Abstract
Simple SummaryEnvironmental conditions are subject to unprecedented changes due to recent progressive anthropogenic activities on our planet. Plants, as the frontline of food security, are susceptible to these changes, resulting in the generation of unavoidable byproducts of metabolism (ROS), which eventually affect their productivity. The response of plants to these unfavorable conditions is highly intricate and depends on several factors, among them are the species/genotype tolerance level, intensity, and duration of stress factors. Defensive mechanisms in plant systems, by nature, are concerned primarily with generating enzymatic and non-enzymatic antioxidants. In addition to this, plant-microbe interactions have been found to improve immune systems in plants suffering from drought and salinity stress.Plants are exposed to various environmental stresses in their lifespan that threaten their survival. Reactive oxygen species (ROS), the byproducts of aerobic metabolism, are essential signalling molecules in regulating multiple plant developmental processes as well as in reinforcing plant tolerance to biotic and abiotic stimuli. However, intensified environmental challenges such as salinity, drought, UV irradiation, and heavy metals usually interfere with natural ROS metabolism and homeostasis, thus aggravating ROS generation excessively and ultimately resulting in oxidative stress. Cellular damage is confined to the degradation of biomolecular structures, including carbohydrates, proteins, lipids, pigments, and DNA. The nature of the double-edged function of ROS as a secondary messenger or harmful oxidant has been attributed to the degree of existing balance between cellular ROS production and ROS removal machinery. The activities of enzyme-based antioxidants, catalase (CAT, EC 1.11.1.6), monodehydroascorbate reductase (MDHAR, E.C.1.6.5.4), dehydroascorbate reductase (DHAR, EC 1.8.5.1), superoxide dismutase (SOD, EC 1.15.1.1), ascorbate peroxidase (APX, EC 1.11.1.11), glutathione reductase (GR, EC 1.6.4.2), and guaiacol peroxidase (GPX, EC 1.11.1.7); and non-enzyme based antioxidant molecules, ascorbate (AA), glutathione (GSH), carotenoids, α-tocopherol, prolines, flavonoids, and phenolics, are indeed parts of the defensive strategies developed by plants to scavenge excess ROS and to maintain cellular redox homeostasis during oxidative stress. This review briefly summarises current knowledge on enzymatic and non-enzymatic antioxidant machinery in plants. Moreover, additional information about the beneficial impact of the microbiome on countering abiotic/biotic stresses in association with roots and plant tissues has also been provided.
Highlights
Constant changes in environmental conditions exacerbates unfavorable, stressful conditions such as salinity, drought, extreme temperatures, waterlogging, or heavy metal stress, severely affecting plant growth, development, and yield through inducing changes in plant physiological and biochemical characteristics [1,2].Plenty of studies have shown that plants are prone to generate both highly reactive oxygen free radicals and slightly reactive non-radicals of oxygen derivatives after subjection to various environmental biotic and/or abiotic stresses [3–5]
monodehydroascorbate reductase (MDHAR) and ascorbate peroxidase (APX) enzymes are co-localised in the mitochondria and peroxisomes, where their reducing and oxidising activity creates a balance between AA and MDHA pool sizes [10,182]
Apart from their presence in plants, carotenoids have been found in algae and photosynthetic microorganisms [152,256]. Their light-harvesting behaviour in chloroplasts encompasses light absorption by antenna molecules (450–570 nm) and transfer of energy to chlorophyll (Chl) molecules and photosynthetic machinery protection [258–260]. Carotenoids exert their antioxidative functioning in photosynthetic apparatus through scavenging singlet oxygen activity, reacting with lipid peroxidation (LPO) to halt the chain reaction of reactive oxygen species (ROS) production, quenching triplet (3Chl*), and exciting (Chl*) Chl molecules to prevent the formation of singlet oxygen, and dissipating excess excitation energy in the xanthophyll cycle [26,261–263]
Summary
Constant changes in environmental conditions exacerbates unfavorable, stressful conditions such as salinity, drought, extreme temperatures, waterlogging, or heavy metal stress, severely affecting plant growth, development, and yield through inducing changes in plant physiological and biochemical characteristics [1,2]. Stressful environmental conditions excessively accelerate cellular ROS concentrations [20] at levels exceeding the antioxidant scavenging capacities [11,17] employed by plants to neutralise excess ROS production [21] This feature potentially leads to oxidative stress along with damage to membrane lipids, proteins, and nucleic acids and eventually results in cell death [4,22,23]. After colonising the rhizosphere or endo-rhizosphere of plants, the PGPRs adopt several direct and/or indirect mechanisms to promote plant growth at the expense of tackling abiotic stresses [33–36] Some of these mechanisms include the induction of osmolyte accumulation [37], the activation of the antioxidant defence system [38], the up/downregulation of stress-responsive genes [39,40], and alteration in root morphology in acquisition of stress tolerance [41,42]. We provided a relatively comprehensive overview of major enzymatic and non-enzymatic antioxidants, pointed out the importance of PGPRs in alleviating plant stress, and summarised some current knowledge on them
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