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Brassinosteroids (BRs) are group of plant steroidal hormones that modulate developmental processes and also have pivotal role in stress management. Biosynthesis of BRs takes place through established early C-6 and late C-6 oxidation pathways and the C-22 hydroxylation pathway triggered by activation of the DWF4 gene that acts on multiple intermediates. BRs are recognized at the cell surface by the receptor kinases, BRI1 and BAK1, which relay signals to the nucleus through a phosphorylation cascade involving phosphorylation of BSU1 protein and proteasomal degradation of BIN2 proteins. Inactivation of BIN2 allows BES1/BZR1 to enter the nucleus and regulate the expression of target genes. In the whole cascade of signal recognition, transduction and regulation of target genes, BRs crosstalk with other phytohormones that play significant roles. In the current era, plants are continuously exposed to abiotic stresses and heavy metal stress is one of the major stresses. The present study reveals the mechanism of these events from biosynthesis, transport and crosstalk through receptor kinases and transcriptional networks under heavy metal stress.

The day by day increase in industrial wastes, sewage irrigation, and pollutants such as organic, inorganic, chemical fertilizers and pesticides have caused the excessive release of toxic metals into the environment either through direct or indirect means. Agricultural soil contamination by these toxic pollutants is becoming a serious environmental problem. However calcium and hydrogen sulfide-mediated signaling pathways regulate many physiological, biochemical, and molecular processes in plant system. These signaling molecules also help the plants to cope up abiotic and biotic stresses and play essential roles in plant growth and development. Calcium and hydrogen sulfide (H2S) signaling activates a cascade of biochemical events that enhance plants’ tolerance to abiotic and biotic stresses, such as controlling reactive oxygen species, activating the antioxidant defense system, cytosolic osmoprotectants accumulation, and induction of Ca2+ increase in cytosol.

Over the years, nanotechnology has emerged with diverse roles in the research field. Its application in agriculture, medicine, public health, cosmetics, bioremediation, and pharmacology has gained immense research interest. The increase in demand and widespread use of nanoparticles (NPs) for agricultural purposes has generated concerns about the living ecosystem and its entities. Metal-based nanoparticles like CuO, CeO2, TiO2, ZnO, CoO, and Ag show adverse effects on plants if they get accumulated in large amount from their respective sources in soil, in sediments, agricultural land and aquatic environments. Most of the nanoparticles had a detrimental impact on the structure and function of photosynthetic apparatus and also caused disruption of grana, etc. The effective use of nanotechnology in agriculture suggests both negative and positive responses of nanoparticles on the growth and development of plants, depending upon the dose, application, and plant species. Continuous exposure of crop plants to nanoparticles and their accumulation in different plant parts, including fruit, grains, and other edible parts, disrupts human health. In recent years, the prime focus of researchers is to understand the mechanism of uptake, transport and biotransformation of nanoparticles in plants at the physiological, biochemical, and molecular level.

Global developmental strategies and population expansion are continuously showing their odd impacts on the living world, thereby causing stresses of multiple natures. To combat these stresses, hydrogen sulfide (H2S) is well-examined signaling molecule that acts as a priming agent and helps in regulating the response of plants to various stressful conditions. Hydrogen sulfide is formed in the plant cells as an intermediate of an assimilatory sulfate reduction. Despite the endogenous release of hydrogen sulfide, its exogenous application has been found to be beneficial in the amelioration of multiple abiotic stresses. These responses are also mediated by the expression of genes and proteins that participate in signaling and metabolic pathways induced through several small signaling molecules known as plant hormones or phytohormones. Phytohormones are also found to be involved in regulation of the protective responses under various abiotic and biotic stress conditions. H2S in crosstalk with these phytohormones significantly ameliorates the abiotic stress in plants. In this chapter we have discussed in detail how H2S in crosstalk with phytohormones helps in the enhancement of defense against abiotic stress in plants.

With the ongoing climatic fluctuations, abiotic stressors are the limiting factors which negatively regulate the growth and yield of crop plants. Among them, drought is a major challenge to the field crops, as it leads to the unavailability of nutrients and minerals to the plant system. Agrochemicals, warmer temperatures, high CO2 levels, and shortage in available water resources typically change the frequency and duration of drought stress. Plant growth-promoting rhizobacteria (PGPRs) are well known to make chemical physical and biological interactions in the rhizosphere region, as these microorganisms feed on the root exudates and further help the plants in growth and development by changing the nutrient dynamics and susceptibility to drought stress. Nutrient like phosphorous (P) is present in organic and inorganic forms but is least available to the plants because they mostly form reactive metal complexes in the soil. However, the soil inoculated with PGPRs significantly improves the solubilization of these phosphates into available form. On the other hand, sulfur (S) is a key limiting element in every ecosystem, and it is required for the synthesis of certain plant hormones. It is also an important constituent of few amino acids. Immobilization of these sulfur compounds to available forms is also triggered by these microorganisms. Finally, the PGPRs act against drought stress by regulating phytohormone status; accumulation of compounds like sugars, amino acids, polyamines, and antioxidants; and stress-related gene expressions. Rhizobacteria not only induce local responses like mineral mobilization, root elongation, and root architecture but also mediate hormonal regulation for the growth and physiological responses in plants under drought stress. This chapter provides a brief overview on plant-rhizobacteria interaction and PGPR-mediated hormonal and nutrient regulation in plants under drought stress.

Rhizosphere comprises inimitable environs, acting as a battleground for PGPR to form an origin of enriched source of unique functionalities for plants. PGPR are closely bound to roots in order to nurture them and protect them from pathogens specifically Root-knot Nematodes (RKNs) through direct and indirect manner. RKNs are mainly endoparasitic to cause dreadful diseases in plants, especially economically important crops. The traditional use of chemical agents for nematode control in agricultural yields has been adversely affecting the soil microbiota as well as physiochemical attributes of soil. Contrastingly, PGPR forms symbiotic associations alongside plants and act as biofertilizers, bioprotectors, and sustainable means for agricultural practices. PGPR are observed to improve nutrient acquisition and release various secondary metabolites like phenols, enzymes, siderophores, hydrogen cyanide (HCN), phytohormones, and hydrolytic enzymes to boost the antioxidant potential of plants damaged by RKNs. Moreover, PGPR also synthesize various antimicrobial agents or toxins in order to directly attack the invading RKNs for activating induced systemic resistance mechanism of plants and enhance antioxidative properties within them. Therefore, all these mechanisms are of paramount importance as they are well-documented biocontrol agents in the field of agronomy. Here, in this chapter, we will discuss the role of PGPR and their interaction along with plant roots within rhizosphere to form RKN resistance. In general, we have elaborated the mechanistic role of PGPR in mediating antioxidant capacity of plants against nematode infestations. Ultimately, we have recapitulated the role of PGPR to be used as alternatives to chemical fertilizers for sustainable agriculture.

Legumes beyond being an imperative source of nutrients have a wide array of antioxidants and other phytochemicals essential for plant growth and development. Melatonin (MEL, 5-acetyl-5-methoxy trptamine) is a ubiquitous phytochemical present in all organisms that plays a multiregulatory role in plant growth and development, senescence, yield, fruit ripening, circadian rhythmes, and response to abiotic stresses. Under stress conditions MEL is accumulates sharply by regulation of its synthesis and varied metabolic pathways. Tryptophan is a precursor for MEL synthesis and four consecutive enzymes mediate synthesis of tryptamine, N-acetylserotonin, and MEL. It is widely documented that production of phytomelatonin is elevated in plants exposed to abiotic stresses including salinity, heavy metal, water, and temperature. It acts as a powerful antioxidant that directly scavenges reactive oxygen species. Moreover, it confers stress tolerance in legumes by activation of antioxidative defense, modulation of transcription factors that are prerequisite for stress resistance and chelation phenomena, and modulation of hormone cross-talk with hormones including salicylic acid, ethylene, and jasmonic acid. Thus the present chapter focuses on biosynthesis of MEL in plants, stressed-induced alteration in endogenous levels of MEL in legumes, the role of exogenously applied MEL in alleviating stress conditions, and underlying mechanisms of MEL-refereed tolerance and detoxification.

Communication among different species across kingdoms occurs through a chain of regulatory molecules that are transferred around cellular boundaries. These molecules are also crucial for defense, virulence, and pathogenesis. In the past, the transport of proteins in long distance communication was observed, but in the present era, the discovery of extracellular vesicles (EVs) has changed our understanding of molecular communication. EVs are not only involved in cell signaling and immunity but also can transfer information by sRNAs, forming a basis for interactions among a wide variety of organisms. Despite extensive research on EVs in other areas, their role in communication between plants and the plant microbiome has been lacking. EVs are potentially involved in protein trafficking along with the transport of lipids and nucleic acids. Interactions between hosts and their microbiomes may also be mediated by EVs, which can be involved in stress responses, immune surveillance and defense, virulence, and signaling, along with many metabolic activities within plant microbiomes. In this review, we have focused on recent information about the role of EVs and the molecules they transport between hosts and microbes. The connection between biofilms and the generation of EVs is also considered. These findings enhance our knowledge about plant-microbiome interactions in terms of immunity and virulence and challenge the conventional viewpoint of inter-kingdom signaling.

The pronounced and major effects of contamination of the environment with heavy metals and other xenobiotic compounds have become a major problem worldwide. Soil contaminated with heavy metals poses serious threat to plants, animals as well as human health. Heavy metals due to their toxicity reduces the soil fertility, affects the plant photosynthetic efficiency, reduces yield of the crops, and causes nutrient imbalance. Phytoremediation an eco-friendly, clean, and green technology helps to remove contaminants from the polluted soils. The use of beneficial microorganisms along with plants is considered as an effective method for increasing the efficiency of remediation of contaminated soils. Earthworms also play an important role in remediation process. Interaction of plants with microflora plays a vital role in bioavailability of the metals and their bioaccumulation in plants.

Metal ions are the natural components of earth’s crust. At low concentrations, metal ions are really essential to the plant system, but their excessive amounts lead to toxicity. Industrial revolution and anthropogenic activities are critically disturbing the level of metal ions in the ecosystem, like soil, water, and plant resources. Elemental and chemically combined forms of these metal ions such as Cd, As, Pb, Hg, Cr, and Zn are toxic to plant system, leading to loss in yield. Also, the heavy metal toxicity hinders the efficacy of photosynthetic apparatus, and causes oxidative stress and abnormal plant growth. Plants overcome such toxicity by assimilating these toxic metals into the root system or transforming these complexes into physiologically tolerant forms. Plants have adopted a series of physiological and molecular mechanisms to mitigate these stress responses that might enable them to withstand higher metal ion levels in soil. Scientific community is highly concerned to understand these mechanisms. Researchers are also keen to know the roles of plant growth regulators auxins, gibberellins, brassinosteroids, ethylene, cytokinin, jasmonic acid, salicylic acid, etc. in signaling pathways associated with the heavy metal tolerance in plants. Recent research has possible evidences of PGR’s role in the mitigation of heavy metal toxicity, as well as shows that it can regulate and integrate the factors associated with growth and development. In this chapter, we have discussed the role of PGRs in alleviating heavy metal stress, their mechanism of action, and cross talk.