Nigrospora oryzae: An endophyte of potential application in agriculture
Authors: Pardeep Kumar, A. Kandan, Jameel Akhtar, Bharat H. Gawade, Akansha Gupta
Division of Plant Quarantine, ICAR-National Bureau of Plant Genetic Resources, Pusa Campus, New Delhi- 110 012

Endophytes are microorganisms which can either complete or at least part of their life cycle inside the plants usually without inducing any symptoms. These include competent, facultative, obligate, opportunistic and passenger endophytes. The effect of endophytic association with the host plant can range from strongly mutualistic to parasitic. Among endophytic species, fungal endophytes appear to be highly coevolved with their host and are thought to be evolved directly from plant pathogenic fungal species. These endophytes are supposed to be latent pathogens at negligible level. Their latent phase represents a specific condition where the fungus can either develop symptoms or cause changes in the physiology of the host plant.

Endophytes can have several functions and may change the function during their life cycle. Plants harbour a variety of endophytes, which in turn protects their host from infections, herbivory and against unfavourable environmental conditions. Some common attributes of endophytes given by Stone et al. (1994) are i) they are internal, at least subticular and derive nutrition from the living host tissue; ii) they establish at least a transitory biotrophic nutritional relationship with their host; iii) the infected host tissues remain symptomless, i.e. disease free during their lifetime. Generally, these fungal species are transmitted through seeds (vertically) which is thought to promote beneficial relationships with the host plant for their development.

Fungal endophytes produce several metabolites, which are not only essential for the plant protection but are also important as medicines. The demand for new and highly effective biocontrol agents to manage pathogens is huge, and the removal of synthetic compounds from the market is also demanding, because of their toxicity to the environment.

Nigrospora oryzae (Berk. & Broome) Petch is a saprotrophic fungus colonizing debris of different living and dead plant species and considered as a weak parasite producing grain spot of rice, sorghum, corn and various grasses. It acts as an endophyte as well as a parasite depending upon host and environmental conditions. The teleomorph of N. oryzae is Khuskia oryzae H. Huds. The monotypic genus Khuskia belongs to the family Trichosphaeriaceae within the order Trichosphaeriales and fungus is widespread in all parts of the world, particularly in the tropical areas.

Morphological characters

N.igrospora oryzae develops as a black shiny conidia borne singly on short fragile conidiophores or in groups. Each conidium is born on hyaline vesicle at the tip of the conidiophores. The conidium shape ranges from spherical to sub-spherical black with the hyphae. Often, the conidia get discharged from condiophores during the course of examination under microscope and it makes the slide preparation more difficult.

Biochemical characters

N.igrospora oryzae has high cellulose and protease activities but lipase activity is not reported. It also produces griseofulvin which is orally non-toxic but active antifungal drug.

Molecular characters

ITS sequences of the N. oryzae are available on NCBI database (www.ncbi.nlm.nih.gov). We can identify endophyte on the basis of ITS sequences.

Application of N. oryzae

Biocontrol agent: Nigrospora oryzae is used as a biocontrol agent in Australia for weedy sporobolus grasses (WSGs). WSGs produce large amounts of long-lived seed and have tough leaves that are difficult for livestock to graze, reducing production. Chemical control of WSGs is expensive and often provides only short term suppression. N. oryzae causes crown rot and suppresses the growth of WSGs. Nigrospora crown rot fungus is generally a saprophyte or is occasionally a secondary cause of disease. The crown rot disease caused up to 78% reduction in tussock size of WSG due to antagonistic effects of Nigrospora fungus in Australia.

Antifungal and antibacterial drug: N.igrospora oryzae is present as endophytes in Emblica officinalis (Amla) and responsible for synthesis of bioactive compounds i.e. griseofulvin which is a non-toxic and an orally active antifungal drug derived from several species of Penicillium. In a study, antimicrobial activity of griseofulvin from N. oryzae was evaluated against two human pathogenic bacteria such as Staphylococcus aureus and Escherichia coli, two human pathogenic fungi like Trichophyton mentagrophytes and Microsporum canis and one plant pathogenic fungus Fusarium oxysporum. The growth of pathogenic fungi F. oxysporum, T. mentagrophytes, M. canis and bacteria S. aureus and E. coli were inhibited at less than 1.5 mg/ml concentration (Rathod et al. 2014).

Protector against plant diseases: The endophytic association may result in protection against insects or other pathogens, instead of causing damage. In a study, it was observed that in the grass Festuca arundinacea Schreb the presence of endophytes reduce the incidence of Puccinia graminis subsp. graminicola Z. Urban and Rhizoctonia zeae Voorhees. Similarly, Epichloe and Neotyphodium species also reported to protectvarious grasses against fungal pathogens, nematodes, insect and mammalian herbivores. Hence, endophytic relationships represent an advantage in the control of pest and diseases in economically important plants.

Impact of N. oryzae: This fungus has also capacity to cause diseases in plants, as it is a latent pathogen. This fungal species has been isolated and identified as a true endophytic pathogen on numerous plants. Generally it causes diseases in plant under stressed conditions, for examples, N. oryzae causes diseases in rice (rice grain spot) and maize (root rot), blueberry (spotted leaves), date palm (leaf spot), Dendrobium (leaf spot) etc. In Plant Quarantine Division, ICAR-National Bureau of Plant Genetic Resources, New Delhi, we have intercepted this fungus during quarantine examination of imported germplasm of various crops and their wild relatives especially rice from different countries.

Conclusions

Nigrospora oryzae is a fast growing fungus which acts as an endophyte, saprophyte and also as a weak pathogen depending upon host and environmental conditions. Apart from its saprophytic and pathogenic effects, there are few studies on different aspects of N. oryzae including potential to be an antibacterial and antifungal drug for animals and plants. It has also been potentially proved for its application as biocontrol agent against WSGs. So, this is high time and there is an urgent need to focus on various biochemical and molecular aspect of N. oryzae to utilize this fungus for welfare of mankind.

References:

1. Rathod DP, Dar MA, Gade AK and Rai MK (2014). Griseofulvin producing endophytic Nigrospora oryzae from Indian Emblica officinalis Gaertn: a new report. Austin J Biotechnol Bioeng. 1(3): 5.

2. Stone, JK, Viret, O, Petrini O and Chapela, IH (1994). Histological studies of host penetration and colonization by endophytic fungi. In: Host wall alterations by parasitic fungi (eds O. Petrini and G.B. Oullette). APS Press, USA: 115-126.


About Author / Additional Info:
Scientist, Plant Quarantine Division, ICAR- National Bureau of Plant Genetic Resources, Pusa Campus, New Delhi- 110 012