Physiological and molecular insight of microbial biostimulants for sustainable agriculture

Abstract

Increased food production to cater the need of growing population is one of the major global challenges. Currently, agro-productivity is under threat due to shrinking arable land, increased anthropogenic activities and changes in the climate leading to frequent flash floods, prolonged droughts and sudden fluctuation of temperature. Further, warm climatic conditions increase disease and pest incidences, ultimately reducing crop yield. Hence, collaborated global efforts are required to adopt environmentally safe and sustainable agro practices to boost crop growth and productivity. Biostimulants appear as a promising means to improve growth of plants even under stressful conditions. Among various categories of biostimulants, microbial biostimulants are composed of microorganisms such as plant growth-promoting rhizobacteria (PGPR) and/or microbes which stimulate nutrient uptake, produce secondary metabolites, siderophores, hormones and organic acids, participate in nitrogen fixation, imparts stress tolerance, enhance crop quality and yield when applied to the plants. Though numerous studies convincingly elucidate the positive effects of PGPR-based biostimulants on plants, yet information is meagre regarding the mechanism of action and the key signaling pathways (plant hormone modulations, expression of pathogenesis-related proteins, antioxidants, osmolytes etc.) triggered by these biostimulants in plants. Hence, the present review focuses on the molecular pathways activated by PGPR based biostimulants in plants facing abiotic and biotic challenges. The review also analyses the common mechanisms modulated by these biostimulants in plants to combat abiotic and biotic stresses. Further, the review highlights the traits that have been modified through transgenic approach leading to physiological responses akin to the application of PGPR in the target plants.

Keywords: PGPR; antioxidants; biostimulant; crop productivity; phytohormones; signaling; stress tolerance; systemic resistance.

Figure 1

(A) Schematic representation of PGPR induced stress tolerance mechanism in plant challenged by abiotic and biotic stresses. Different elicitors released by PGPRs modulates endogenous phytohormones which in turn influences secondary metabolites, osmolytes production, activity of antioxidant enzymes and PR proteins. These combined metabolic pathways imparts stress tolerance and promotes plant growth under stressed environment. (B) PGPR-based direct and indirect mechanism involved in activating cascade of abiotic and biotic stress signaling in plants. The activation events are represented by arrows, inhibition process is represented by bar while dashed arrows represent signaling cascade. IAA, indole-3-acetic acid; ACC, 1-Amino Cyclopropane-1-Carboxylate; AUX/IAA, auxin/indole-3-acetic acid; ARF, auxin response factor; ERF, ethylene response factor; P5CS1, Δ¹ - pyrroline-5-carboxylate synthase1; P5CS2, Δ¹ - pyrroline-5-carboxylate synthase 2; DREB2b/DREB1/DREB3, drought-responsive element binding protein 2b, drought-responsive element binding protein 1, drought-responsive element binding protein 3; MPK3/MPK6, mitogen–activated protein kinase 3, mitogen-activated protein kinase 6; RD29A/RD29B, response-to-desiccation 29A, response-to-desiccation 29B; PDF1.2, protodermal factor 1.2; CH1, chitinase; WRKY, W-box domain binding transcription factor; TIP2, tonoplast intrinsic protein 2; LTP1, lipid transfer protein 1; AQP7, aquaporin 7; GST6, glutathione S-transferase; DIP1, dehydration stress-inducible protein 1; DHN1, dehydrin 1; RAB21, responsive to ABA protein 21; PR, pathogenesis related proteins; LOX, lipoxygenase; AOS2, allene oxide synthase 2; PAL, phenylalanine ammonia lyase; ICS1, isochorismate synthase 1; NH1, Arabidopsis NPR1 homolog 1; PAD4, p hytoalexin-deficient 4; AOS, antioxidant scavenging; SOD, superoxide dismutase; CAT, catalase; APX, ascorbate peroxidase; POX, peroxidases; ROS, reactive oxygen species.

Figure 2

Model representing common physiological responses observed in wheat crop via transgenic approach and PGPRs application. Different colored symbols in the figure indicates the conditions of the experiment (Pots, Filter Paper, Hydroponics, Greenhouse potting mix).