Adaptation and tolerance of wheat plants to drought mediated by natural growth regulators Bacillus spp.: mechanisms and practical importance (review)

Environmental abiotic factors leading to water deficiency significantly limit the production of major crops worldwide (Z. Ahmad et al., 2018). In the face of rapid population growth and climate change, it is important to ensure food security, which is mainly possible by increasing the productivity of strategically important crops, including wheat, which is used for human consumption in many regions of the world and provides more than 50 % of food energy needs (S. Asseng et al., 2019). Application of beneficial growth-stimulating bacteria Bacillus spp. are effective, environmentally friendly and safe natural strategy for protecting plants from stresses resulting in water deficiency (M. Kaushal et al., 2019; A. Hussain et al., 2020; M. Camaille et al., 2021). To date, the growth-stimulating and protective effect of Bacillus spp. under various abiotic stresses are indicated for a wide range of plants (S. Moon et al., 2017; H.G. Gowtham et al., 2020; N. Shobana et al., 2020), including wheat (G. Sood et al., 2020; U. Rashid et al., 2021). The mechanisms of this physiological action of Bacillus spp. on host plants remain largely unknown. Presumably, it is due to i) competition for space and nutrients with plant pathogens and increased availability of macro- and micronutrients (S. Danish et al., 2019; D. Miljakovic et al., 2020; А. Kumar et al., 2021), ii) production of a wide range of bioactive components and protective compounds (M. Saha et al., 2016; R. Çakmakçı et al., 2017; N. Ilyas et al., 2020), and iii) induction of plant systemic tolerance to stresses (I.A. Abd El-Daim et al., 2019; C. Blake et al., 2021; U. Rashid et al., 2021). The efficacy of the same Bacillus strain may vary, depending on many factors including a spectrum of the synthesized compounds, strains, plant species, ecological and geographical origin, varietal characteristics, the types of stresses during the growing season, etc. (A. Khalid et al., 2004; G. Salem et al., 2018; O. Lastochkina et al., 2020). This review summarizes an information on the current state of research and the latest available information on plant-microbe interactions with a focus on protecting wheat against drought. In particular, the mechanisms underlying Bacillus -mediated adaptation and tolerance of wheat plants to drought are under consideration. It is shown that Bacillus spp. can induce wheat drought tolerance due to i) synthesis of compounds which provide protection against osmotic and oxidative stresses (D. Miljakovic et al., 2020; R. Çakmakçı et al., 2017), ii) intracellular transmission and enhancement of protective signals by a cascade of mediators, iii) regulation of the protective protein gene expression and interorgan transduction with the participation of the main phytohormones, their biosynthesis in the whole plant (U. Rashid et al., 2021), and iv) numerous compounds involved in increasing the bioavailability of macro- and microelements and productivity (А. Hussain et al., 2020; А. Kumar et al., 2021). Bacillus spp. can positively influence plant photosynthesis and water exchange (I.A. Abd El-Daim et al., 2019), as well as drought tolerance of wheat genotypes of different agroecological groups (L.I. Pusenkova et al., 2020). The joint use of Bacillus bacteria with other natural growth regulators enhance their effectiveness and stability of action (M. Zafar-ul-Hye et al., 2019). The listed commercial bacillary biologicals are effective on wheats. The review contributes to the understanding of the fundamental mechanisms of wheat- Bacillus spp. interactions under drought, the development of Bacillus -based biologicals and their use in ecologically oriented technologies for wheat growing under changing climate conditions.