The lack of clean planting material is a major constraint for banana production in sub-Saharan Africa. The banana weevil, Cosmopolites sordidus, is the most important insect pest of highland bananas and plantains. Banana weevil larvae tunnel into the rhizome (underground stem), resulting in reduced nutrient uptake, weakening the stability of the plant, and causing plants to break. The burrowing nematode, Radopholus similis is globally the most important nematode pest to banana production and a major constraint in East Africa. Nematodes kill the roots by feeding on them. The destruction of root tissues reduces yield, and ultimately destroys plant anchorage, resulting in a typical symptom of toppling of the whole plant.

Banana in the region is traditionally propagated by means of suckers, which are rhizomes cut off from the motherplant. These suckers are infested with soilborne pests such as the banana weevil and the burrowing nematode. Consequently, the use of suckers as a primary means of banana propagation decreases yield and plantation longevity, with dire consequences for food security in the region. In the laboratory, banana can now be produced axenically through tissue culture. Apart from viruses, tissue culture banana plantlets are by definition a pest- and disease-free planting material (Fig. 1). When establishing new fields, tissue culture plantlets will reduce damage by banana pests and diseases. Pest infestation or reinfestation, however, remains a vital concern. Enhancing banana tissue culture plantlets is currently being investigated by IITA to extend the benefits of clean planting material. 

An endophyte is an organism that lives within plant tissues. This association is often mutualistic: endophytes provide the plant with antagonism against pests and diseases. Natural entry of an endophyte into a plant is a process that can be manipulated. Once inside the plant, an endophyte occupies a niche with relatively low competition from other microorganisms, provided the endophyte gets there first. Early entry is key, and can be effected through inoculation during propagation of the plant material. In other words, the endophyte becomes an intrinsic component of the planting material when sold to growers.

At IITA, research into endophyte-enhanced banana tissue culture was initiated in 1997 under funding by the German Federal Ministry for Economic Cooperation and Development (BMZ). Research into endophyte-enhanced banana tissue culture has so far yielded a wealth of information.

The quest for endophytes

The most critical step in the quest for endophytes is accurate isolation. Since the aim is to find strains that will target specific pests and diseases, IITA’s approach is to isolate endophytes from apparently healthy plants growing among high levels of pests or diseases. In addition, the endophytic flora varies with plant cultivar, host plant age and ecological conditions. To isolate endophytes, plant samples are surface-sterilized and plated onto specialized culture media in the laboratory. After identification and purification, they are stored. The most frequently isolated endophytes belong to the genus Fusarium. IITA has therefore focused primarily on this genus. 

Based on morphological and molecular analysis, sampling a handful of plants yields a vast array of endophytic strains. Hence, a rapid, easy, and cheap laboratory screening protocol was devised to test as many strains as possible against both the banana weevil and the burrowing nematode. Only a handful of strains that show high antagonism against the target pests are further tested. Merging concurrent research into the development of efficient inoculation techniques that yield high colonization, banana tissue culture plantlets are inoculated with these best endophytic strains for screenhouse assessment against the target pests. At present, IITA’s best strains are undergoing field testing against both pests.

This logical flow of biopesticide research has been carried out hand-in-hand with upstream research among the partners in the project. Foremost, much effort has been focused towards determination of endophytic modes of action. Recent interest at IITA has focused on the induction of resistance by endophyte infection of the plant against pests and diseases. Induced resistance is the activation of biochemical and structural plant defence mechanisms following contact with elicitors, such as endophytes. Induced plant resistance, elicited by an endophyte, provides a susceptible plant cultivar with pest or disease resistance. "Immunization" of plants with endophytes therefore can help complement current breeding programs. In addition, many endophytes are individual strains of F. oxysporum, which are ubiquitous soil organisms but can also be destructive wilt pathogens. It has therefore been essential to establish early the non-pathogenic nature of IITA’s promising strains, and determine markers to identify them (Fig. 2).

Our partners

The project is comprised of seven partners in five countries: the University of Bonn, Germany; the National Agricultural Research Organization (NARO), Uganda; the University of Pretoria, South Africa; Makerere University, Uganda; Wageningen University, The Netherlands; the Catholic University of Leuven, Belgium; and the Biologische Bundesanstalt fur Land- und Forstwirtschaft, Germany. IITA does not possess a sufficient in-house tissue culture production facility to accommodate its endophyte-based research. IITA has developed partnerships with key players involved in tissue culture production: a commercial tissue culture laboratory Agro-Genetic Technologies Ltd (AGT), with strong support from NARO, Uganda, and Jomo Kenyatta University of Science and Technology (JKUAT), Kenya. These partnerships have proven to be pivotal in steering endophyte-enhanced technology towards the ultimate client: the farmer (Figs 3 and 4).