**2. Plant growth promoting fungi (PGPF)**

*Organic Agriculture*

agriculture.

fitted to organic agriculture.

the field and be cost effective [6].

intensification produces more food in a way that the future production potential of farmland is being undermined and the environment is being affected. An increasingly degraded soil, overwhelming health hazards from soil and water pollution, disturbed natural microbial populations are a few of the direct implications in chemical-intensive agriculture. To avoid these potentially harmful effects of agrochemicals in agriculture, alternative approaches must be persuaded. An ecocentric approach that provides both environmental and economic benefits is increasingly needed. Organic farming is one of many such approaches that promote agroecosystem health, ensuring sustainable intensification in

The uniqueness of microorganisms and the dynamic part played by them in sustaining agricultural ecosystems have made them likely candidates for playing a central role in organic-based modern agriculture. Fortunately, plant roots harbor an abundant association of beneficial microorganisms. Root exudates are the largest source of carbon that attracts the microbial populations and allow them to forge an intimate association with host plants [3]. In response, the rhizosphere microbial populations play versatile roles in transforming, mobilizing and solubilizing soil nutrients, which are crucial for plant growth and development. Among the diverse rhizosphere microbial population, fungi known as plant growth promoting fungi (PGPF) are receiving a growing attention in recent days. Over the decades, varieties of PGPF have been studied including those belong to genera *Trichoderma, Penicillium, Phoma* and *Fusarium* [4]. Studies have shown that PGPF modulate plant growth and enhance resilience to plant pathogens without environmental contamination [5]. The positive effects of PGPF on plant and environment make them well

The course of plant growth promotion by PGPF is a complex process and often cannot be attributed to a single mechanism. A variety of direct and indirect mechanisms, including solubilization of minerals, synthesis of phytohormones, production of volatile organic compounds, exploitation of microbial enzymes, increases in nutrient uptake, amelioration of abiotic stresses and suppression of deleterious phytopathogens are involved. These wide arrays of interconnected mechanisms help PGPF maintaining rhizosphere competence and stability in host performance. Compared to the large number of PGPF identified in the laboratory, only a small fraction of them is in agricultural practice worldwide. Inconsistent performance of the inoculated PGPF under field conditions limits the commercial application of them. Development of appropriate formulation could improve the performance in the field and pave the way for commercialization of the PGPF. An ideal formulation of PGPF should fit with existing application technologies, protect biological actives from stress, ensure viability, remains unaffected after storage under ambient conditions, ensure microbial actives in

Considering the aspects discussed above, the need for superior PGPF to supplement inorganic chemical fertilizers as one of the crucial steps of moving toward organic farming practices has been highlighted. Inclusion of new techniques in these processes has been vital to the development of novel PGPF applications. This review will therefore attempt to shed light on the recent findings related to the impact of PGPF on plant growth and yield, duration of their effects, host specificity of the cooperation, root colonization mechanisms, their modes of action and commercial formulation for enhancement of plant growth and yield. The knowledge produced from this review could be very useful to those who are apprehensive about environmental protection and agricultural

**66**

sustainability.

Plants have intricate relationships with an array of microorganisms, particularly rhizosphere fungi and bacteria, which can lead to an increase in plant vigor, growth and development as well as changes in plant metabolism [7]. The group of rhizosphere fungi that colonize plant roots and enhance plant growth is referred to as PGPF [4]. PGPF are heterogeneous group of nonpathogenic saprotroph fungi. They can be separated into endophytic, whereby they live inside roots and exchange metabolites with plants directly, and epiphytic, whereby they live freely on the root

#### **Figure 1.**

*Beneficial interaction between plant and plant growth promoting fungi (PGPF). PGPF can modulate plant growth and development through the production of phytohormones and volatile compounds. PGPF also influence plant nutrition via solubilization of phosphorus and mineralization of organic substrates. PGPF modify plant functioning against biotic and abiotic stresses by negating their harmful effects.*

surface and free-living PGPF, which live outside plant cells, i.e., in the rhizosphere [5]. PGPF establish a non-obligate mutualism with a broader range of host plants. That is why symbiotic mycorrhizal fungi are not considered as PGPF, although they are known to improve growth of the plants [8]. Moreover, PGPF encompass a diverse taxonomic group in comparison to mycorrhiza. They are often involved in a range of complex interactions with plants and develop distinct strategies to mediate improvements in seed germination, seedling vigor, plant growth, flowering and productivity of host plants (**Figure 1**). PGPF are not only associated with the root to mediate positive effects on plant growth and development but also have beneficial effects on suppressing phytopathogenic microorganisms [9]. Not every organism identified as PGPF will improve plant growth under all conditions or in association with all plant hosts [10]. Some PGPF biocontrol inoculants usually contain necrotrophic mycoparasites such as *Trichoderma* spp. [11], while a limited number such as *Sphaerodes mycoparasitica* is biotrophic mycoparasitic agent [12]. Therefore, PGPF are considered one of the potential active ingredients in both biofertilizer and mycofungicide formulation.
