AMF inoculation and phosphorus supplementation alleviates drought induced growth and photosynthetic decline in Nicotiana tabacum by up-regulating antioxidant metabolism and osmolyte accumulation

Highlights

• Physiological and biochemical processes in Nicotiana tabacum were affected by drought stress resulting in altered growth.

• AMF inoculation allevaited the drought induced decline in photosynthesis by up-regulating the antioxidant metabolism and osmolyte accumulation.

• Enhanced drought tolerance was related to the elevated levels of phytohormones like ABA and IAA, and lowered ROS.

Abstract

Arbuscular mycorrhizal fungi (AMF) improves phosphorus (P) uptake in plants and regulates growth under environmental stresses. Present trials were undertaken to study the effect of phosphorus (P, 40 mg kg⁻¹ soil) supplementation and AMF (Glomus versiforme) inoculation on the growth performance of tobacco (Nicotiana tabacum L. variety Yunyan 87) under drought stress. Supplementation of P and inoculation of AMF significantly enhanced growth, biomass accumulation and root activity of tobacco plants under normal conditions and mitigated the drought induced decline. Chlorophyll and carotenoid contents, photosynthesis and PSII efficiency increased due to AMF and P treatment attaining maximal values due to their combined treatment. Drought resulted in generation of reactive oxygen species (ROS) like superoxide (O₂⁻) and hydrogen peroxide (H₂O₂) resulting in increased lipid peroxidation hence reducing membrane stability. Plants inoculated with AMF and/or supplemented with P exhibited reduced accumulation of ROS and lipid peroxidation. Content of osmolytes including proline, sugars and free amino acids were enhanced due to P and AMF treatment under normal as well as drought conditions that influenced the leaf relative water content. Reduction in oxidative damage in AMF and P treated plants was apparently linked with the stimulation of antioxidant system in them. Activities of SOD, CAT, APX, POD and GR, and the content of AsA and GSH improved significantly due to P and AMF treatment both under normal and drought conditions thereby ameliorating the oxidative effects. Further, accumulation of phenols increased more obviously due to P and/or AMF inoculation than in drought. Increased osmolyte and phenol content due to P and/or AMF helped tobacco plants to counteract the drought mediated damage by strengthening the non-enzymatic antioxidant system. P and/or AMF ameliorated the decline in contents of nitrogen, potassium and phosphorous, and activity of nitrate reductase more due to their combined treatments. Drought induced decline in nicotine content was mitigated because of P and/or AMF treatment. Additionally, mycorrhizal inoculation along with P supplementation increased indole-3-acetic acid (IAA) and abscisic acid (ABA) concentrations both in roots and leaves under drought stress. Hence P supplementation along with AMF inoculation mitigates the photosynthetic alteration and growth in tobacco plants by up-regulating antioxidant system and osmolyte accumulation.

Introduction

Global climatic change has increased the risk of abiotic stresses eventually reducing the crop production (Meena et al., 2017). It is pertinent to mention that drought is common environmental factor limiting crop yield (Mathur et al., 2019). Drought brings physiological and biochemical alterations in plants that affect their potential for increased production (Duque and Setter, 2019).

Drought reflects to defficiency or absence of enough soil moisture required for plant production (Hasanuzzaman et al., 2013). Most probably drought stress effects plant growth by affecting ion uptake and enzyme activity (Ahanger et al., 2017a,b). Drought decreases soil water potential thereby causes obstruction in water uptake by roots ultimately reducing the growth of plants (Rubin et al., 2017). The recent climate change trend significantly influences crop growth and productivity all over the world. With rising temperatures leading to melting of glaciers, drought is expected to occur more frequently thereby influencing the vegetation pattern and sustainability (Khosravi et al., 2017). Drought exposure initiates several alterations in the metabolism of plants through increased generation of toxic molecules like ROS (Czyczyło-Mysza and Myśków, 2017). Among the toxic radical species generated in different cellular organelles are included hydroxyl ion (OH⁻), hydrogen peroxide (H₂O₂) and superoxide (O₂⁻) (Mittler, 2002). Radicals affect plant structural and functional stability by oxidizing proteins, lipids and nucleic acids (Ahmad, 2010). Plants are fortified by various physiological and molecular processes to overcome with abiotic factors (Ahmad et al., 2014), amongst which eliciting of antioxidant enzymes e.g. ascorbate peroxidase (APX), superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) etc. has a vital role in plants stress tolerance (Hanin et al., 2016). Specific mechanisms are initiated to avert the drought mediated growth decline which includes: (a) osmoregulation through greater accumulation of compatible solutes leading to maintenance of tissue water content and (b) up-regulation of ROS neutralizing mechanisms (Ahanger et al., 2014b). Therefore it is important to devise management strategies for improving these indigenously existing tolerance mechanisms for protecting growth and improving productivity of crop plants in changing environments. Prime strategies include proper mineral fertilization or the use of beneficial microbes that directly or indirectly strengthen the tolerance mechanisms. AMF are the beneficial and important microorganisms exiting in the soil and have the ability to develop symbiotic association with nearly 90% of the plant species including bryophytes, ferns and flowering plants (Ahanger et al., 2014a; Begum et al., 2019a). AMF regulates plant growth and development by surrounding the roots with the extra-radical mycelium (Gosling et al., 2006) and improving the absorption of mineral nutrients (Schnepf et al., 2011). Plants colonized with AMF exhibit greater enzyme activity, mineral nutrition and photosynthesis (Begum et al., 2019a; Hazzoumi et al., 2015; Balliu et al., 2015). Several studies revealed that AMF inoculation modifies root morphology, influences the expression of genes and improve plant tolerance to abiotic stresses (Posta and Hong Duc, 2019; Hashem et al., 2016a; Khanna et al., 2019). Plants colonized with AMF exhibit improved growth, water uptake and yield (Miransari, 2011). It has been reported that drought stress tolerance enhances by AMF in various crops (Ruiz-Lozano et al., 2016; Augé, 2001; Yooyongwech et al., 2016). AMF leads to maintenance of moisture content and modulates the stomatal behavior of plants leading to improved plant–water relations (Augé et al., 2014; Ruiz-Lozano, 2003). AMF inoculation alleviates stress damage to growth and biomass accumulation by declining the generation of toxic radicals like H₂O₂ thereby preventing membrane damage by up-regulating the activities of antioxidant enzymes (Benhiba et al., 2015) and the stimulation of osmolytes in plants (Yooyongwech et al., 2013; Chitarra et al., 2016). Furthermore, AMF modify the hydraulic properties of plant roots (Zhao et al., 2015).

Phosphorus (P) is a macronutrient that have a major role in plant productivity (Becquer et al., 2014). Though, concentrations of inorganic phosphate are low in the soil results in limited use proficiency of plants (Shen and Wang, 2011). To cope with phosphorus deficiency, plants have evolved multiple strategies like enhancing the soil-root interface for the purpose to increase phosphorus transport or create symbiotic associations with AMF in the rhizosphere (López-Arredondo et al., 2014). It is evident that the inoculation of AMF improves uptake of nitrogen and phosphorus resulting in increased leaf area and growth (Balliu et al., 2015). The P uptake rate was considerably improved in the AMF inoculated plants of maize (Zea mays) as reported by Garcés-Ruiz et al. (2017) and Begum et al. (2019b). Increased seedling weight through increased water contents, intercellular CO₂ and P and N contents in Leymuschinensis due to AMF has been reported (Lin et al., 2018). AMF inoculation expedites the acquisition and translocation of mineral nutrients that are not easily accessible to plants especially P (Zarei et al., 2006).

In china and elsewhere in the world, tobacco is a famous economic crop (Poltronieri, 2016). China is amongst the biggest global tobacco producers and supports the economy of many central and western poverty-stricken areas in china. A significant increase in consumption of tobacco during the last 20 years was recorded, with more than 34.8 million packets of cigarettes were being produced and sold annually (Paskett et al., 2015; Peedin, 2011; Kist, 2018). Teng and Wang (2012) have reported that tobacco leaves contain numerous high-quality soluble proteins. The tobacco production and quality was affected by various abiotic stresses including drought stress (Bahrami-Rad and Hajiboland, 2017; Rizhsky et al., 2002) Thus, it is very much obvious to learn the importance of tobacco in detail.

Northwest area of China falls in semi-arid zone therefore drought conditions occur frequently and severely inhibits the plant growth thus leading to significant damage to the country economy (Zhu, 2011; Yang et al., 2011). Research studies have described the links between AMF and tobacco (Andrade et al., 2013; Hua et al., 2014; Yuan et al., 2016), while others have witnessed improved growth, photosynthesis and antioxidants functioning due to AMF (Del-Saz et al., 2017; Langeroodi et al., 2017). However, positive role of AMF in drought stress mitigation in tobacco through modulations in tolerance mechanisms under different P regimes have not been worked out. We hypothesize that AMF inoculation and P supplementation can improve drought stress tolerance by up-regulating antioxidant and osmolyte metabolism, root activity, mineral uptake and assimilation in tobacco. To get the target objectives experimental trials were undertake that are described below.