Fungi live in diverse habitats and have adapted to ecological niches, including plant systems. Plants and fungi have established complex mutualistic relationships, and wild plants are almost always colonized by endophytic, parasitic and mycorrhizal fungi (Rodriguez et al. 2009; Patkar and Naqvi 2017). Fungi produce a variety of bioactive compounds that play an important role in the physiological activities of the host plant, influencing the growth of the hosts. This can even lead to an increased tolerance to abiotic and biotic stresses of the plants (Pineda et al. 2010). Many studies have shown that fungi enhance plant growth through the solubilization of insoluble minerals in soil and secretion of plant growth regulators (Bilal et al. 2018; Chanclud and Morel 2016; Ju´nior et al. 2017; Khan et al. 2012). Growth promotion by plant growth regulators or phytohormones production, are signal molecules acting as chemical messengers and play a fundamental role in plants. Plant growth hormones produced by symbiotic fungi may greatly influence processes including seed germination, root initiation, stem and leaf growth, phototropism, flowering and fruit growth (Khan et al. 2015b; Petracek et al. 2003). On the other hand, many fungal pathogens produce phytohormones during host invasion and colonization, which are mainly involved in plant defense response  (Kazan and Lyons 2014; Spence et al. 2015).

The use of plant growth hormones in agriculture and horticulture has been growing significantly. One of the best studied plant growth regulators is indole-3-acetic acid (IAA), a principal auxin involved in apical dominance, root initiation, cell division and cell enlargement (Benjamins and Scheres 2008; Vessey 2003). Several fungal strains belonging to various families of Ascomycota, Basidiomycota and Mucoromycota have been reported to produce IAA (Chandra et al. 2018; Hasan 2002; Waqas et al. 2012). Recently, Numponsak et al. (2018) reported that the IAA
containing crude extract of a strain of Colletotrichum fructicola increased coleoptile elongation of rice, corn and rye, in a similar manner as the commercial IAA standard. Moreover, when the spore suspension of C. fructicola was applied in rice seedlings, it accelerated the growth of seedlings and enhanced their biomass and chlorophyll content.

Gibberellins were first discovered in the culture filtrate of the pathogen “Gibberella” (now classified in Fusarium) fujikuroi, which causes disease in rice plants (Hedden and Sponsel 2015). This hormone is used to accelerate the processes of seed germination, stem elongation, leaf expansion, flower initiation and fruit development (Yamaguchi 2008). Gibberellic acid can induce bolting and flowering in rosette species and rescue dwarf mutants of maize and peas (Hedden and Sponsel 2015).

Cytokinins (CKs) were reportedly found in pathogenic fungi such as Leptosphaeria maculans, Magnaporthe oryzae, and in mycorrhizal fungi (Crafts and Miller 1974; Chanclud et al. 2016; Trda´ et al. 2017). This hormone plays a significant role in plant physiological processes including apical dominance, cell division, leaf senescence, chloroplast biogenesis, vascular and shoot differentiation, programmed cell-death and anthocyanin production (Fahad et al. 2014).

Abscisic acid (ABA) is another important plant growth hormone, which was discovered as secondary metabolite of various fungi, including species of Aspergillus, Botrytis, Cercospora, Penicillium and Rhizopus (Shi et al. 2017). Abscisic acid plays a significant role in plant responses and adaptations to various environmental stresses, thus increasing crop yields (Devinar et al. 2013; Narusaka et al. 2003). Moreover, some fungi are able to produce ethylene (ET), salicylic acid (SA) and jasmonic acid (JA) hormones that regulate plant defense against pathogens as well as plant growth and development. Trichoderma species have been reported to simultaneously induce the ET, SA and JA pathways following pathogen attacks in Arabidopsis thaliana, grape, tomato and melon (Jogaiah et al. 2018). The production of plant hormones by fungi depends on their growth conditions, such as temperature, pH, incubation
period, growth dynamics and internal physiology (Bilal et al. 2018; Khan et al. 2012). Optimization using statistical approach is necessary to improve yield for production of phytohormones and other bioactive metabolites in the industry level (Albermann et al. 2013). Although plant growth regulators are widely found in plants, fungi and bacteria, they are being produced by chemical synthesis at the commercial scale. The high cost and low productivity of the proceesses available to access microbial-derived plant hormones limit their extensive application and thus restrict the development of the industry (Shi et al. 2017). Currently, ABA is produced with high yielding strains of B. cinerea and has been available commercially since 2009 (Rademacher 2015). Furthermore, in case of GAs production, Fusarium (Giberella) fujikuroi produces relatively high titers and therefore it is the principally utilized strain for industrial production. The possibility of the chemical synthesis of GAs was studied, and it was found that the compounds were too complex and the process too expensive to be a commercially viable alternative (Mander 2003). In order to increase the GAs titer, the mutagenesis of Fusarium fujikuroi and the use of cell immobilization together with extractive fermentation techniques were successfully applied in the production of GAs (Eleazar et al. 2000; Lale et al. 2006). In general, 60% to 90% of the total applied fertilizer was lost and the remaining 10% to 40% were taken up by plants. Only Trichoderma spp. and mycorrhizal fungi are commercially produced and applied in crop production.

Crop production not only faces the challenges of climate change and the diseases that affect them, but also increases in food demands due to the burgeoning global population. Biotic and abiotic stresses are also important limitations on global crop productivity. Hence plant hormones are playing an increasingly significant role in the horticulture and agricultural field. However, the low efficiency of fungal fermentation processes continues to preclude the cost reductions necessary for industrial-scale production. Thus, the study of fungal genome sequencing may help to shed light on the presence of the hormonal biosynthesis pathways. The molecular biology and transcriptomic analyses of fungal-derived plant hormones may provide more details related to the effects of phytohormones on plants, and ultimately effect the increased productivity of these hormones.