Mechanistic Insights of Plant Growth Promoting Bacteria Mediated Drought and Salt Stress Tolerance in Plants for Sustainable Agriculture

Abstract

Climate change has devastating effects on plant growth and yield. During ontogenesis, plants are subjected to a variety of abiotic stresses, including drought and salinity, affecting the crop loss (20-50%) and making them vulnerable in terms of survival. These stresses lead to the excessive production of reactive oxygen species (ROS) that damage nucleic acid, proteins, and lipids. Plant growth-promoting bacteria (PGPB) have remarkable capabilities in combating drought and salinity stress and improving plant growth, which enhances the crop productivity and contributes to food security. PGPB inoculation under abiotic stresses promotes plant growth through several modes of actions, such as the production of phytohormones, 1-aminocyclopropane-1-carboxylic acid deaminase, exopolysaccharide, siderophore, hydrogen cyanide, extracellular polymeric substances, volatile organic compounds, modulate antioxidants defense machinery, and abscisic acid, thereby preventing oxidative stress. These bacteria also provide osmotic balance; maintain ion homeostasis; and induce drought and salt-responsive genes, metabolic reprogramming, provide transcriptional changes in ion transporter genes, etc. Therefore, in this review, we summarize the effects of PGPB on drought and salinity stress to mitigate its detrimental effects. Furthermore, we also discuss the mechanistic insights of PGPB towards drought and salinity stress tolerance for sustainable agriculture.

Keywords: antioxidant defense; biostimulants; osmotic stress; plant–microbe interaction; reactive oxygen species; water deficit.

Figure 1

Drought and salinity-induced ROS generation in plants. Drought and salinity stress generates ROS via Fenton and Haber-Weiss reactions. ROS production by abiotic stresses modulates the enzymes (such as inducing NADPH oxidase and decreasing the antioxidant glutathione pool), activating calcium-dependent systems and altering iron-mediated processes. This led to a higher damage of ROS, thereby causing oxidative stress and damaging the cellular organelles.

Figure 2

Schematic representation of plant growth-promoting bacteria (PGPB)—mediated drought and salinity stress tolerance in plants. During drought stress, the plant itself and PGPB are able to detoxify ROS into stable nonreactive compounds. SOD—superoxide dismutase; CAT—catalase; GR—glutathione reductase; GPX—glutathione peroxidase; MAMPs—microbe-associated molecular patterns; NFs—nodulation factors. PGPB modulates the signaling pathways involved in drought and salt response biochemically and molecularly. Drought response is primarily regulated by ABA, which controls other signaling pathways such as SA, IAA, JA, and GA. SA—salicylic acid; ABA—abscisic acid; JAs—jasmonic acid; GAs—gibberellins; IAA—indole-3-acetic acid. PGPR also modulates transcription factors (TFs) that are essential in the drought and salt response and tolerance. NAM, ATAF, NAC, MYB/MYC, and WRKY—transcription factors; NF-Y—nuclear factor-Y; ERF—ethylene-responsive element-binding factor; LCOs—Lipo-chitooligosaccharides; BNF—Biological Nitrogen Fixation; AHP—cytokinin-related genes; AOC1—allene oxide cyclase; HKT—High-affinity K⁺ transporters; NHX1—vacuolar Na⁺/H⁺ antiporter gene; BADH1—Betaine aldehyde dehydrogenase 1; V-ATPase—Vacuolar-H⁺-pyrophosphatase; USP—Cytosolic universal stress protein; SDR1—salt and drought-responsive gene; LEA—late embryogenesis abundant; TIP1—Tonoplast AQP gene; SRP—Salt-responsive protein-encoding gene; SOS1—Salt overly sensitive gene. Figure created with BioRender.com (https://app.biorender.com/biorender-templates)—accessed on 27 January 2022.

Figure 3

Mechanisms of plant growth-promoting bacteria (PGPB)-induced tolerance of drought and salinity stress. Plant inoculated with PGPB experienced growth-promoting attributes like EPS and ESP production that modulate cellular water homeostasis. PGPB also induces the accumulation and synthesis of various osmoprotectants like trehalose, proline, glycine, phenols, flavonoids, and so on that help in scavenging ROS and RNS in cells. PGPB are also responsible for maintaining the ion homeostasis (Na⁺/K⁺) and removing the toxic ions from the cell. EPS—extracellular polymeric substances; ESP—exopolysaccharide; PL—polysaccharide lipid; LP—lipopolysaccharide protein; Na⁺—sodium ion; K⁺—potassium ion. Figure created with BioRender.com (https://app.biorender.com/biorender-templates)—accessed on 21 November 2021.