Multifaceted Potential of Plant Growth Promoting Rhizobacteria (PGPR): An Overview

Introduction

Plants have a very intricate relation with the soil as most of their advanced representatives are anchored to it through their root system. The soil acts as an important substrate and pool of minerals, nutrients and water for the plants. Out of the entire soil pool, the region which is in immediate vicinity of the plant is of utmost importance as it is in this region, all the interaction of the plant with the soil takes place. The importance of this zone was felt by the scientists long back and in 1904, German agronomist and plant physiologist Lorenz Hiltner first coined the term “rhizosphere” to illustrate the plant-root interface, a part of the word having its genesis from Greek word “rhiza”, indicating root. According to Hiltner, ‘rhizospehere’ is the area around the root of a plant which is dwelled by a unique variety of microorganisms (McNear and David, 2013). The rhizosphere can further be divided into three zones viz. The innermost is the endorhizosphere which includes portions of cortex and endodermis which is generally inhabited by microbes and cations. The middle region forms the rhizoplane which is directly adjacent to the root and includes the epidermal region and mucilage. The outermost region is the ectorhizosphere which extends from rhizoplane into the soil (McNear and David, 2013; Chaparro et al., 2014). Soil contains various types of microorganisms of which bacteria are most predominant. The type and number of bacteria may vary due to abiotic factors like moisture, temperature, soil nutrient and the presence of other flora in the soil (Glick et al., 1999). The interaction of these bacteria with the plants may be beneficial, harmful or neutral (Lynch and Whipps, 1990). A plant under field condition is not a stand-alone member but often forms a community with microorganisms called the phytomicrobiomes. The rhizomicrobiome (association between microbe and root) among them is most extensively studied (Backer et al., 2018).

As mentioned earlier, the rhizosphere is inhabited by a wide range of bacteria and majority of them are associated with the plant roots. These root associated bacteria that often enters into a symbiotic relation with the plants are called rhizobacteria. The definition of rhizobacteria can be further refined based on their beneficial activity. This lead to the introduction of a new phrase namely ‘plant growth promoting rhizobacteria (PGPR) which may be defined as the soil bacteria which inhabits on or around the root surface and directly or indirectly involved in promotion of plant growth and development through production of growth promoting substances or sequestering minerals or secreting a number of regulatory chemicals around the rhizosphere (Ahemad and Kibret, 2014). The term PGPR was first introduced by Kloepper and coworkers in the late 1970s (Tailor and Joshi, 2014) and from then onwards there has a continuous increase in interest on PGPR and its beneficial activities. In the 1990s, the original definition of PGPR was revised in order to accommodate a number of bacteria that have beneficial activity towards the plant but are present outside the rhizosphere (Bashan and Holguin, 1998; Goswami et al., 2016). Importance of PGPR was soon felt in the scientific world for the betterment of agricultural yield and sustainable development.

The hike in global population has resulted in an increased demand in food supply. In the 20^th century Green Revolution resulted in the attainment of global food security mainly through chemical inputs and improved crop variety. But the extensive use of chemical fertilizers, pesticides and herbicides took a heavy toll on the environment. To fulfil this global demand of food, fibre and fuel with minimised environmental stress, taking climate change into consideration, put forward the concept of ‘Fresh’ Green revolution or Bio- Revolution which emphasises on the utilization of phytomicrobiome for biological inputs and crop improvements (Timmusk et al., 2017).