**2.5 Toxin**

Toxic metals have been added to agriculture soils as a result of increased reliance on chemical fertilizers and sewage wastewater irrigation, as well as increasing industrialization, having detrimental consequences on the soil–plant environment system [16]. These metals bioaccumulate and slowly enter plants through air, water, and progression of the food chain over a certain period of time [17].

#### **3. Crop plants and biotic stresses**

Plants are subjected to a variety of biotic stress caused by various living organisms such as fungi, viruses, bacteria, nematodes, and insects [2]. These biotic stress agents induce a variety of diseases, infections, and damage to crop plants, lowering agricultural yields. However, different strategies for overcoming biotic stressors have been created through research methodologies. The biotic stresses in plants can be overcome by studying the genetic mechanism of the agents causing these stresses [18]. Genetically modified plants have proven to be a great effort against biotic stresses in plants by developing resistant varieties of crop plants.

Plant-parasitic nematodes feed on the contents of plant cells and can feed on all sections of the plant, but they predominantly cause soil-borne illnesses and affect the root system. They cause wilting and stunting, which are signs of nutritional inadequacy. Viruses cause not only local but also systemic damage to plants, causing stunting, chlorosis, and deformities in many areas of the plant, despite the fact that they rarely kill their hosts [19]. Plants are harmed when insects feed or lay eggs on them. Viruses can be transmitted to plants by piercing-sucking insects *via* their stylets. There are two types of fungus parasites: nectrotrophs, which use toxins to kill host cells, and biotrophs, which do not. They induce vascular wilts, leaf spots, and cankers, among other symptoms, and can infect different sections of the plant when combined with bacteria [20].

#### **4. Plant defenses against abiotic stresses**

Plants use five general botanical defenses against abiotic stresses. These include cuticle, unsaturated fatty acids, reactive species scavengers, molecular chaperones, and compatible solute, which are also an economic important trait [21].

#### **4.1 Cuticle**

This is the exterior translucent lipid structure in land plants, which seals the aerial surface of their organs. It is coated by cuticular waxes and is described as a hydrophobic layer. As the primary interface between plant and environment, the cuticle plays critical role in restricting liquid and gas fluxes, defending pathogen and insect attacks, and resisting various abiotic stresses. It is an elegant innovation of land plants to deploy an outermost shield derived from simple molecules, which is fundamental to their success in terrestrial colonization [22]. Wax accumulation in the cuticle is closely associated with multiple stress tolerance [23].

#### **4.2 Unsaturated fatty acids**

Unsaturated fatty acids containing 16 or 18 carbon atoms are the key ingredients of the membrane and the prime stocks for the cuticle. The unsaturated nature of fatty acids is a major determinant of membrane fluidity [21]. Dysfunction of biomembrane due to protein deactivation and ion leakage are caused by cold-driven rigidification and heat-driven fluidization, which makes membrane fluidity susceptible to various abiotic stresses, especially at high temperatures [24]. An increase *Stresses in Plants: Biotic and Abiotic DOI: http://dx.doi.org/10.5772/intechopen.100501*

in the level of normal alkanes with a decrease in the level of primary alcohols can lead to cold susceptibility, which can cause growth retardation, while an increase in the levels of both n-alkanes and primary alcohols resulted in better viability, where drought and freezing will have no effect on plant growth [25]. When polyunsaturated fatty acids are liberated by lipase form glycerolipids, they serve as raw materials for the synthesis of oxylipins, a bioactive molecule that is involved in the diverse physiological processes of stress resistance [26].

#### **4.3 Reactive species scavengers**

The reactive species scavengers include reactive carbonyl species (ROS) and reactive oxygen species (RCS). The ROS and RCS are interwoven, due to the fact that RCS can arise from ROS-induced lipid peroxidation, while ROS can be raised by RCS activities the other way round. Abiotic stresses can trigger a burst in both ROS and RCS thereby turning the two scavengers into a general defenses. Plants utilize both enzymatic and non-enzymatic means to developed sophisticated ROS scavenging system [21]. The application of excessive nitrogen fertilization in crop cultivation depresses the ROS scavenging system causing the increase in stress susceptibility [27].

#### **4.4 Molecular chaperones**

Molecular chaperones are induced to facilitate protein folding, assembly, transport, and degradation. Heat shock protein (HSP), which are good examples of molecular chaperon, is employed by all living organisms to counteract all detrimental conditions that can induce protein damage, wherein they function to prevent aggregation of denatured proteins, assist in their refolding, or present them to lysosomes or proteasomes for proteolysis, thereby restoring cellular homeostasis [28].

#### **4.5 Compatible solutes**

They are electrical neutral small organic compounds with high solubility and low toxicity. The molecules include sugar, amino acids, and their derivatives [21]. In an abiotic stress, these metabolites may accrue to act as osmoprotectants against dehydration, scavengers of RS, and/or stabilizers of proteins and membranes [29].
