolactones in regulation of various physiological and developmental processes in plants. Various pathways reveal the versatile role of brassinosteroids in various hormonal interactions.

The present work demonstrated the nematicidal potential of Streptomyces hydrogenans strain DH16 (a strain with strong antagonism against fungal phytopathogens and insect pest) against Meloidogyne incognita. The culture supernatant and solvent extract significantly inhibited egg hatching (almost 100%) along with J2 mortality of more than 95% after 96 h. The nematicidal activity of 10-(2,2-dimethyl-cyclohexyl)-6,9-dihydroxy-4,9-dimethyl-dec-2-enoic acid methyl ester (SH2; a new antifungal compound) purified from this streptomycete was also evaluated using different concentrations. The juvenile mortality of the nematode increased with increasing concentration and exposure time and reached the maximum (95%) after 96 h at concentration of 100 μg/ml. After 160 h of incubation, egg hatch of 16% was observed at concentration of 100 μg/ml as compared to control where 100% egg hatching was achieved. However, at the highest concentration of the compound (200 μg/ml), 100% J2 mortality and 0% egg hatching were observed after 72 and 160 h of incubation, respectively. In vivo pot experiments further revealed the nematicidal potential of S. hydrogenans where soil drenching with its culture supernatant and cells effectively controlled root galls, egg masses in nematode infested tomato plants and at the same time promoted the growth of tomato plants. Additionally, in the absence of nematodes, soil drenching with culture supernatant and cells significantly enhanced the various agronomic traits of plants as compared to control plants. Thus, the outcomes of the current study endorse the potential of S. hydrogenans strain DH16 and its metabolites to be developed as safe nematicidal and plant growth promoting agents.

Plant growth and development is under the control of mutual interactions among plant hormones. The five classical categories of plant hormones include auxins, cytokinins, gibberellins, abscisic acid and ethylene. Additionally, newer classes of plant hormones have been recognized like brassinosteroids, jasmonic acid, salicylic acid and polyamines. These hormones play significant roles in regulating the plant growth and development. Various receptors and key signaling components of these hormones have been studied and identified. At genetic level, crosstalk among the various plant hormones is found to be antagonistic or synergistic. In addition, components of signaling pathway of one plant hormone interact with the signaling components of other hormone. Thus, an attempt has been made to review the literature regarding the role of plant hormones in plant physiology and the common molecular players in their signaling and crosstalk.