Date Published: October 18, 2018
Publisher: Public Library of Science
Author(s): Tosapon Numponsak, Jaturong Kumla, Nakarin Suwannarach, Kenji Matsui, Saisamorn Lumyong, Sabrina Sarrocco.
Endophytic fungi are known to produce indole-3-acetic acid (IAA), which can stimulate plant growth. Twenty-seven isolates of endophytic fungi were isolated from Coffea arabica in northern Thailand. Only one isolate (CMU-A109) produced IAA in vitro. This isolate was identified as Colletotrichum fructicola based on morphological characteristics and molecular phylogenetic analysis of a combined five loci (internal transcribed spacer of ribosomal DNA, actin, β-tubulin 2, chitin synthase and glyceraldehyde-3-phosphate dehydrogenase genes). Identification of a fungal IAA production obtained from indole 3-acetamide (IAM) and tryptophan 2-monooxygenase activity is suggestive of IAM routed IAA biosynthesis. The highest IAA yield (1205.58±151.89 μg/mL) was obtained after 26 days of cultivation in liquid medium supplemented with 8 mg/mL L-tryptophan at 30°C. Moreover, the crude fungal IAA could stimulate coleoptile elongation of maize, rice and rye. This is the first report of IAA production by C. fructicola and its ability to produce IAA was highest when compared with previous reports on IAA produced by fungi.
Endophytic fungi are defined as the fungi that colonize plant tissues without causing any disease-related symptoms [1–3]. Colonization by endophytic fungi can improve the ecological adaptation of the host plant by enhancing its growth and tolerance to biotic and abiotic stresses [4, 5]. Several studies have reported the potential of endophytic fungi to produce bioactive compounds that protect the host plant against microbial influences and promote plant growth in both volatile and non-volatile substance productions [6–8]. Many studies have shown that plant growth promotion may be attributed to the secretion of plant-growth promoting secondary metabolites (phytohormones e.g. auxins, cytokinins, ethylene and gibberellins, and siderophore) and the ability to mobilize insoluble phosphate and provide nitrogen to their host plants by endophytic fungi [9–13].
IAA production by plant-associated microorganisms have been broadly studied and reported [7, 12, 20, 33]. In this study, C. fructicola CMU-A109 isolated from arabica coffee produced IAA in liquid medium supplemented with L-Trp at the IAA level 662.96 μg/mL. The results are similar to those of previous studies that found that the pure culture of endophytic fungi (e.g. Alternaria alternata, Aspergillus fumigatus, Chaetomium globosum, Chrysosporium pseudomerdarium, C. acutatum, C. gloeosporioides, Fusarium spp., M. cinnamomi, Paecilomyces spp., Penicillium spp., Phoma spp. and Tulasnella sp.) could produce IAA after being cultured in liquid medium supplemented with L-Trp [7, 10–13, 16, 17, 19, 49]. However, the present study provides the first report of IAA produced by C. fructicola. Indole derivatives as intermediate compounds give clues for identification of the IAA biosynthetic pathway. Reports on the IAA biosynthetic pathway of microbes are limited  and there is a need to more clearly understand the IAA production pathway of microorganisms. Consequently, a different biosynthesis pathway of IAA has been proposed for various microbial species [21, 22, 50] and it has been determined that a single species contained more than one pathway [22, 51]. Our study found that C. fructicola produced IAA from IAM, which was confirmed by the fungal IAA production from IAM and tryptophan 2-monooxygenease activities. Similarly, previous studies have reported synthesis of IAA via the IAM pathway in C. gloeosporioides f. sp. aeschynomene, F. proliferatum, F. verticillioides, F. fujikuroi and F. oxysporum [25, 31, 52]. However, Chung et al.  and Shilts et al.  reported synthesis of IAA by multiple pathways in C. acutatum (IAM and IPyA pathways). IAA production through the IPyA pathway was found in Piriformospora indica, Rhodosporidium paludigenum, Rhizoctonia cerealis, R. solani, Ustilago maydis and U. esculenta [26, 27, 30, 33, 54].
The endophytic fungus, C. fructicola CMU-A109 isolated from leaf tissues of coffee plant produced IAA in vitro via the IAM pathway. The highest IAA yield was obtained after 26 days of cultivation in liquid medium supplemented with 8 mg/mL L-Trp at 30°C. Crude fungal IAA stimulated coleoptile elongation as plays an important role in plant-growth promotion. Further study of this fungal isolate is required to evaluate its plant growth promoting abilities for the purposes of developing fungal inoculum. In addition, any pathogenicity needs to be tested.